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Practice Test 24 - High School Chemistry

20 questions. Answer them in any order, then submit to see your score and every explanation.

  1. A student-run café wants reusable containers to store fresh orange juice (pH about 3–4) in a refrigerator for up to a week. The container will be washed daily with hot water and dish soap. The container material must not react with acidic liquids, must not leach harmful chemicals into drinks, and must remain stable during repeated washing. Which set of chemical properties should be prioritized?
  2. A student mixes 50 mL of hydrochloric acid with 50 mL of sodium hydroxide in a foam cup. The temperature of the mixture rises from 21°C to 32°C within one minute, and the cup feels warm. Based on these observations, is the reaction exothermic or endothermic, and what is the direction of energy flow?
  3. A student heats water in a pot until it boils. Many bubbles rise to the surface and steam forms above the water. Which observation is NOT evidence of a chemical reaction?
  4. Students investigate: How does pH affect the color intensity produced by the reaction of red cabbage indicator with a solution? They prepare three buffer solutions at pH 4, pH 7, and pH 10. Into three test tubes they place 10.0 mL of each buffer, then add 2.0 mL of the same red cabbage indicator extract to each tube. After gently swirling each tube for 10 seconds, they use a colorimeter to record absorbance. Identify the independent variable, the dependent variable, and one appropriate controlled variable.
  5. A food-truck kitchen needs a mixing container for a salty, acidic sauce (vinegar + salt). Requirements: must be food-safe (non-toxic), should resist corrosion/metal taste, and should survive repeated dishwashing up to ~75°C. Budget is moderate (not unlimited). Which material is the best compromise? Options: - Stainless steel: excellent corrosion resistance; food-safe; high durability; higher cost. - Plain carbon steel: rusts/corrodes in acidic salty solutions; durable; low cost. - Polycarbonate plastic: food-safe; can absorb odors/stain over time; can warp with repeated high-heat washing; moderate cost.
  6. Students investigate: How does temperature affect the rate of the reaction between sodium thiosulfate and hydrochloric acid? For each trial, they place 50.0 mL of sodium thiosulfate solution into a flask and warm or cool it to 15°C, 25°C, 35°C, or 45°C using a water bath. They then add 5.0 mL of 1.0 M HCl, start a stopwatch, and swirl the flask the same way each time. The flask is placed over a paper with a black "X," and the student records the time until the "X" can no longer be seen. Identify the independent variable, dependent variable, and one controlled variable from the procedure.
  7. Arrange these halogens in order of decreasing reactivity (most reactive to least reactive): fluorine (F, Group 17 Period 2), chlorine (Cl, Group 17 Period 3), bromine (Br, Group 17 Period 4).
  8. Oxygen (O) is in group 16 and has 6 valence electrons (electron configuration ends in 2s² 2p⁴). In many compounds, oxygen forms two bonds (for example, H₂O). How does oxygen's valence electron structure explain this behavior?
  9. A city replaced old cast-iron pipes in a section of its drinking-water system with copper pipes. The decision was made after routine water tests found elevated iron levels and reddish-brown water complaints, especially after periods of low flow. Which reasoning best justifies choosing copper over cast iron for this application using chemical properties and evidence?
  10. A white solid is added to water and stirred. The solid disappears and the water becomes clear. The outside of the cup feels noticeably colder. Which observation is NOT evidence of a chemical change?
  11. Two clear, colorless liquids are poured into the same beaker. The mixture immediately turns cloudy and a white solid appears and settles to the bottom. What evidence indicates a chemical change occurred?
  12. Balance the chemical equation (smallest whole-number coefficients): Zn + HCl → ZnCl2 + H2
  13. A word equation describes a nuclear change: hydrogen nuclei + hydrogen nuclei → helium nucleus + energy What type of nuclear process does this represent?
  14. Three beakers each contain 50 mL of HCl at 25°C. A student adds the same mass and size of zinc granules to each beaker. Only the acid concentration is changed. Observed results: - 0.5 M HCl: slow bubbling; reaction finishes in ~120 s - 1.0 M HCl: moderate bubbling; reaction finishes in ~60 s - 2.0 M HCl: rapid bubbling; reaction finishes in ~30 s What is the best particle-level explanation for the faster reaction in 2.0 M HCl?
  15. A teacher says: "Chemical reactions typically involve energies of a few electron volts (eV) per reaction because they involve electrons and chemical bonds. Nuclear reactions are typically in the mega–electron volt (MeV) range per reaction because they involve changes in the nucleus." Which statement best connects this to the real-world observation that a nuclear plant uses much less fuel mass than a coal plant for the same electrical output?
  16. Iron(III) chloride forms by the balanced synthesis equation 2Fe+3Cl2→2FeCl3\mathrm{2Fe + 3Cl_2 \rightarrow 2FeCl_3} What is the mole ratio of Fe\mathrm{Fe} to Cl2\mathrm{Cl_2}?
  17. A maintenance team must choose a lubricant for metal parts in a food-processing conveyor where occasional contact with water and cleaning chemicals occurs. They selected a silicone-based grease instead of a vegetable-oil-based lubricant. After 3 months, the silicone grease remained stable with no rancid odor and maintained smooth motion, while the vegetable-oil lubricant developed a strong odor and sticky residue that increased friction. Which justification best supports the selection?
  18. A documentary mentions that the Sun has shone for billions of years because it is powered by nuclear fusion, not by "burning" like a fire. Which statement best explains why fusion can power the Sun so long compared with chemical burning of the same mass of material?
  19. An oxide ion is written as O2−\mathrm{O^{2-}}, and the isotope is oxygen-16 (16^{16}O). How many protons, neutrons, and electrons does 16O2−\mathrm{^{16}O^{2-}} have?
  20. Which scenario describes a chain reaction, where neutrons released from one event can trigger more events of the same type in nearby nuclei?

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