Check Atom Conservation - Middle School Physical Science

1. What does it mean for atoms to be conserved in a chemical reaction model?

   Each element has the same number of atoms in reactants and products. Atoms cannot be created or destroyed in chemical reactions.

2. What is the correct conclusion if a model shows different numbers of an element on each side?

   The model violates atom conservation and is not balanced. Unequal atom counts indicate an unbalanced equation.

3. What must stay the same in a chemical reaction model: number of atoms, atom sizes, or element types?

   Number of atoms of each element and the element types must stay the same. Atoms rearrange but don't appear/disappear; sizes may vary in models.

4. Identify the total number of atoms (all elements) in one molecule of $C_6H_{12}O_6$.

   $24$ atoms. Sum all subscripts: 6 C + 12 H + 6 O = 24 atoms total.

5. What is the correct conclusion if a model shows different numbers of oxygen atoms before and after?

   The model violates conservation of atoms and is incorrect. Different atom counts mean matter was created or destroyed.

6. Identify whether this model conserves atoms: reactants have $2$ O atoms; products have $2$ O atoms.

   Yes, oxygen atoms are conserved. Equal O atoms on both sides confirms conservation.

7. Identify what a subscript changes in a chemical formula such as $H_2O$.

   It shows how many atoms of that element are in one molecule. Subscripts indicate atom ratios within a single molecule.

8. Identify what must match to verify conservation: total atoms or total molecules on each side?

   Total atoms of each element (not total molecules). Conservation applies to atoms, not molecular groupings.

9. What is the correct conclusion if at least one element has different atom counts in reactants and products?

   The model does not conserve atoms (it is not balanced). Unequal counts violate conservation of mass.

10. Identify whether atoms are conserved in the model: reactants $2H_2 + O_2$, products $2H_2O$.

    Conserved; $H=4$ and $O=2$ on both sides. Count: $2(2)=4$ H atoms and $2$ O atoms on each side.

11. Find the missing coefficient to conserve atoms: $2Na + Cl_2 \rightarrow \_NaCl$.

    $2$. Need $2NaCl$ to match $2$ Na and $2$ Cl atoms.

12. What is the total number of H atoms in $3H_2$?

    $6$ H atoms. Coefficient $3$ times $2$ H atoms per molecule equals $6$.

13. What is the correct method to verify atom conservation using particle models?

    Count atoms of each element on both sides and compare totals. This systematic approach ensures all atoms are accounted for.

14. What is the total hydrogen atom count in $2\text{H}_2\text{O}$?

    $4$ hydrogen atoms. $2 \times 2 = 4$ (coefficient times subscript).

15. Identify whether $H_2 + O_2 \rightarrow H_2O$ conserves atoms by counting O atoms.

    No; O atoms are $2$ on reactants and $1$ on products. $O_2$ has $2$ O atoms; $H_2O$ has only $1$ O atom.

16. Which option is conserved in a correct model: number of molecules or number of each type of atom?

    Number of each type of atom. Molecules can change in number, but atoms must be conserved.

17. What is the correct way to verify conservation using particle models of reactants and products?

    Count atoms of each element on both sides and compare totals. This ensures the law of conservation of mass is satisfied.

18. For $N_2 + 3H_2 \rightarrow 2NH_3$, what is the total number of N atoms on the product side?

    $2$ N atoms. Coefficient $2$ times $1$ N atom in $NH_3$ equals $2$.

19. What is the total atom count in one molecule of $\text{C}_2\text{H}_6$?

    $8$ atoms total. $2$ carbon $+ 6$ hydrogen $= 8$ total atoms.

20. What is the key difference between rearrangement of atoms and change of atoms in reaction models?

    Atoms rearrange into new groupings; the atom identities and totals stay the same. Chemical reactions only rearrange existing atoms into new compounds.

21. What is the correct method to verify conservation of atoms using models of a reaction?

    Count atoms of each element on both sides and compare totals. This systematic approach ensures no atoms are missed.

22. Identify the atom counts for $2H_2O$: how many $H$ atoms are in the products?

    $4$ $H$ atoms. $2H_2O$ means 2 molecules × 2 H atoms each = 4 total.

23. Identify the conserved feature when bonds break and new bonds form in a reaction model.

    Atoms are rearranged; the number of each type of atom stays constant. Conservation means same atoms before and after, just reorganized.

24. Identify what a subscript changes in a chemical formula model: element type or atoms per molecule?

    Subscript changes the number of atoms per molecule of that element. Subscripts define the chemical formula and cannot be changed.

25. What conclusion is correct if the number of each type of atom matches in reactant and product models?

    The model shows atom conservation. Equal atom counts verify the law of conservation of mass.

26. What is the atom count for one molecule of $C_2H_6$?

    $2$ C atoms and $6$ H atoms. Subscripts show atom counts per molecule.

27. Identify the correct conclusion if a model shows a new element appearing only in the products.

    The model violates conservation of atoms and is incorrect. Elements cannot spontaneously appear in chemical reactions.

28. Identify whether atoms are conserved in $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

    Yes, atoms are conserved. C: 1→1, H: 4→4, O: 4→4; all elements balance.

29. What does it indicate in a model if atoms appear to be created or destroyed during the reaction?

    The model violates conservation of atoms and must be revised. Matter cannot be created or destroyed in chemical reactions.

30. Identify whether $2CO + O_2 \rightarrow 2CO_2$ conserves atoms for C and O.

    Yes; C: $2=2$, O: $4=4$. $2CO$ has $2$ C; $2CO_2$ has $2$ C. $2CO$ has $2$ O, $O_2$ has $2$ O (total $4$); $2CO_2$ has $2×2=4$ O.

31. Which statement best describes what happens to atoms during a chemical reaction?

    Atoms are rearranged into new groupings; they are not created or destroyed. This follows the law of conservation of mass.

32. What is the formula to find total atoms of an element from coefficient and subscript?

    Total atoms $=$ coefficient $\times$ subscript (use subscript $=1$ if none). Multiply to find total atoms when counting for conservation.

33. Identify whether atoms are conserved in the model: $2H_2 + O_2 \rightarrow 2H_2O$.

    Conserved; H is $4$ on both sides and O is $2$ on both sides. Both sides have equal counts for each element type.

