Practice Test 39 - High School Chemistry

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

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1. Bromine (Br) is a group 17 element. A neutral bromine atom becomes a bromide ion, Br. Which explanation correctly links group number to the ions charge and electron change?
2. A student spills lemon juice into a cup of milk during lunch, and the milk forms small clumps. The cafeteria manager wants to know whether this is a chemical change (important for deciding if the milk is still safe to serve in recipes) or just a physical change. Design an investigation to test the question: Does adding an acid (lemon juice) to milk cause a chemical change? Which plan best identifies variables, uses fair comparisons, and collects observable evidence of chemical change?
3. Ammonia forms by the reaction $\,N_2 + 3H_2 \rightarrow 2NH_3\,$. If 2.0 mol $N_2$ and 4.0 mol $H_2$ are available, which reactant is limiting?
4. Arrange these halogens in order of decreasing reactivity (most reactive to least reactive), given their periodic positions: chlorine (Cl, Group 17 Period 3), bromine (Br, Group 17 Period 4), iodine (I, Group 17 Period 5).
5. Students examine the question: "How does light intensity affect the rate of a photochemical reaction?" They place identical test tubes containing 10.0 mL of a light-sensitive dye solution at the same concentration into three locations: 10 cm, 20 cm, and 40 cm from the same lamp. They keep the room temperature the same and start all trials at the same time. Every minute for 10 minutes, they use a colorimeter to record the dye's absorbance. What is the independent variable?
6. A student claims: "Increasing acid concentration increases the reaction rate between magnesium ribbon and hydrochloric acid." In each trial, the same length of magnesium ribbon was placed into $50.0\,\text{mL}$ of HCl at $25.0^\circ\text{C}$. The time for the magnesium to completely disappear was recorded. Do the data support the claim?
7. A student observes that a reaction in a test tube is slow at room temperature but becomes much faster when the test tube is warmed in a hot-water bath. Using collision theory, what is the best particle-level reason the reaction speeds up when heated?
8. A student engineering team needs a transparent cover for a UV-sterilization box. Requirements: must transmit UV light reasonably well, must not release toxic fumes when warmed to ~50°C, and must resist cracking if cleaned with alcohol. Budget is limited. Materials: - Acrylic (PMMA): clear; can crack with some solvents; decent UV transmission; low-to-moderate cost. - Polycarbonate (PC): very impact-resistant; can yellow under UV over time; generally tolerates alcohol better; moderate cost. - PVC sheet: low cost; can release irritating fumes if overheated; poorer UV transmission.
9. Ammonia forms from nitrogen and hydrogen: $\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}$ If the reaction uses 1 mol of nitrogen gas ($\mathrm{N_2}$), how many moles of hydrogen gas ($\mathrm{H_2}$) are required to match the coefficient ratio?
10. A student asks: "Does a catalyst change how fast hydrogen peroxide decomposes?" They set up two 125 mL flasks, each containing 30.0 mL of 3% H2O2 at 25°C. Flask 1 receives 0.50 g of manganese(IV) oxide (MnO2), and Flask 2 receives no solid. They immediately fit each flask with a gas syringe and record the volume of oxygen gas produced every 10 seconds for 1 minute. What is being deliberately changed between the two setups?
11. A student heated 50.0 mL of water in a beaker on a hot plate and recorded temperature every 2 minutes. Table: Temperature vs. time

    Time (min)	Temperature (°C)
    0	21.5
    2	31.0
    4	40.2
    6	49.8

    Which statement best describes the trend shown by the data?
12. A reaction between two kinds of molecules in a gas mixture is observed to proceed slowly even though the molecules collide often. Which particle-level idea from collision theory best explains why many collisions do not lead to product formation?
13. The decomposition of water can be written as: $\mathrm{2H_2O(l) \rightarrow 2H_2(g) + O_2(g)}$ $\Delta H = +572\ \mathrm{kJ}$ Based on the value of $\Delta H$, what must be true about heat flow during this reaction at constant pressure?
14. A student built a small battery holder for AA batteries using copper contacts because copper is a good conductor. The holder was used in a humid greenhouse. Observations after 1 month: - The copper contacts developed a green coating. - The device started flickering unless the batteries were rotated. - Cleaning the contacts temporarily restored good connection. - A test holder with nickel-plated contacts stayed shiny and maintained stable connection. Which refinement is most evidence-based?
15. Fluorine (F) has electron configuration $[He]2s^2\,2p^5$ and chlorine (Cl) has electron configuration $[Ne]3s^2\,3p^5$. These elements show similar chemical behavior (both commonly form $-1$ ions). Which electron-configuration feature best explains their similarity?
16. A student pours a clear liquid (vinegar) into a cup containing a white powder (baking soda). The mixture immediately foams with many bubbles, the cup feels cooler to the touch, and the white powder disappears. Which observation is the best evidence that a chemical reaction occurred?
17. A student places steel wool in a dry jar and measures its mass. Then the student adds a small amount of water to make the steel wool damp, leaves the jar open to air for 3 days, and measures again. Measurements and observations: - Day 0 mass of steel wool: 2.00 g; appearance: shiny gray. - Day 3 mass of steel wool: 2.16 g; appearance: reddish-brown coating present. - No bubbling was observed during the 3 days. - The steel wool is harder to pull apart after 3 days. Did a chemical reaction occur? Choose the best CER justification.
18. In the nuclear equation $^{210}_{84}\mathrm{Po} \rightarrow {}^{206}_{82}\mathrm{Pb} + X,$ what is $X$?
19. A student claims: "A new substance formed when solution C was heated." Evidence/observations: 1) The solution changed from pale yellow to dark brown after 5 minutes of heating. 2) The mass of the open beaker decreased by $1.2\,\text{g}$. 3) A sweet smell was noticed during heating. 4) When the heat was turned off, the solution slowly returned to pale yellow after 20 minutes. Is the evidence sufficient to support the claim? Choose the best evaluation.
20. In a simplified view of methane combustion, some bonds break (C-H and O=O) and others form (C=O and O-H). Without using numbers, which statement best describes why combustion is typically exothermic?

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