Question 1

Which of the following best explains why mass is conserved in a chemical reaction that occurs in a closed system?

Accepted Answer

Atoms are rearranged into new substances, but the total number and type of atoms remain constant.

Question 2

A student burns magnesium in an open crucible and observes a mass increase. Which is the best explanation for this observation?

Accepted Answer

Oxygen from the air combines with magnesium to form magnesium oxide.

Question 3

Which of the following chemical equations is balanced?

Accepted Answer

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

Question 4

Which statement best describes a synthesis reaction?

Accepted Answer

Two or more substances combine to form a single compound.

Question 5

A student mixes solutions of silver nitrate and sodium chloride, forming a white precipitate. Which type of reaction has occurred?

Accepted Answer

Double displacement

Question 6

Which of the following is an example of a combustion reaction?

Accepted Answer

$$\mathrm{CH_4$ + 2O_2 \rightarrow CO_2$ + 2H_2O}$$

Question 7

Which law states that a chemical compound always contains the same proportion of elements by mass?

Accepted Answer

Law of Definite Proportions

Question 8

In a closed system, 10 g of reactant A reacts with 15 g of reactant B to form product C. What is the mass of product C?

Accepted Answer

25 g

Question 9

Which of the following best describes what happens to atoms during a chemical reaction?

Accepted Answer

Atoms are rearranged to form new substances.

Question 10

A student observes that the mass of a system decreases during a reaction in an open container. What is the most likely explanation?

Accepted Answer

A gas is produced and escapes from the system.

Question 11

Which of the following is the correct balanced equation for the reaction of aluminum with oxygen to form aluminum oxide?

Accepted Answer

$$\mathrm{4Al + 3O_2 \rightarrow 2Al_2O_3}$$

Question 12

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

Accepted Answer

A change in state from solid to liquid due to melting

Question 13

Which of the following best describes a decomposition reaction?

Accepted Answer

A single compound breaks down into two or more simpler substances.

Question 14

A student balances the equation $$\mathrm{Fe + O_2$ \rightarrow Fe_2O_3} as$$ $$\mathrm{2Fe + 3O_2 \rightarrow 2Fe_2O_3}. $$What is wrong with this equation?

Accepted Answer

The equation is not balanced for iron and oxygen atoms.

Question 15

Which of the following is an example of a single displacement reaction?

Accepted Answer

$$\mathrm{Zn + 2HCl \rightarrow $$ZnCl_2$$ + H_2$}$$

Question 16

Which of the following best describes the role of models in understanding chemical reactions?

Accepted Answer

Models help visualize the rearrangement of atoms and conservation of mass.

Question 17

Which of the following is a correct application of the Law of Conservation of Mass in a real-world scenario?

Accepted Answer

The total mass of reactants in a sealed reaction vessel equals the total mass of products.

Question 18

A student observes bubbles forming when vinegar is added to baking soda. What does this observation most likely indicate?

Accepted Answer

A gas is being produced in a chemical reaction.

Question 19

Which of the following equations represents a double displacement reaction?

Accepted Answer

$$\mathrm{Na_2SO_4 + $$BaCl_2$$ \rightarrow 2NaCl + $$BaSO_4$$}$$

Question 20

Which of the following best explains why balancing chemical equations is necessary?

Accepted Answer

To satisfy the Law of Conservation of Mass by having equal numbers of each type of atom on both sides.

Observing and Modeling Chemical Reactions: Conservation of Mass and Types of Reactions

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