34. Which feature in a particle model usually identifies different elements: color or position?

    Color (or a distinct symbol) identifies different elements. Position shows arrangement; visual differences show element types.

35. Find and correct the error: "To conserve atoms, you may change subscripts in formulas."

    Correct: You may change coefficients, not subscripts. Subscripts define the substance; changing them creates different compounds.

36. Choose the correct statement for $2\text{CO} + \text{O}_2 \rightarrow 2\text{CO}_2$: are C and O conserved?

    Yes; C: $2=2$ and O: $4=4$. Both elements balance: $2$ C and $4$ O on each side.

37. Identify whether atoms are conserved: Reactants $\text{CH}_4 + \text{O}_2$, Products $\text{CO}_2 + 2\text{H}_2\text{O}$.

    Not conserved. Reactants have $2$ O; products have $4$ O.

38. Choose the correct conclusion if reactant and product models have equal counts for every element.

    The model satisfies conservation of atoms. Equal counts for all elements confirms a balanced reaction.

39. For $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$, are H atoms conserved?

    Yes; $4$ H atoms on each side. Left: $4$ H in $CH_4$; Right: $4$ H in $2H_2O$.

40. Identify the correct way to fix a model that does not conserve atoms without changing atom types.

    Adjust the number of molecules (coefficients), not subscripts. Changing subscripts creates different compounds entirely.

41. Which statement is correct: atoms are created, destroyed, or rearranged in chemical reactions?

    Atoms are rearranged; they are not created or destroyed. This follows the law of conservation of mass.

42. Which feature must match on both sides of a reaction model to confirm atom conservation?

    The count of each type of atom (each element) must match. Conservation requires equal atom counts for each element type.

43. Which statement best describes atoms during a chemical reaction: created, destroyed, or rearranged?

    Atoms are rearranged; they are not created or destroyed. This is the law of conservation of mass.

44. Find the missing reactant coefficient to conserve atoms: $x$ N$_2$ + $3$ H$_2$ → $2$ NH$_3$.

    $x = 1$. Products have 2 N, so need $1$ N₂ (2 N atoms) in reactants.

45. What does it mean for atoms to be conserved in a chemical reaction model?

    Each element has the same number of atoms in reactants and products. Conservation means no atoms are created or destroyed during reactions.

46. What is a coefficient in a chemical equation model, and what does it change?

    A number before a formula; it changes molecule count, not atoms per molecule. Coefficients multiply entire formulas, not individual atoms.

47. Which statement best describes what changes during a chemical reaction at the particle level?

    Atoms rearrange into new groupings; atoms are not created or destroyed. This reflects the law of conservation of mass.

48. Which option best describes a correct particle model: atoms created, atoms destroyed, or atoms rearranged?

    Atoms rearranged. Reactions rearrange atoms; they can't create or destroy them.

49. What is the total number of oxygen atoms in $2\text{Al}_2(\text{SO}_4)_3$?

    $24$ oxygen atoms. $2 \times 3 \times 4 = 24$ (coefficient × parentheses × subscript).

50. Identify what a subscript represents in a chemical formula such as $H_2O$.

    The number of atoms of that element in one molecule. Subscripts show atom count within a single molecule.

51. What is a subscript in a chemical formula model, and what does it change?

    A small number in a formula; it changes atoms per molecule. Subscripts show how many atoms of that element per molecule.

52. Verify atom conservation for $2H_2 + O_2 \rightarrow 2H_2O$: conserved or not conserved?

    Conserved. Both sides have 4 H atoms and 2 O atoms.

53. Identify whether atoms are conserved: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

    Conserved (C: $1=1$, H: $4=4$, O: $4=4$). Each element's atom count matches on both sides.

54. For $CaCO_3 \rightarrow CaO + CO_2$, are all element counts conserved?

    Yes; Ca $1$, C $1$, and O $3$ on each side. Decomposition redistributes atoms without changing totals.

55. Find the missing coefficient $x$ to conserve atoms: $xN_2 + 3H_2 \rightarrow 2NH_3$.

    $x=1$. Products have $2$ N atoms, so reactants need $1 \times N_2 = 2$ N atoms.

56. What is the correct first step to verify atom conservation using particle models?

    Count each element's atoms in reactants and in products. Systematic counting ensures accurate verification of conservation.

57. Which number can you change to balance an equation without changing the substances: coefficient or subscript?

    Coefficient. Changing subscripts would create different substances.

58. What is the correct conclusion if the reactant model has $6$ O atoms and the product model has $5$ O atoms?

    Atoms are not conserved; the model is incorrect or incomplete. Missing an O atom violates conservation laws.

59. Which statement best describes atoms in a correct product model: created, destroyed, or rearranged?

    Rearranged. Atoms can't be created or destroyed in chemical reactions.

60. Choose the correct coefficient to conserve atoms in $\_\_Na + Cl_2 \rightarrow 2NaCl$.

    $2$. This balances $Na$ to match $2$ $Na$ in products.

61. Which quantity must stay the same to verify atom conservation in a model: atoms or molecules?

    Atoms (of each element) must stay the same; molecules may change. Individual atoms are conserved, but they rearrange into new molecules.

62. Identify whether atoms are conserved: reactants have $2$ H and $1$ O; products have $2$ H and $2$ O.

    No, oxygen atoms are not conserved. Products have one extra O atom, violating conservation.

63. What conclusion is correct if each element's atom count matches on both sides of a model?

    The model shows atoms are conserved. Equal counts confirm the law of conservation of mass.

64. What is the total number of hydrogen atoms in $2\text{H}_2\text{O}$?

    $4$ hydrogen atoms. Multiply coefficient $2$ by subscript $2$.

65. Identify the conserved count for $2H_2 + O_2 \rightarrow 2H_2O$: how many O atoms on each side?

    $2$ O atoms on each side. $O_2$ gives $2$ O; $2H_2O$ gives $2×1=2$ O.

66. What does a subscript in a chemical formula tell you when checking conservation?

    It tells how many atoms of that element are in one molecule. Subscripts show atom ratios within a single molecule.

67. Which change is allowed in a chemical reaction model: atom type, atom count, or atom arrangement?

    Atom arrangement (bonds) can change; atom type and count cannot. Chemical reactions only break and form bonds between existing atoms.

68. Which side has more atoms in this incorrect model: Reactants total $7$ atoms; Products total $6$ atoms?

    Reactants have more atoms. The difference shows one atom was lost, violating conservation.

