Question 1

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

Accepted Answer

A solid melts into a liquid when heated.

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

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

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 compound.

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 NOT one of the seven diatomic elements?

Accepted Answer

Sulfur

Question 8

When balancing chemical equations, 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 best describes a precipitate?

Accepted Answer

An insoluble solid formed in a solution during a chemical reaction.

Question 10

A reaction is observed to absorb heat from the surroundings and the temperature of the solution decreases. What type of reaction is this?

Accepted Answer

Endothermic

Question 11

Which of the following is the 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 12

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

Accepted Answer

The total number of atoms of each element must be the same on both sides of the equation.

Question 13

Which of the following is an example of a physical change?

Accepted Answer

Boiling water to produce steam.

Question 14

If you observe fizzing and bubbling when two substances are mixed, what is the most likely explanation?

Accepted Answer

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

Question 15

Which of the following best explains why $$\mathrm{H_2O_2} is $$not the same as $$\mathrm{H_2O}$$?

Accepted Answer

$$\mathrm{H_2O_2}$$ has a different chemical composition and properties than $$\mathrm{H_2O}.$$

Question 16

Which of the following is the correct way to write the physical state of sodium chloride dissolved in water in a chemical equation?

Accepted Answer

$$\mathrm{NaCl (aq)}$$

Question 17

A student balances the equation $$\mathrm{C_2H_6 + O_2$ \rightarrow CO_2$ + H_2O}$$ and gets $$\mathrm{2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O}. $$What should the student do next?

Accepted Answer

Check if the coefficients can be divided by a common factor to get the smallest whole number ratio.

Question 18

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

Accepted Answer

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

Question 19

Which of the following is the correct representation of chlorine as it appears in a chemical reaction?

Accepted Answer

$$\mathrm{Cl_2$}$$

Question 20

Which of the following is NOT a correct observation indicating a chemical reaction?

Accepted Answer

A liquid evaporates into a gas at room temperature.

Chemical Reactions: Evidence, Equations, and Balancing

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