IB Chemistry Quiz: Understand Particulate Nature Of Matter
20 questions · exam conditions
0:00
Understand Particulate Nature Of MatterQuestion 1 of 20

Ammonia (NH₃) and hydrogen chloride (HCl) are volatile substances that react to form a white solid, ammonium chloride (NH₄Cl). If cotton wool plugs soaked in aqueous solutions of these two substances are placed at opposite ends of a sealed glass tube, a white ring of NH₄Cl forms. Given the relative molecular masses Mᵣ(NH₃) = 17 and Mᵣ(HCl) = 36.5, what can be deduced?

The white ring will form exactly in the middle of the tube.
The white ring will form closer to the hydrogen chloride end.
The white ring will form closer to the ammonia end.
The average kinetic energy of the NH₃ molecules is greater than that of the HCl molecules.
← Back to quizzes

IB Chemistry Quiz

IB Chemistry Quiz: Understand Particulate Nature Of Matter

Practice Understand Particulate Nature Of Matter in IB Chemistry with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Understand Particulate Nature Of Matter, giving you a quick way to practice the rules, question types, and explanations that matter most for IB Chemistry.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

Ammonia (NH₃) and hydrogen chloride (HCl) are volatile substances that react to form a white solid, ammonium chloride (NH₄Cl). If cotton wool plugs soaked in aqueous solutions of these two substances are placed at opposite ends of a sealed glass tube, a white ring of NH₄Cl forms. Given the relative molecular masses Mᵣ(NH₃) = 17 and Mᵣ(HCl) = 36.5, what can be deduced?

  1. The white ring will form exactly in the middle of the tube.
  2. The white ring will form closer to the hydrogen chloride end. (correct answer)
  3. The white ring will form closer to the ammonia end.
  4. The average kinetic energy of the NH₃ molecules is greater than that of the HCl molecules.
Explanation: At the same temperature, gas molecules have the same average kinetic energy. Since kinetic energy is ½mv², lighter molecules must have a higher average speed. Ammonia (Mᵣ = 17) is lighter than hydrogen chloride (Mᵣ = 36.5), so NH₃ molecules will diffuse faster down the tube than HCl molecules. Therefore, the ammonia gas will travel a greater distance in the same amount of time, and the white ring will form where the gases meet, which will be closer to the end where the slower-moving HCl was placed. D is incorrect because at the same temperature, the average kinetic energies are equal.

Question 2

A student separates a mixture of two miscible liquids with boiling points of 85 °C and 115 °C using distillation. Which statement correctly describes what happens in the distillation flask as the temperature first reaches 85 °C?

  1. Only the liquid with the 85 °C boiling point will vaporize; the other liquid remains completely unvaporized.
  2. The vapor phase will consist of a mixture of both liquids, but will be enriched in the more volatile component. (correct answer)
  3. Both liquids will vaporize in equal proportions, so no separation will be achieved.
  4. The temperature of the liquid in the flask will remain constant at 85 °C until all of the first component has boiled away.
Explanation: When a mixture of miscible liquids is heated, both components will exert a vapor pressure and vaporize to some extent at any given temperature. The liquid with the lower boiling point (the more volatile component) will vaporize more readily. Therefore, the vapor above the liquid will contain molecules of both substances, but it will be richer (have a higher mole fraction) in the component with the lower boiling point (85 °C). This enrichment allows for separation by condensation. A is an oversimplification. D is only true for a pure substance, not a mixture.

Question 3

An insulated container holds 50 g of liquid water at 25 °C. An identical insulated container holds 50 g of steam at 100 °C. The containers are cooled at the same constant rate. Which statement is correct regarding the temperature change over the first minute of cooling?