69. Find the total number of $H$ atoms in the reactants of $2H_2 + O_2 \rightarrow 2H_2O$.

    $4$ $H$ atoms. $2H_2$ means $2 \times 2 = 4$ hydrogen atoms total.

70. Find and correct the missing coefficient to conserve O: $2H_2 + O_2 \rightarrow H_2O$.

    Use coefficient $2$: $2H_2 + O_2 \rightarrow 2H_2O$. $O_2$ has $2$ O atoms that must equal products' $2×1=2$ O atoms.

71. What does a coefficient in front of a chemical formula represent in counting atoms?

    The number of molecules (or formula units) of that substance. Coefficients multiply all atoms in the formula.

72. Which option is valid evidence of conservation in models: equal total atoms or equal total molecules?

    Equal total atoms of each element, not equal total molecules. Molecule counts can differ; atom counts must match.

73. Identify whether atoms are conserved in $H_2 + O_2 \rightarrow H_2O$.

    No; oxygen atoms are not conserved. Reactants have $2$ $O$, products have $1$ $O$.

74. What does it mean for a chemical reaction model to conserve atoms?

    Each element has the same number of atoms in reactants and products. Conservation means no atoms are created or destroyed during reactions.

75. Which change is allowed when balancing a model: changing subscripts or changing coefficients?

    Changing coefficients only. Changing subscripts would create different compounds.

76. What is the correct conclusion if a model shows different numbers of an element on each side?

    The model does not conserve atoms and is incorrect. Unequal atom counts violate the law of conservation of mass.

77. Find and correct the error: "Balance by changing $\text{H}_2\text{O}$ to $\text{H}_2\text{O}_2$ to add oxygen."

    Correct: change coefficients, not subscripts or formulas. Changing formulas creates different compounds, violating reaction identity.

78. Identify whether atoms are conserved in the model: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

    Conserved; C $1$, H $4$, and O $4$ on both sides. All three elements have matching counts on both sides.

79. What is the second step to check atom conservation using particle models of reactants and products?

    Count each type of atom in the product model and compare to reactants. Compare product counts to reactant counts for verification.

80. Identify whether atoms are conserved: reactants $2H_2 + O_2$, products $2H_2O$.

    Conserved (H: $4=4$, O: $2=2$). Both sides have $4$ H atoms and $2$ O atoms total.

81. What is the first step to verify atom conservation using reactant and product models?

    Count atoms of each element on both sides and compare totals. Systematic counting ensures no atoms are missed or double-counted.

82. Verify atom conservation for $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$: conserved or not conserved?

    Conserved. Both sides have 1 C, 4 H, and 4 O atoms.

83. Identify what is conserved in a correct particle model of a chemical reaction: atoms, bonds, or molecules?

    Atoms. Only atoms are conserved; bonds and molecules can change.

84. Choose the missing coefficient to conserve atoms: $\_\_Al + 3Cl_2 \rightarrow 2AlCl_3$.

    $2$. Need $2$ Al atoms to match $2$ Al in products.

85. What does the law of conservation of atoms state for a chemical reaction model?

    Each element has the same number of atoms in reactants and products. This principle ensures mass is conserved during chemical reactions.

86. Find the missing coefficient to conserve atoms: $\_\_\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$.

    $1$. Products have $2$ N atoms, so reactants need $1 \times \text{N}_2$.

87. What is the total number of sulfur atoms in $4\text{H}_2\text{SO}_4$?

    $4$ sulfur atoms. Multiply coefficient $4$ by subscript $1$.

88. What must be true about the types of atoms when comparing reactant and product models?

    The same elements must appear on both sides. No new elements can appear or disappear in chemical reactions.

89. Find and correct the error: $2H_2 + O_2 \rightarrow H_2O$ does not conserve atoms. What is the product coefficient?

    Change product to $2H_2O$. Products need $2H_2O$ to match $4$ H and $2$ O atoms.

90. Which side must change to fix a model where reactants have $1$ C atom and products have $2$ C atoms?

    Product side must be corrected (or reactant side must be increased). Carbon atoms must be equal; adjust the deficient side.

91. What does a subscript change in a chemical formula: molecule count or atoms per molecule?

    A subscript changes the number of atoms per molecule. Subscripts show atom ratios within one molecule/formula unit.

92. Which change fixes an unbalanced model: changing coefficients or changing subscripts?

    Changing coefficients. Subscripts define compounds; only coefficients can be adjusted.

93. What is the meaning of a coefficient in a chemical equation when comparing models?

    It tells how many whole molecules (or formula units) are present. Coefficients scale entire formulas, not individual atoms.

94. What is the correct conclusion if any element's atom count differs between reactants and products?

    The model does not conserve atoms (the equation is unbalanced). Unequal counts violate conservation laws; balancing is needed.

95. What is the total number of oxygen atoms in $3\text{CO}_2$?

    $6$ oxygen atoms. Multiply coefficient $3$ by subscript $2$.

96. Identify whether atoms are conserved: reactants have $2$ Na and $1$ Cl; products have $2$ Na and $1$ Cl.

    Yes, both Na and Cl atom counts match. All element counts match, confirming conservation.

97. Identify whether atoms are conserved: $4\text{Fe} + 3\text{O}_2 \rightarrow 2\text{Fe}_2\text{O}_3$.

    Yes; Fe: $4=4$ and O: $6=6$. Both Fe and O atoms balance on each side of the equation.

98. Find the missing coefficient to conserve atoms: $\_N_2 + 3H_2 \rightarrow 2NH_3$.

    $1$. Products have $2$ N atoms, so need $1N_2$ (which has $2$ N).

99. What is the key visual sign in particle models that a chemical reaction occurred?

    Atoms are connected in different groupings (new molecules) after the reaction. Bonds break and reform to create different molecular structures.

100. Identify whether atoms are conserved if reactants have $2$ C and $6$ H and products have $2$ C and $6$ H.

     Conserved. Equal counts for all elements means conservation.

101. Find the total number of O atoms in $3\text{CO}_2$ when checking conservation in a model.

     $6$ O atoms. $3 \times 2 = 6$ oxygen atoms total.

102. What is the correct method to check atom conservation using particle models?

     Count atoms of each element on both sides and compare totals. This systematic approach ensures no atoms are missed or double-counted.

