Practice Test 19 - High School Chemistry

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

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1. Two liquids are mixed in a beaker. The mixture turns from colorless to yellow and becomes warm to the touch within a minute, even though no external heat is applied. Which combination of observations suggests a chemical change?
2. Two different pairs of reactant molecules are compared. Pair 1 collides very often, but only a few collisions form products. Pair 2 collides less often, but a larger fraction of collisions forms products. Which statement best describes the key collision-theory idea illustrated by this comparison?
3. Three alkali metals—lithium (Li, Group 1 Period 2), sodium (Na, Group 1 Period 3), and potassium (K, Group 1 Period 4)—are each placed separately into identical beakers of cold water. Which metal is expected to react most vigorously with the water?
4. A student heats a small amount of candle wax in a metal spoon until it melts. Observations: - Solid wax becomes a clear liquid. - No bubbles form in the wax. - When the heat is removed, the liquid wax cools and becomes solid again. - The wax has the same smell before and after melting. Based on the evidence, did a chemical reaction occur? Choose the best CER justification.
5. A student mixes vinegar with baking soda in a cup. The mixture immediately foams with many bubbles and makes a fizzing sound. After a short time, the bubbling stops. Which observation is evidence of a chemical reaction rather than a physical change?
6. A student repeated the same neutralization trial three times to check reliability. In each trial, $25.0\,\text{mL}$ of $1.00\,\text{M}$ HCl was mixed with $25.0\,\text{mL}$ of $1.00\,\text{M}$ NaOH in a foam cup. The initial temperature of both solutions was $22.0^\circ\text{C}$. The highest temperature reached was recorded.

   Trial	Highest temperature reached (°C)
   1	28.6
   2	28.7
   3	28.5

   Which statement best describes the data collection quality?​
7. A piece of steel wool is left in a damp place for several days. It changes from gray and shiny to a reddish-brown coating, and the surface becomes flaky. Which observation best indicates a chemical reaction occurred?
8. A student mixes 50.0 mL of hydrochloric acid solution (HCl(aq)) with 50.0 mL of sodium hydroxide solution (NaOH(aq)) in a sealed cup. The measurements are shown. Based on the data, what does it indicate about whether a chemical reaction occurred?

   Property	Before mixing	After mixing (in sealed cup)
   Mass of cup + contents	152.40 g	152.40 g
   Temperature	22.0°C (both)	29.5°C
   Appearance	clear + clear	clear (no solid)

9. Radium-226 decays as shown: $^{226}_{88}\text{Ra} \rightarrow {}^{222}_{86}\text{Rn} + {}^{4}_{2}\text{He}.$ Which statement best describes what is emitted?
10. In beta decay, a nucleus emits an electron ($^{0}_{-1}\text{e}$). If $^{A}_{Z}\text{X} \rightarrow ^{A}_{Z+1}\text{Y} + ^{0}_{-1}\text{e},$ what happens to the mass number $A$?
11. A lab needs a spatula/scoop to handle small amounts of solid oxidizer (like potassium nitrate) and also occasionally scoop table salt. The tool should not react with oxidizers, should be easy to clean, and should be reasonably durable. Cost matters, but safety is the top priority. Which material is the best trade-off? Options: - Steel: strong; can rust; generally compatible with many solids but may corrode if stored wet. - Aluminum: lightweight; can react with some strong oxidizers under certain conditions; forms oxide layer but still not preferred for oxidizers. - PTFE (Teflon): very chemically inert; easy to clean; more expensive and less stiff than metal.
12. A potassium ion is written as ³⁹K⁺ (potassium-39 with a $1+$ charge). How many electrons does ³⁹K⁺ have?
13. A lab needs bottles for storing concentrated hydrochloric acid (HCl). The lab chose borosilicate glass bottles with acid-resistant caps instead of metal containers. Over a year, the glass bottles showed no pitting or discoloration, and the acid concentration stayed within 1% of the labeled value. Which justification best explains why glass was appropriate?
14. A neutral atom of oxygen-16 (O-16) is given. How many protons, neutrons, and electrons does it have?
15. Two alkaline earth metals—magnesium (Mg, Group 2 Period 3) and calcium (Ca, Group 2 Period 4)—are placed in separate containers of water. Which is expected to react faster with water based on periodic trends in Group 2?
16. A student needs the safest (least reactive) alkali metal for a demonstration with water. The choices are sodium (Na, Group 1 Period 3), potassium (K, Group 1 Period 4), and rubidium (Rb, Group 1 Period 5). Which should be least reactive with water?
17. In the balanced reaction: $2\text{KClO}_3 \rightarrow 2\text{KCl} + 3\text{O}_2$ If $4.0\ \text{mol}$ of $\text{KClO}_3$ decompose completely, how many moles of $\text{O}_2$ are produced?
18. An engineering club built a simple solar oven. They used ordinary clear soda-lime glass as the top cover because it was cheap and transparent. During testing, the oven reached $120^\circ\text{C}$ inside. After three heating cycles, fine cracks formed near the corners of the glass cover, and one cover broke when cooled quickly by a breeze. Which refinement best targets the failure mode suggested by the evidence?
19. A graph plots concentration (y-axis) versus time (x-axis) for a reactant $[B]$. At an early time, the curve is steeply decreasing. At a later time, the curve is still decreasing but much more gently. At which time is the reaction rate faster, and why?
20. Use the periodic trend for first ionization energy. Consider these group 1 elements: - Li (period 2) - Na (period 3) - Cs (period 6) Which element has the highest first ionization energy?

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