  1. The temperature of the liquid water will decrease more rapidly than the temperature of the steam.
  2. The temperature of the steam will remain at 100 °C for a period of time. (correct answer)
  3. Both containers will show the same rate of temperature decrease.
  4. The steam will immediately drop to a temperature below 100 °C.
Explanation: When the steam is cooled, it must first undergo a phase change from gas to liquid (condensation). This is an exothermic process that releases a large amount of latent heat of vaporization. As heat is removed from the container at a constant rate, this latent heat must be removed first before the temperature of the resulting liquid can begin to drop. Therefore, the temperature of the steam will plateau at 100 °C until all of it has condensed to liquid water. The liquid water at 25 °C, however, will begin to cool immediately as heat is removed.

Question 4

A substance is tested in a laboratory. It cannot be separated by filtration, chromatography, or distillation. When an electric current is passed through the molten substance, two new substances are formed at the electrodes. What is the classification of the original substance?

  1. A homogeneous mixture
  2. A heterogeneous mixture
  3. An element
  4. A compound (correct answer)
Explanation: The substance cannot be separated by physical means (filtration, chromatography, distillation), which indicates it is a pure substance, not a mixture. The fact that it can be broken down into simpler substances by a chemical process (electrolysis) means it is a compound. An element is a pure substance that cannot be broken down by chemical means.

Question 5

An aqueous solution contains dissolved caffeine (polar) and chlorophyll (non-polar). The two solutes need to be separated from each other and the water. Which process would be most effective for the initial separation of chlorophyll from the aqueous solution?

  1. Simple distillation to boil off the water, leaving a solid mixture of caffeine and chlorophyll.
  2. Paper chromatography using water as the mobile phase.
  3. Solvent extraction with a non-polar solvent like hexane. (correct answer)
  4. Filtration using a fine filter paper.
Explanation: Solvent extraction works on the principle of 'like dissolves like'. Chlorophyll is non-polar and will be much more soluble in a non-polar solvent (like hexane) than in the polar aqueous solution. Caffeine is polar and will preferentially remain in the aqueous layer. By adding hexane and shaking in a separating funnel, the chlorophyll will move into the hexane layer, which can then be physically separated from the water layer containing the caffeine. Distillation (A) would remove water but not separate the solutes. Chromatography (B) could work but extraction is often better for a bulk initial separation. Filtration (D) is incorrect as both substances are dissolved.

Question 6

A pure, colourless liquid, substance Q, is heated in an open container. It begins to boil at 110 °C and produces two different colourless gases, R and S. The boiling point remains constant at 110 °C until all of Q has vaporized. How should Q, R, and S be classified?

  1. Q is a compound that decomposes upon boiling; R and S could be elements or compounds. (correct answer)
  2. Q is a homogeneous mixture that separates upon boiling; R and S are its pure components.
  3. Q is an element undergoing a phase change; R and S are gaseous isotopes of Q.
  4. Q is a compound undergoing a phase change; R and S are molecules of gaseous Q.
Explanation: Since Q is a pure substance that produces two different substances (R and S) upon heating, it must be undergoing a chemical change (decomposition), not just a phase change. This means Q is a compound. A pure substance has a sharp, constant boiling point, but if it decomposes upon boiling, it's a chemical reaction occurring at that temperature. The products, R and S, could be simpler compounds or elements (e.g., 2H₂O₂ (l) → 2H₂O (g) + O₂ (g)). Choice B is incorrect because a mixture would typically boil over a range of temperatures. Choice C is incorrect as an element cannot decompose into different substances. Choice D is incorrect because a simple phase change would only produce gaseous molecules of Q, not two different substances R and S.

Question 7

When solid iron and solid sulfur are mixed at room temperature, they form a heterogeneous mixture. When this mixture is heated, solid iron(II) sulfide is formed in a highly exothermic reaction. Which statement is correct?