103. Identify whether atoms are conserved in this model count: Reactants $C:1, O:2$; Products $C:1, O:1$.

     No, oxygen atoms are not conserved. Reactants have $2$ O atoms but products have only $1$.

104. Identify whether $N_2 + 3H_2 \rightarrow 2NH_3$ conserves atoms for N and H.

     Yes; N: $2=2$, H: $6=6$. $N_2$ gives $2$ N; $2NH_3$ gives $2×1=2$ N. $3H_2$ gives $6$ H; $2NH_3$ gives $2×3=6$ H.

105. What does conservation of atoms mean in a chemical reaction model?

     Each element has the same number of atoms in reactants and products. Conservation means atoms can't be created or destroyed in reactions.

106. Find the missing product atom count to conserve atoms: Reactants $C:2$; Products $C:?$.

     $2$. Carbon atoms must be equal on both sides of the equation.

107. Which quantity must match on both sides to verify conservation: molecules or atoms of each element?

     Atoms of each element must match on both sides. Conservation applies to atoms, not molecules.

108. Identify whether atoms are conserved: reactants have $2$ O and $2$ H; products have $1$ O and $2$ H.

     No, oxygen atoms are not conserved. Oxygen count mismatch violates conservation.

109. Identify what a coefficient changes in a model: atoms inside a molecule or number of molecules?

     A coefficient changes the number of molecules (or formula units). Coefficients multiply entire formulas, not individual atoms.

110. What does it mean if a model shows the same atom count but different grouping of atoms in products?

     A chemical reaction occurred; atoms were rearranged into new molecules. Chemical reactions form new substances through atom rearrangement.

111. Find the missing product count: Reactants have $7$ N atoms; products show $5$ N atoms. How many N atoms are missing?

     $2$ N atoms are missing. $7 - 5 = 2$ nitrogen atoms unaccounted for.

112. Identify whether this model conserves atoms: reactants $1$ H$_2$ + $1$ O$_2$; products $1$ H$_2$O.

     No, it does not conserve atoms. H: 2→2 ✓, but O: 2→1, missing one O atom.

113. What is the correct way to count atoms for $2H_2O$ in a model or equation?

     $4$ H atoms and $2$ O atoms. Coefficient $2$ multiplies all atoms in $H_2O$.

114. What is the atom count for one formula unit of $CaCl_2$?

     $1$ Ca atom and $2$ Cl atoms. No subscript means one atom of that element.

115. Which part of a chemical formula tells how many atoms are in one molecule: coefficient or subscript?

     Subscript. Subscripts show atom count within a single molecule.

116. Which statement best describes atoms in a chemical reaction: created, destroyed, or rearranged?

     Atoms are rearranged into new combinations. Chemical reactions only break/form bonds between existing atoms.

117. What is the correct way to count atoms for $3CO_2$ in a model or equation?

     $3$ C atoms and $6$ O atoms. Multiply coefficient $3$ by subscript $2$ for O atoms.

118. Find the missing coefficient $x$ to conserve atoms: $2Al + 3Cl_2 \rightarrow xAlCl_3$.

     $x=2$. Reactants have $2$ Al atoms, so products need $2$ $AlCl_3$ molecules.

119. What is the first step to check atom conservation using particle models of reactants and products?

     Count each type of atom in the reactant model. Start by inventorying atoms on the reactant side.

120. What does conservation of atoms mean in a chemical reaction model?

     Each element has the same number of atoms in reactants and products. This is the fundamental law of conservation of mass in chemistry.

121. Choose the correct statement about mass when atoms are conserved in a closed system model.

     Total mass of reactants equals total mass of products. Same atoms means same total mass (conservation of mass).

122. Identify the element that is not conserved: $C_2H_6 + O_2 \rightarrow CO_2 + H_2O$.

     C and H are not conserved (coefficients are missing). Without coefficients, atom counts don't match for C and H.

123. What does a coefficient in a chemical equation model represent?

     The number of molecules (or formula units) of that substance. Coefficients multiply the entire formula.

124. Which statement best describes what atoms do during a chemical reaction?

     Atoms rearrange to form new molecules; atoms are not created or destroyed. This describes conservation of mass - matter cannot be created or destroyed.

125. Identify the total number of H atoms in $3H_2O$.

     $6$ H atoms. Each $H_2O$ has 2 H atoms, multiplied by coefficient 3.

126. Identify whether this model conserves atoms: reactants $2$ CO + $1$ O$_2$; products $2$ CO$_2$.

     Yes, it conserves atoms. C: 2→2, O: 4→4 (2+2→4), perfectly balanced.

127. Identify whether atoms are conserved in $N_2 + 3H_2 \rightarrow 2NH_3$.

     Yes; $N$ and $H$ atoms are conserved. Count shows $2$ $N$ and $6$ $H$ on each side.

128. What is the total number of O atoms in $2H_2O$?

     $2$ O atoms. Coefficient $2$ times $1$ O atom per molecule equals $2$.

129. Identify the correct atom count for $2H_2O$ when verifying conservation with models.

     $4$ H atoms and $2$ O atoms. $2 \times 2 = 4$ H atoms and $2 \times 1 = 2$ O atoms.

130. Find the missing product coefficient to conserve atoms: $2\text{H}_2 + \text{O}_2 \rightarrow ule{1.5cm}{0.15mm}\text{H}_2\text{O}$.

     $2$. $2\text{H}_2\text{O}$ gives $4$ H and $2$ O to match reactants.

131. Which is compared to check conservation: number of molecules or number of atoms?

     Number of atoms of each element, not the number of molecules. Conservation applies to atoms, as molecules can break and reform.

132. Which option indicates a balanced model: $2$ H atoms reactants and $2$ H atoms products, or $2$ and $3$?

     $2$ and $2$. Equal H atoms on both sides shows conservation.

133. Which option best describes a balanced particle model: same molecules or same atoms per element?

     Same atoms per element, not necessarily the same molecules. Conservation applies to atoms, not molecular arrangements.

134. What is the most common counting mistake when comparing reactant and product particle models?

     Ignoring subscripts or coefficients when counting atoms. Both numbers affect total atom count in different ways.

