Chemistry: Chemical Equations and Reactions

Which of the following is true regarding the law of conservation of mass?

• It dictates that the number of molecules on each side of a chemical equation must be the same.

• It dictates that the number of atoms of each element must be the same on both sides of a chemical equation.

• It states that the mass of the reactants must remain constant in order for a chemical reaction to proceed.

• It does not apply to chemical reactions.

Answer explanation

The number of molecules does not have to be the same because lots of molecules could be needed to make one new one. However, the number of each type of atom does need to be the same.

Which of the following is NOT true regarding a properly written chemical equation?

• It must be balanced.

• It must contain proper chemical formulas.

• It must show the formation of a precipitate or a gas.

• It should represent known data.

Answer explanation

There are many signs that a chemical reaction has taken place. Formation of a precipitate or a gas is not required, but could be an indicator that a reaction has occurred.

In a chemical equation, the formula of a substance in water solution is followed by the symbol

• (l)

• (s)

• (g)

• (aq)

Answer explanation

aq = aqueous solution which means that the substance is dissolved in water

s = solid

l = liquid

g = gas

• 1, 1, 1, 1

• 1, 2, 2, 1

• 2, 1, 2, 1

• 2, 1, 1, 2

What type of chemical reaction results in the formation of a single product?

• combustion

• synthesis

• decomposition

• displacement

Answer explanation

Synthesis reactions have the general form: A + B --> AB

• Zn

• H

• S

• O

Which of the following equations represents a decomposition reaction?

Answer explanation

Decomposition reactions have the general form AB --> A + B

• double displacement reaction

• synthesis reaction

• decomposition reaction

• combustion reaction

Answer explanation

Double displacement reactions have the general form: AB + CD --> AD + CB

• double displacement reaction

• synthesis reaction

• decomposition reaction

• combustion reaction

Answer explanation

Combustion reactions are typically some fuel + O2 gas. When the fuel contains only C, H (and possibly O), the products are CO2 and H2O. This makes these reactions easy to identify.

A solid produced by a chemical reaction in solution that separates from the solution is called

• a precipitate

• a reactant

• a molecule

• the mass of a product