  1. The mixture of iron and sulfur is a compound, while iron(II) sulfide is an element.
  2. The formation of iron(II) sulfide from the mixture is a physical change.
  3. The properties of the iron and sulfur mixture are an average of the properties of its components.
  4. The iron(II) sulfide has chemical and physical properties that are different from those of iron and sulfur. (correct answer)
Explanation: The reaction between iron and sulfur forms a new substance, the compound iron(II) sulfide. A key characteristic of a chemical change is that the product has distinct properties (e.g., non-magnetic, different melting point) from its constituent elements. This is what distinguishes a compound from a mixture. A is incorrect as the classification is reversed. B is incorrect because a new substance is formed, which is a chemical change. C is not always true for mixtures, and the components retain their individual properties (e.g., iron is still magnetic).

Question 8

A student is given a mixture of sand (insoluble in water), ammonium chloride (soluble in water), and naphthalene (insoluble in water but soluble in hot ethanol). Which sequence of techniques would be most effective for separating the three components to recover each as a pure, dry solid?

  1. Add water to the mixture and filter; evaporate the filtrate to dryness; add hot ethanol to the residue, filter while hot, and then cool the new filtrate. (correct answer)
  2. Add hot ethanol to the mixture and filter; evaporate the filtrate to dryness; add water to the residue, filter, and then cool the new filtrate.
  3. Use a magnet to remove the sand; dissolve the remaining mixture in water; perform distillation to separate the ammonium chloride and naphthalene.
  4. Perform fractional distillation on the mixture; collect the naphthalene first, followed by the ammonium chloride, leaving the sand behind.
Explanation: The correct procedure involves using solubility differences. Step 1: Add water to dissolve the ammonium chloride. Filter to separate the insoluble sand and naphthalene (residue) from the aqueous ammonium chloride solution (filtrate). Step 2: Evaporate the water from the filtrate to recover solid ammonium chloride. Step 3: Add hot ethanol to the residue to dissolve the naphthalene, leaving the sand. Filter this hot mixture to separate the sand. Step 4: Cool the ethanol filtrate to cause the naphthalene to crystallize out, which can then be collected. Choice A correctly outlines this logic.

Question 9

In a paper chromatography experiment, a solvent is used to separate a mixture of two amino acids, alanine and lysine. Alanine is less polar than lysine. The stationary phase is cellulose (paper), which is polar. Which statement correctly predicts the result?

  1. Both amino acids will have the same Rƒ value because they are both polar molecules.
  2. Lysine will have a higher Rƒ value because it will be more strongly adsorbed by the stationary phase.
  3. Alanine will have a higher Rƒ value because it is less strongly adsorbed by the polar stationary phase. (correct answer)
  4. The Rƒ values cannot be predicted without knowing the identity of the mobile phase.
Explanation: In paper chromatography, the stationary phase (paper) is polar. Polar molecules will adsorb more strongly to the polar stationary phase and thus travel more slowly up the paper. Less polar molecules will be more soluble in a typical non-polar or moderately polar mobile phase and adsorb less strongly to the paper, allowing them to travel further. Since alanine is less polar than lysine, it will be less strongly adsorbed by the polar paper and will travel further with the mobile phase, resulting in a higher Rƒ value. Lysine, being more polar, will interact more strongly with the paper and have a lower Rƒ value.

Question 10

Two identical, sealed containers hold krypton (Kr) and neon (Ne) gas at the same temperature. The pressure in the container with krypton is twice the pressure in the container with neon. Which statement is correct?

  1. The average kinetic energy of the krypton atoms is twice that of the neon atoms.
  2. The average speed of the krypton atoms is greater than that of the neon atoms.
  3. The number of krypton atoms is approximately twice the number of neon atoms. (correct answer)
  4. The mass of krypton gas is approximately the same as the mass of neon gas.
Explanation: Since both gases are at the same temperature, their atoms have the same average kinetic energy (A is incorrect). Because krypton atoms are much more massive than neon atoms, for their kinetic energies (½mv²) to be equal, the neon atoms must have a higher average speed (B is incorrect). According to the ideal gas law (PV=nRT), pressure is directly proportional to the number of moles (n) when volume (V) and temperature (T) are constant. Since the pressure of Kr is twice the pressure of Ne, the container must contain twice the number of moles (and therefore atoms) of Kr (C is correct). Since there are twice as many Kr atoms and each Kr atom is much heavier than a Ne atom, the mass of krypton gas will be significantly more than twice the mass of neon gas (D is incorrect).