135. Identify whether atoms are conserved: reactants have $2$ H and $1$ O; products have $2$ H and $1$ O.

     Yes, atoms are conserved. Both sides have equal H and O atoms.

136. Identify the common modeling mistake if products show fewer atoms than reactants for one element.

     An atom was lost or not included; the model violates conservation. Missing atoms indicate an incomplete or incorrect model.

137. What is the key rule for comparing reactant and product particle models for conservation?

     Count atoms of each element on both sides and compare totals. This ensures conservation by matching atom inventories.

138. What does conservation of atoms mean when comparing reactant and product particle models?

     Each element has the same number of atoms before and after the reaction. Matter cannot be created or destroyed in chemical reactions.

139. Identify what a coefficient changes in a chemical equation model: subscript or molecule count?

     Coefficient changes the number of molecules (or formula units). Coefficients multiply entire formulas, not individual elements.

140. Identify whether atoms are conserved in $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

     Yes; $C$, $H$, and $O$ atoms are conserved. Count confirms $1$ $C$, $4$ $H$, and $4$ $O$ on each side.

141. Identify whether atoms are conserved in this model count: Reactants $H:4, O:2$; Products $H:4, O:2$.

     Yes, atoms are conserved. All element counts match between reactants and products.

142. Choose the missing coefficient to conserve atoms: $2Na + \_\_Cl_2 \rightarrow 2NaCl$.

     $1$. Need $2$ Cl atoms to match $2$ Cl in products.

143. Identify the element that is not conserved: $\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3$.

     Iron (Fe). Reactants have $1$ Fe, products have $2$ Fe.

144. Find the total number of $H$ atoms in the products of $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

     $4$ $H$ atoms. $2H_2O$ contains $2 \times 2 = 4$ hydrogen atoms.

145. What is the total oxygen atom count in $3\text{CO}_2$?

     $6$ oxygen atoms. $3 \times 2 = 6$ (coefficient times subscript).

146. Find the missing reactant coefficient to conserve atoms: $ ule{1.5cm}{0.15mm}\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$.

     $1$. $1\text{N}_2$ provides $2$ N atoms needed for $2\text{NH}_3$.

147. What is the correct conclusion if each element's atom count matches in reactants and products?

     The model shows atom conservation (the equation is balanced). Equal atom counts verify the law of conservation of mass.

148. Find the missing coefficient to conserve atoms: $2\text{Na} + \_\_\text{Cl}_2 \rightarrow 2\text{NaCl}$.

     $1$. Products have $2$ Cl atoms, so reactants need $1 \times \text{Cl}_2$.

149. Identify whether atoms are conserved: $2CO + O_2 \rightarrow 2CO_2$.

     Conserved; $C=2$ and $O=4$ on both sides. Carbon and oxygen atoms balance on both sides.

150. Identify whether this model conserves atoms: reactants have $4$ H atoms; products have $2$ H atoms.

     No, hydrogen atoms are not conserved. Unequal H atoms violates conservation law.

151. What does it mean for a chemical reaction model to conserve atoms?

     Each element has the same number of atoms in reactants and products. Conservation means no atoms are created or destroyed during reactions.

152. What should you conclude if a model shows $6$ oxygen atoms in reactants and $5$ in products?

     Atoms are not conserved; the model (or equation) is not balanced. Unequal atom counts violate conservation laws.

153. Which option verifies conservation for oxygen: reactants $5$ O, products $5$ O; or reactants $5$ O, products $4$ O?

     Reactants $5$ O, products $5$ O. Equal oxygen counts satisfy conservation requirement.

154. Find the missing coefficient to conserve atoms: $\text{H}_2 + ule{1.5cm}{0.15mm}\text{Cl}_2 \rightarrow 2\text{HCl}$.

     $1$. $1\text{Cl}_2$ provides $2$ Cl atoms for $2\text{HCl}$.

155. Which option is allowed in a correct model: splitting atoms, merging atoms, or breaking bonds?

     Breaking and forming bonds is allowed; atoms do not split or merge. Chemical reactions rearrange atoms via bond changes only.

156. Identify whether atoms are conserved: reactants $H_2 + O_2$, products $H_2O$.

     Not conserved; oxygen atoms differ ($2$ vs $1$). Reactants have $2$ O atoms but products have only $1$.

157. Identify the element not conserved if reactants have $4$ O atoms but products have $2$ O atoms.

     Oxygen (O). Missing 2 O atoms indicates oxygen wasn't conserved.

158. Which quantity must stay the same in a closed system reaction model: atom type or molecule type?

     Atom type (and atom count) must stay the same; molecules may change. Atoms rearrange but don't disappear; molecules can form/break.

159. What conclusion is correct if oxygen atoms differ between reactant and product models?

     The model violates atom conservation and is incorrect or incomplete. Atoms cannot be created or destroyed in chemical reactions.

160. Find the total number of H atoms in $2\text{H}_2\text{O}$ when checking conservation in a model.

     $4$ H atoms. $2 \times 2 = 4$ hydrogen atoms total.

161. What is the correct check to confirm a model matches the law of conservation of mass?

     Verify equal counts of each type of atom in reactants and products. Mass conservation requires atom conservation for each element.

162. Find the H atom totals in $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ on each side.

     H totals: reactants $4$, products $4$. $CH_4$ has $4$ H; $2H_2O$ has $2×2=4$ H.

163. What does it mean in a model if atoms are rearranged into new groupings after a reaction?

     Bonds change, but the atoms themselves are the same. Chemical reactions only rearrange atoms into new molecules.

164. Identify whether atoms are conserved: Reactants $2\text{H}_2 + \text{O}_2$, Products $2\text{H}_2\text{O}$.

     Conserved. Both sides have $4$ H and $2$ O atoms.

165. Identify what must be equal to confirm a model shows atom conservation for each element.

     Reactant atom count = product atom count for that element. Conservation requires equal atom counts on both sides.

166. Find the missing coefficient to conserve atoms: $2Na + Cl_2 \rightarrow xNaCl$.

     $x = 2$. Need 2 NaCl to balance 2 Na atoms from reactants.

167. Identify what a coefficient changes in a chemical equation or particle model.

     It multiplies the number of molecules (and all atoms in them). Coefficients scale entire molecules proportionally.

168. Which element is not conserved in this model count: Reactants $Na:2, Cl:2$; Products $Na:2, Cl:3$?

     Chlorine (Cl). Reactants have $2$ Cl atoms but products have $3$.

