Question 1

Which of the following observations is NOT evidence that a chemical reaction has occurred?

Accepted Answer

A solid dissolves completely in water with no other changes.

Question 2

Which of the following best describes an exothermic reaction?

Accepted Answer

A reaction that releases heat to the surroundings.

Question 3

Which of the following is a diatomic molecule as it appears in a chemical equation?

Accepted Answer

$$\mathrm{O_2$}$$

Question 4

What is the correct way to balance the following equation?

$$\mathrm{H_2$ + O_2$ \rightarrow H_2O}$$

Accepted Answer

$$2\mathrm{H_2$} + \mathrm{O_2$} \rightarrow 2\mathrm{H_2O}$$

Question 5

Which of the following statements about coefficients and subscripts in chemical equations is correct?

Accepted Answer

Coefficients change the amount of molecules; subscripts change the identity of the substance.

Question 6

A student mixes two clear solutions and observes the formation of a yellow solid. What is the best explanation for this observation?

Accepted Answer

A chemical reaction occurred, producing a precipitate.

Question 7

Which of the following is the correct representation of chlorine gas in a chemical equation?

Accepted Answer

$$\mathrm{Cl_2$ (g)}$$

Question 8

When balancing a chemical equation, which of the following rules must always be followed?

Accepted Answer

Never change the subscripts in a chemical formula.

Question 9

Which of the following is a correct balanced equation for the reaction of acetic acid with sodium bicarbonate to produce sodium acetate, water, and carbon dioxide?

Accepted Answer

$$\mathrm{CH_3COOH (aq) + $$NaHCO_3$$ (s) \rightarrow CH_3COONa (aq) + H_2O (l) + CO_2$ (g)}$$

Question 10

Which of the following best describes the physical state symbol (aq) in a chemical equation?

Accepted Answer

A substance dissolved in water, forming an aqueous solution.

Question 11

A reaction produces a large amount of heat and light, as shown in a laboratory demonstration. Which type of evidence for a chemical reaction is this?

Accepted Answer

An energy change is observed.

Question 12

Which of the following elements is NOT naturally found as a diatomic molecule?

Accepted Answer

Carbon

Question 13

If you observe fizzing and bubbling when magnesium metal is added to hydrochloric acid, what is the most likely explanation?

Accepted Answer

A gas is being produced as a result of a chemical reaction.

Question 14

Which of the following is the correct balanced equation for the combustion of hydrogen gas in oxygen to form water?

Accepted Answer

$$2\mathrm{H_2$ (g)} + \mathrm{O_2$ (g)} \rightarrow 2\mathrm{H_2O (l)}$$

Question 15

A student balances the equation $$\mathrm{C_2H_6 + O_2$ \rightarrow CO_2$ + H_2O}$$ and gets $$\mathrm{C_2H_6 + 3O_2 \rightarrow 2CO_2 + 3H_2O}. $$What should the student do next?

Accepted Answer

Check if the coefficients are the smallest whole number ratio.

Question 16

Which of the following is a correct statement about balancing polyatomic ions in chemical equations?

Accepted Answer

Balance polyatomic ions as a single unit if they appear unchanged on both sides.

Chemical Reactions: Evidence, Equations, and Balancing

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