Question 11

At standard temperature and pressure, one mole of any gas occupies approximately 22.4 L. A student argues that this supports the idea that gas particles have negligible volume because "if the particles had significant size, different gases would occupy different volumes." Which analysis of this reasoning is most accurate?

  1. The reasoning is correct; equal molar volumes directly prove that gas particles themselves occupy negligible space compared to container volume.
  2. The reasoning is flawed; equal molar volumes result from equal numbers of particles with similar kinetic energies, regardless of individual particle size. (correct answer)
  3. The reasoning is partially correct; while particle volume is negligible, equal molar volumes actually result from identical intermolecular forces between different gases.
  4. The reasoning is incorrect; different gases do occupy different volumes, but the differences are too small to measure with standard laboratory equipment.
Explanation: Correct answer B: Equal molar volumes occur because gases have the same number of particles (Avogadro's number) and similar average kinetic energies at the same temperature and pressure. The negligible particle volume is a separate assumption of ideal gas behavior. A is wrong because equal volumes don't directly prove negligible particle size. C is wrong because intermolecular forces in ideal gases are assumed negligible, not identical. D is wrong because different ideal gases do occupy essentially the same molar volume at STP.

Question 12

A sealed container holds a mixture of nitrogen gas and solid iodine crystals at room temperature. Purple iodine vapor gradually appears in the gas phase until equilibrium is reached. Based on particulate behavior, what can be concluded about the energy distribution among iodine molecules in the solid phase?

  1. All iodine molecules in the solid have identical kinetic energies that are insufficient to overcome intermolecular forces holding the crystal together.
  2. The kinetic energies of solid iodine molecules are negligible compared to potential energies, which determine whether sublimation occurs at equilibrium.
  3. Iodine molecules in the solid phase have no kinetic energy until they absorb thermal energy from nitrogen gas molecules through collisions.
  4. Most iodine molecules have similar kinetic energies, but some molecules have sufficient energy to escape intermolecular attractions in the solid phase. (correct answer)
Explanation: This question tests your understanding of kinetic molecular theory and phase equilibria, specifically how molecular energy distribution enables phase transitions like sublimation. When you observe purple iodine vapor gradually appearing from solid crystals, you're witnessing sublimation at equilibrium. This occurs because molecules in any phase have a distribution of kinetic energies - not all molecules move at the same speed. According to Maxwell-Boltzmann distribution, most molecules cluster around an average energy, but there's always a fraction with much higher energies. Those high-energy molecules can overcome the intermolecular forces holding the crystal together and escape to the gas phase. Option D correctly describes this reality: most iodine molecules have similar kinetic energies insufficient for escape, but some have enough energy to sublime. This explains why sublimation occurs gradually until equilibrium, not all at once. Option A is wrong because it claims all molecules have identical energies, which contradicts kinetic molecular theory - molecular energies always follow a distribution. Option B incorrectly suggests kinetic energies are negligible; while potential energy matters, it's the kinetic energy distribution that determines which molecules can escape. Option C is fundamentally flawed because it states solid molecules have no kinetic energy, when in reality they're constantly vibrating and have thermal motion even in the solid phase. Remember: whenever you see phase transitions occurring gradually at constant temperature, think energy distribution. Not all molecules have the same energy - it's the high-energy "tail" of the distribution that drives sublimation, evaporation, and other phase changes.

Question 13

Two identical balloons are filled with equal moles of gas at the same temperature. Balloon X contains helium (M=4.00 g/molM = 4.00 \text{ g/mol}), and Balloon Y contains carbon dioxide (M=44.0 g/molM = 44.0 \text{ g/mol}). After several hours, Balloon X has noticeably deflated while Balloon Y remains nearly unchanged. Which particulate explanation best accounts for this observation?