169. What is the total number of H atoms in $2H_2O$?

     $4$ H atoms. Coefficient $2$ times $2$ H atoms per molecule equals $4$.

170. What does conservation of atoms mean in a chemical reaction model?

     Each element has the same number of atoms in reactants and products. This follows the law of conservation of mass.

171. Identify the atom counts for $2H_2 + O_2$: how many $H$ atoms are in the reactants?

     $4$ $H$ atoms. $2H_2$ means 2 molecules × 2 atoms each = 4 total.

172. Identify the conserved elements in the reaction model $2H_2 + O_2 \rightarrow 2H_2O$.

     Both $H$ and $O$ are conserved. Count shows $4$ $H$ and $2$ $O$ on each side.

173. What must you do to subscripts when a coefficient is placed in front of a formula?

     Multiply every atom count in the formula by the coefficient. The coefficient distributes to all atoms in the formula.

174. What is the first step to verify conservation using a reactant and product particle diagram?

     List and count each type of atom in the reactants. Start with reactant inventory to establish what must be conserved.

175. Which element is not conserved in this model count: Reactants $N:2, H:6$; Products $N:2, H:4$?

     Hydrogen (H). Reactants have $6$ H atoms but products have only $4$.

176. Which change is allowed in a reaction model while still conserving atoms: rearrangement or creation of atoms?

     Rearrangement of atoms is allowed; creation or destruction is not. Chemical reactions only break and form bonds between existing atoms.

177. Which particle count must match on both sides to verify conservation: molecules or atoms?

     Atoms (of each element), not necessarily molecules. Individual atoms must be counted, as molecules can break apart and reform.

178. Which change is allowed when balancing a model: changing coefficients or changing subscripts?

     Changing coefficients is allowed; changing subscripts is not allowed. Subscripts define compounds; only coefficients balance equations.

179. What is the correct conclusion if each element has equal atom counts in reactants and products?

     The model shows atoms are conserved. Equal counts confirm the law of conservation of mass.

180. Which is conserved in typical chemical reactions: total mass or total volume of gas?

     Total mass is conserved. Mass conservation follows from atom conservation; gas volumes can change.

181. Find the missing reactant atom count to conserve atoms: Reactants $O:?$; Products $O:6$.

     $6$. Oxygen atoms must balance between reactants and products.

182. Identify what a coefficient in a chemical equation represents in a particle model.

     The number of molecules or formula units shown for that substance. Coefficients multiply entire formulas in balanced equations.

183. What does conservation of atoms mean in a chemical reaction model?

     Each element has the same number of atoms in reactants and products. This follows the law of conservation of mass in chemical reactions.

184. For $2H_2 + O_2 \rightarrow 2H_2O$, which element count is $4$ on both sides?

     Hydrogen (H). Left: $2H_2$ has $4$ H; Right: $2H_2O$ has $4$ H.

185. Identify what changes and what stays the same during a chemical reaction in particle models.

     Bonds change; the types and numbers of atoms stay the same. Atoms rearrange but their identities and quantities remain constant.

186. Identify whether atoms are conserved in $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$.

     Conserved. Both sides have $4$ H atoms and $2$ O atoms.

187. Identify the conserved items in a reaction model: atom types, atom counts, or particle colors?

     Atom types and atom counts of each type are conserved. Both the types and quantities must remain constant.

188. What must be the same on both sides of a reaction model to verify conservation of atoms?

     The count of each type of atom (each element) must match on both sides. Conservation requires equal atom counts for each element.

189. Identify what a coefficient represents in a model or chemical equation.

     The number of molecules (or formula units) of that substance. Coefficients multiply entire formulas, not individual atoms.

190. Identify whether atoms are conserved in $Na + Cl_2 \rightarrow NaCl$.

     No; $Cl$ atoms are not conserved. Reactants have $2$ $Cl$, products have $1$ $Cl$.

191. For $2H_2 + O_2 \rightarrow 2H_2O$, which element count is $2$ on both sides?

     Oxygen (O). Left: $O_2$ has $2$ O; Right: $2H_2O$ has $2$ O.

192. Which part of a chemical equation changes the number of molecules shown in a model: coefficient or subscript?

     Coefficient. Coefficients multiply the entire molecule count.

193. What conclusion is correct if the reactant and product models have equal counts for every element?

     Atoms are conserved; the model is consistent with conservation of matter. Equal counts verify the law of conservation of mass.

194. Identify whether atoms are conserved: $CH_4 + O_2 \rightarrow CO_2 + 2H_2O$.

     Not conserved; oxygen atoms differ ($2$ vs $4$). Reactants have $2$ O but products need $4$ O total.

195. Identify whether atoms are conserved in $\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$.

     Conserved. C: $3=3$, H: $8=8$, O: $10=10$ on both sides.

196. What does it mean for a chemical reaction model to conserve atoms?

     Each element has the same number of atoms in reactants and products. Conservation means atoms are neither created nor destroyed.

197. Identify whether atoms are conserved: $2Na + Cl_2 \rightarrow 2NaCl$.

     Conserved; $Na=2$ and $Cl=2$ on both sides. Each side has equal sodium and chlorine atoms.

198. Identify the atom counts for $2H_2O$: how many $O$ atoms are in the products?

     $2$ $O$ atoms. $2H_2O$ means 2 molecules × 1 O atom each = 2 total.

199. What is the first step to verify atom conservation from particle models of a reaction?

     Count atoms of each element on the reactant side. Start by inventorying each element type before the reaction.

200. Identify what a coefficient changes in a chemical equation or model.

     A coefficient multiplies the number of whole molecules (or formula units). It acts as a multiplier for the entire compound, not individual atoms.

201. Identify the total number of O atoms in $2CO_2$.

     $4$ O atoms. Each $CO_2$ has 2 O atoms, multiplied by coefficient 2.

202. What does a coefficient in a chemical equation tell you when comparing models?

     How many of that molecule or formula unit are present. Coefficients multiply the entire formula, affecting all atoms in it.

203. Identify whether atoms are conserved: reactants have $1$ C and $4$ H; products have $1$ C and $2$ H.

     No, hydrogen atoms are not conserved. Products are missing $2$ H atoms compared to reactants.