  1. Helium atoms have higher average kinetic energy than carbon dioxide molecules, causing more frequent and energetic collisions with the balloon material.
  2. Carbon dioxide molecules form stronger intermolecular attractions with the balloon material, preventing them from escaping through microscopic pores.
  3. Helium atoms have smaller molecular diameter and higher average velocity, increasing their probability of escaping through microscopic openings in the balloon. (correct answer)
  4. Carbon dioxide molecules have greater mass and momentum, allowing them to maintain pressure against the balloon walls more effectively than helium atoms.
Explanation: Correct answer C: At the same temperature, lighter helium atoms have higher average velocities than heavier CO₂ molecules (Graham's law). Combined with their smaller size, helium atoms more easily effuse through tiny pores in the balloon material. A is wrong because both gases have the same average kinetic energy at the same temperature. B is wrong because gas escape is due to effusion through pores, not intermolecular attractions with balloon material. D is wrong because pressure depends on particle collisions per unit time and area, not just momentum.

Question 14

At 25°C25°\text{C}, liquid water molecules have an average kinetic energy, yet the water doesn't spontaneously boil. However, some water molecules at the surface do evaporate. Which statement best explains this apparent contradiction using principles of particulate behavior?

  1. Surface molecules have access to thermal energy from air molecules, giving them higher kinetic energies than molecules in the liquid interior.
  2. All molecules have the same average kinetic energy, but surface molecules experience weaker intermolecular forces due to fewer neighboring molecules.
  3. Surface molecules undergo chemical decomposition at the liquid-gas interface, converting to vapor phase through bond breaking rather than physical evaporation.
  4. Individual molecules have a range of kinetic energies around the average, and surface molecules with sufficient energy can overcome intermolecular attractions. (correct answer)
Explanation: This question tests your understanding of kinetic molecular theory and phase transitions at the particle level. The key insight is recognizing that "average" kinetic energy doesn't mean all molecules have identical energies. At any given temperature, molecules in a liquid have a distribution of kinetic energies around the average value - this is called the Maxwell-Boltzmann distribution. While most molecules have energies near the average, some have significantly higher or lower energies due to random collisions. For evaporation to occur, surface molecules need enough kinetic energy to overcome the intermolecular forces holding them in the liquid phase. Even though the average energy isn't sufficient for boiling, the highest-energy molecules in the distribution can still escape as vapor. This explains why evaporation happens at temperatures below the boiling point. Option A incorrectly suggests surface molecules gain extra energy from air - temperature equilibrium means air and liquid surface have the same average kinetic energy. Option B correctly notes that surface molecules experience weaker forces (fewer neighbors), but wrongly claims all molecules have identical kinetic energies rather than a distribution. Option C describes chemical decomposition rather than the physical process of evaporation, where no bonds break within H2OH_2O molecules. Option D correctly identifies both crucial factors: the energy distribution around the average (some molecules have much higher energies) and the fact that only surface molecules with sufficient energy can overcome intermolecular attractions to evaporate. Remember: whenever you see questions about phase changes below boiling/melting points, think about energy distributions rather than average values alone.

Question 15

A student observes that when equal masses of aluminum powder and aluminum foil are separately added to hydrochloric acid, the powder reacts much more rapidly than the foil. The student concludes that "breaking aluminum into smaller pieces changes its chemical properties." Which analysis of this conclusion is most accurate?

  1. The conclusion is incorrect; the chemical properties remain identical, but smaller particles provide greater surface area for contact with acid molecules. (correct answer)
  2. The conclusion is correct; smaller particles have different electronic configurations that make them more chemically reactive than larger particles of the same substance.
  3. The conclusion is partially correct; while chemical properties change slightly, the primary factor is that smaller particles have higher kinetic energy.
  4. The conclusion is correct; mechanical subdivision creates new chemical bonds at particle surfaces that are more reactive than bonds within the bulk material.
Explanation: This question tests your understanding of the difference between chemical and physical properties, particularly how particle size affects reaction rates without changing the fundamental nature of a substance. The key insight is that aluminum's chemical properties—its electron configuration, bonding behavior, and intrinsic reactivity—remain identical whether it's in powder or foil form. What changes dramatically is the surface area available for reaction. When you break aluminum into smaller pieces, you expose more surface atoms to the hydrochloric acid molecules. Since chemical reactions occur at the interface between reactants, more surface area means more simultaneous collision sites, leading to faster reaction rates. This is a physical effect, not a chemical one. Option A correctly identifies this distinction: the chemical properties are unchanged, but increased surface area accelerates the reaction. Option B incorrectly claims that particle size alters electronic configuration—electrons in aluminum atoms remain the same regardless of how you divide the metal. Option C mentions kinetic energy changes, but smaller particles at the same temperature don't inherently possess higher kinetic energy than larger ones. Option D suggests that mechanical subdivision creates new, more reactive bonds, which is false—breaking aluminum doesn't change the nature of metallic bonding. When you encounter reaction rate questions on the IB exam, always distinguish between factors that change intrinsic chemical properties (like temperature affecting molecular energy) versus those that change physical conditions (like surface area or concentration). Particle size is a classic physical factor that affects reaction kinetics without altering chemical identity.

Question 16

A student observes that when sodium chloride dissolves in water, the solution conducts electricity, but when sugar dissolves in water, the resulting solution does not conduct electricity. Based on the particulate nature of matter, which statement best explains the fundamental difference between these two dissolution processes?

  1. Sodium chloride molecules break apart into smaller molecules, while sugar molecules remain intact as larger molecular units in solution.
  2. Sodium chloride dissociates into separate charged particles that move independently, while sugar molecules disperse as neutral particles between water molecules. (correct answer)
  3. Sugar particles form hydrogen bonds with water molecules that prevent electrical conduction, while sodium chloride particles do not form such bonds.
  4. Sodium chloride has a lower molecular mass than sugar, allowing its particles to move more freely and conduct electricity through the solution.
Explanation: Correct answer B: NaCl is an ionic compound that dissociates into Na⁺ and Cl⁻ ions when dissolved, creating mobile charged particles that can conduct electricity. Sugar is a molecular compound that dissolves as intact neutral molecules. A is wrong because NaCl doesn't consist of molecules that break apart - it's ionic. C is wrong because hydrogen bonding doesn't prevent conduction; the lack of charged particles does. D is wrong because molecular mass doesn't determine conductivity - charge does.

Question 17

When a small amount of potassium permanganate (KMnO4\text{KMnO}_4) is placed in one corner of a large container of still air, the purple color gradually spreads throughout the container over several hours. A student claims this demonstrates that "air molecules are moving randomly and colliding with the potassium permanganate particles." What is the most accurate evaluation of this claim?

  1. The claim is entirely correct; air molecules collide with KMnO4\text{KMnO}_4 particles, causing them to move randomly throughout the container by Brownian motion.
  2. The claim describes the correct mechanism; random molecular motion and collisions explain how KMnO4\text{KMnO}_4 molecules disperse from high to low concentration regions.
  3. The claim is partially correct about random motion, but the spreading occurs because KMnO4\text{KMnO}_4 molecules sublime and then move independently through kinetic motion. (correct answer)
  4. The claim incorrectly identifies the mechanism; the spreading results from convection currents caused by density differences, not random molecular motion.
Explanation: Correct answer C: The observation involves sublimation of solid KMnO₄ into gaseous molecules, which then diffuse through random kinetic motion. While air molecules do move randomly, the primary mechanism is the random motion of the KMnO₄ molecules themselves after sublimation. A is wrong because it overemphasizes Brownian motion over sublimation and diffusion. B is wrong because it doesn't account for the phase change from solid to gas. D is wrong because the primary mechanism is molecular diffusion, not convection.