204. Identify what a subscript changes in a chemical formula or particle model.

     It changes the number of atoms of that element in one molecule. Subscripts define the molecular composition.

205. Which quantity must stay the same in reactant and product models: molecules or atoms?

     Atoms (of each element) must stay the same; molecules may change. Atoms rearrange but don't appear or disappear in reactions.

206. What conclusion is correct if one element has different atom counts in reactants and products?

     The model is not balanced; it violates conservation of atoms. Unequal atom counts prove the equation is unbalanced.

207. Choose the missing coefficient to conserve atoms: $\_\_H_2 + N_2 \rightarrow 2NH_3$.

     $3$. Need $6$ H atoms total to match $6$ H in products.

208. Identify the total number of O atoms in $3O_2$.

     $6$ O atoms. Coefficient 3 multiplies the 2 O atoms in each $O_2$ molecule.

209. Identify what a coefficient changes in a chemical equation or particle model.

     It multiplies the number of whole molecules (or formula units) shown. Coefficients scale entire formulas, not individual atoms.

210. What does a subscript in a chemical formula represent in a particle model?

     The number of atoms of that element in one molecule or formula unit. Subscripts show atom ratios within compounds.

211. Choose the correct coefficient to conserve $O$ in $2H_2 + O_2 \rightarrow \_\_H_2O$.

     $2$. This gives $2$ $O$ atoms in products to match reactants.

212. Identify the element not conserved: reactants $2$ Na and $1$ Cl; products $1$ Na and $1$ Cl.

     Na (sodium) is not conserved. Reactants have $2$ Na but products only have $1$ Na.

213. What is the correct check after counting atoms in reactants of a particle model?

     Count the same atom types in products and match each total. Conservation requires identical atom counts on both sides.

214. Choose the correct statement for a conserved model: Reactants have $2$ H atoms; products must have how many H atoms?

     $2$ hydrogen atoms. Conservation requires equal H atom counts on both sides.

215. State the rule for counting atoms of an element when a coefficient is present, such as $3H_2$.

     Total atoms = coefficient × subscript for that element. Multiply coefficient by subscript to find total atoms.

216. Identify whether atoms are conserved in the model: $CH_4 + O_2 \rightarrow CO_2 + 2H_2O$.

     Not conserved; O is $2$ on left and $4$ on right. Left has $2$ O atoms, right has $4$ O atoms total.

217. Which side has more O atoms in $C + O_2 \rightarrow CO$: reactants or products?

     Reactants; O atoms are $2$ vs $1$ on products. $O_2$ provides $2$ O atoms; $CO$ uses only $1$ O atom.

218. Which option best describes a balanced chemical equation in terms of atom conservation?

     Each element has equal atom counts on reactant and product sides. Balance means atoms are conserved for all elements.

219. Which quantity must stay the same on both sides to verify atom conservation in models?

     The count of each type of atom (each element). Individual elements must be tracked separately for conservation.

220. What is the key difference between a reactant model and a product model in a chemical reaction?

     Reactants show starting particles; products show ending particles. Models show the transformation from initial to final substances.

221. Identify whether atoms are conserved in the model: $N_2 + 3H_2 \rightarrow 2NH_3$.

     Conserved; N is $2$ and H is $6$ on both sides. Left: $2$ N and $6$ H; Right: $2$ N and $6$ H.

222. What conclusion is correct if any element's atom count differs between reactants and products?

     The model violates conservation of atoms. Unequal counts mean atoms were created or destroyed (impossible).

223. Identify whether atoms are conserved in the model: $H_2 + O_2 \rightarrow H_2O$.

     Not conserved; O atoms are $2$ on left and $1$ on right. Left has $2$ O atoms, right has only $1$ O atom.

224. What conclusion is correct if at least one element has different atom counts in reactants and products?

     Atoms are not conserved; the model or equation is incorrect. Unequal counts violate the law of conservation of mass.

225. Which statement best describes atoms during a chemical reaction: created, destroyed, or rearranged?

     Atoms are rearranged; they are not created or destroyed. This reflects the law of conservation of mass.

226. What changes in a chemical reaction model while total atoms of each element stay constant?

     Atoms rearrange into new combinations; bonds change but atoms remain. Matter is neither created nor destroyed, only rearranged.

227. Identify whether atoms are conserved: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

     Conserved; $C=1$, $H=4$, $O=4$ on both sides. Count each element: all match on both sides.

228. What is the quickest method to verify atom conservation in two particle models?

     Count each element's atoms on reactant side and product side and compare. This systematic approach ensures no atoms are missed in the comparison.

229. Identify whether atoms are conserved in the model: reactants $H_2 + O_2$, products $H_2O$.

     Not conserved (O atoms: $2$ on left, $1$ on right). Reactants have $2$ O atoms but products have only $1$.

230. Find the missing coefficient to conserve atoms: $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + \_\_\text{H}_2\text{O}$.

     $2$. Reactants have $4$ H atoms, needing $2$ water molecules.

231. Identify what a subscript changes in a chemical formula model (for example, in $H_2O$).

     A subscript changes the number of atoms of that element in one molecule. Subscripts define the chemical composition of a single molecule.

232. Identify whether atoms are conserved in $H_2 + O_2 \rightarrow H_2O$.

     No, atoms are not conserved. H: 2→2 balances, but O: 2→1 doesn't balance.

233. Verify atom conservation for $H_2 + O_2 \rightarrow H_2O$: conserved or not conserved?

     Not conserved. Reactants have 2 H and 2 O; products have 2 H and 1 O.

234. Identify the conserved count for $2H_2 + O_2 \rightarrow 2H_2O$: how many H atoms on each side?

     $4$ H atoms on each side. $2H_2$ gives $2×2=4$ H; $2H_2O$ gives $2×2=4$ H.

235. What must stay the same when comparing reactant and product particle models?

     The count of each type of atom (each element) must match on both sides. Conservation requires equal atom counts, not equal molecule counts.

236. Find the missing product coefficient to conserve atoms: $2$ H$_2$ + $1$ O$_2$ → $x$ H$_2$O.

     $x = 2$. Need 4 H and 2 O in products; $2$ H₂O provides both.

237. Find and correct the conservation error: $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$ (change coefficients only).

     $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$. Balances Mg and O atoms on both sides.

238. Identify the total number of H atoms in $2H_2$.

     $4$ H atoms. Coefficient 2 multiplies the 2 H atoms in each $H_2$ molecule.

239. Identify what is conserved in a correctly drawn chemical reaction model: atoms, molecules, or bonds.

     Atoms (and total mass) are conserved. Molecules and bonds change, but atoms remain constant.

240. Identify whether atoms are conserved if reactants have $4$ O atoms and products have $3$ O atoms.

     Not conserved. Different oxygen counts violate conservation.

241. What must be true about the count of each element when a chemical equation is balanced?

     Each element has equal atom counts on reactant and product sides. Balancing ensures conservation of mass and atoms.

242. What is the total oxygen atom count in $4\text{Al}_2(\text{SO}_4)_3$?

     $48$ oxygen atoms. $4 \times 3 \times 4 = 48$ (coefficient $\times$ subscript $\times$ subscript).

243. Find the total number of $O$ atoms in the reactants of $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$.

     $4$ $O$ atoms. $2O_2$ gives $2 \times 2 = 4$ oxygen atoms.

244. What is the correct method to verify atom conservation using particle models?

     Count atoms of each element on both sides and compare totals. This systematic approach ensures no atoms are missed in the comparison.

245. What is the fastest reliable method to check atom conservation using reactant and product models?

     Count each element's atoms on both sides and compare totals. Systematic counting ensures no atoms are missed or double-counted.

246. Find the missing coefficient to conserve atoms: $2H_2 + O_2 \rightarrow \_H_2O$.

     $2$. Need $2H_2O$ to balance: $4$ H and $2$ O on each side.

247. Which option best describes a balanced chemical equation?

     It has equal numbers of each type of atom on both sides. This satisfies the law of conservation of mass.

248. What does a subscript in a chemical formula tell you when counting atoms in a model?

     How many atoms of that element are in one molecule. Subscripts indicate atom ratios within a single molecule.

249. Identify the atom counts for $2H_2 + O_2$: how many $O$ atoms are in the reactants?

     $2$ $O$ atoms. $O_2$ has 2 oxygen atoms per molecule.

250. What does it indicate if the number of atoms of an element differs between reactants and products in a model?

     The model violates conservation of atoms and is incorrect. Unequal atom counts mean the reaction equation is unbalanced.

251. For $N_2 + 3H_2 \rightarrow 2NH_3$, what is the total number of H atoms on the reactant side?

     $6$ H atoms. Coefficient $3$ times $2$ H atoms in $H_2$ equals $6$.

252. Find the O atom totals in $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ on each side.

     O totals: reactants $4$, products $4$. $2O_2$ has $2×2=4$ O; $CO_2$ has $2$ O and $2H_2O$ has $2×1=2$ O, total $4$.

253. Find the total number of $H$ atoms in the reactants of $N_2 + 3H_2 \rightarrow 2NH_3$.

     $6$ $H$ atoms. $3H_2$ means $3 \times 2 = 6$ hydrogen atoms.

254. Which part of a chemical formula tells how many atoms of an element are in one molecule or unit?

     The subscript. Subscripts show atom count per molecule (e.g., $\text{H}_2$ has 2 H atoms).

255. What does a coefficient in a chemical equation tell you about a particle model?

     It tells the number of molecules (or formula units) present. Coefficients scale the entire formula, not individual atoms.

256. Identify whether atoms are conserved: Reactants $Mg:1, O:2$; Products $Mg:1, O:1$.

     No, oxygen atoms are not conserved. Reactants have $2$ O atoms but products have only $1$.

257. Identify the conserved quantities in a closed-system reaction model: mass, atoms, or temperature.

     Mass and atoms are conserved; temperature is not necessarily conserved. Temperature can change as energy is absorbed or released.

258. Identify whether this model conserves atoms: reactants $1$ CO$_2$; products $1$ CO + $1$ O.

     Yes, it conserves atoms. C: 1→1, O: 2→2 (1+1), all atoms accounted for.

259. Find the total number of $O$ atoms in the products of $2H_2 + O_2 \rightarrow 2H_2O$.

     $2$ $O$ atoms. $2H_2O$ contains $2 \times 1 = 2$ oxygen atoms.

260. What does a subscript in a chemical formula represent when counting atoms in a model?

     The number of that atom in one molecule or formula unit. Subscripts show atom ratios within a single molecule.

261. For $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$, are C atoms conserved?

     Yes; $1$ C atom on each side. Left: $1$ C in $CH_4$; Right: $1$ C in $CO_2$.

262. What does a subscript in a chemical formula represent in a particle model?

     The number of atoms of that element in one molecule. Subscripts show atom count within the molecule.

263. For $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$, are O atoms conserved?

     Yes; $4$ O atoms on each side. Left: $4$ O in $2O_2$; Right: $2$ O in $CO_2$ + $2$ O in $2H_2O$.

264. Identify whether this model conserves atoms: reactants $1$ Ca + $1$ Cl$_2$; products $1$ CaCl.

     No, it does not conserve atoms. Products show CaCl but Cl₂ has 2 Cl atoms; one is missing.

265. Identify whether atoms are conserved: Reactants $H:2, Cl:2$; Products $H:2, Cl:2$ but molecules differ.

     Yes, atoms are conserved even if molecules differ. Conservation depends on atom counts, not molecular arrangements.

266. What should you do after counting reactant atoms to verify conservation in a model?

     Count atoms of each element on the product side and compare. Compare totals to ensure no atoms were created or destroyed.

267. Which quantity must stay the same to verify atom conservation: atoms, molecules, or phases?

     Atoms (of each element) must stay the same. Molecules can form/break, but individual atoms are conserved.

268. Which option best indicates a correct particle model: same atom counts or same molecule counts on both sides?

     Same atom counts for each element. Conservation requires atom counts, not molecule counts.

269. What is a common sign that a particle model does NOT conserve atoms?

     At least one element has different atom counts in reactants vs. products. Unequal atom counts violate conservation law.

270. What is the total sulfur atom count in $4\text{Al}_2(\text{SO}_4)_3$?

     $12$ sulfur atoms. $4 \times 3 = 12$ (coefficient $\times$ subscript in parentheses).

271. Identify whether atoms are conserved in $\text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O}$.

     Not conserved (oxygen atoms do not match). Reactants have $2$ O atoms, products have $1$.

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