Question 18

A sealed flask contains both liquid water and water vapor at equilibrium at 60°C60°\text{C}. A student claims that "molecules are constantly moving between liquid and gas phases, but we can't observe any macroscopic changes because the rates of evaporation and condensation are equal." Which aspect of particulate theory does this claim most directly illustrate?

  1. The concept that equilibrium represents a dynamic balance where opposing microscopic processes occur simultaneously at equal rates. (correct answer)
  2. The principle that molecular motion ceases at equilibrium, allowing the system to maintain constant macroscopic properties without energy input.
  3. The idea that intermolecular forces become negligible at equilibrium, allowing molecules to move freely between phases without energy barriers.
  4. The principle that molecular kinetic energies become uniform at equilibrium, preventing further phase changes from occurring in either direction.
Explanation: When you encounter questions about equilibrium systems, focus on the fundamental concept that equilibrium is dynamic, not static. The student's observation perfectly captures this principle—molecules are continuously switching between liquid and gas phases, but the system appears unchanged because evaporation and condensation occur at identical rates. This scenario directly illustrates dynamic equilibrium, where opposing microscopic processes balance each other perfectly. At 60°C60°\text{C}, water molecules in the liquid phase gain enough kinetic energy to escape into the vapor phase (evaporation), while simultaneously, vapor molecules lose energy and return to the liquid phase (condensation). The equal rates create the illusion of a motionless system at the macroscopic level, even though intense molecular activity continues. Answer A correctly identifies this dynamic balance concept. Answer B is fundamentally wrong—molecular motion never ceases at equilibrium; it's the constant motion that maintains equilibrium. Answer C misrepresents intermolecular forces, which remain significant and actually determine the equilibrium position; they don't become negligible. Answer D incorrectly suggests uniform kinetic energies prevent phase changes, when in reality, the distribution of kinetic energies (following Maxwell-Boltzmann distribution) enables continuous phase transitions in both directions. Study tip: Remember that "equilibrium = dynamic balance" is a core theme throughout IB Chemistry. Whether you're dealing with phase equilibria, chemical equilibria, or solubility equilibria, the system always involves continuous molecular-level processes occurring at equal rates, creating apparent stability at the observable scale.

Question 19

According to the kinetic molecular theory, increasing the temperature of an ideal gas in a container of fixed volume increases the pressure because...

  1. the gas molecules expand and occupy more space.
  2. the number of gas molecules in the container increases.
  3. the molecules collide with the walls more frequently and with greater force. (correct answer)
  4. the attractive forces between the molecules are overcome.
Explanation: Increasing the temperature increases the average kinetic energy of the gas molecules, meaning they move faster. Faster-moving molecules will collide with the container walls more often (increased frequency). Additionally, because their momentum is greater, each collision will exert a greater force. The combination of more frequent and more forceful collisions results in an increase in pressure. A is incorrect as individual molecules do not expand. B is incorrect for a sealed container. D is incorrect as attractive forces are assumed to be negligible in an ideal gas.

Question 20

During the melting of a pure crystalline solid at a constant pressure, which statement is correct?

  1. The temperature increases as the average potential energy of the particles increases.
  2. The temperature remains constant as the average kinetic energy of the particles increases.
  3. The temperature increases as both the average kinetic and potential energies of the particles increase.
  4. The temperature remains constant as the average potential energy of the particles increases. (correct answer)
Explanation: During a phase change, such as melting, the energy added (latent heat of fusion) is used to overcome the intermolecular forces holding the particles in the fixed lattice structure. This increases the potential energy of the particles. The temperature, which is a measure of the average kinetic energy of the particles, remains constant until all the solid has melted. Therefore, the temperature is constant while the average potential energy increases.