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.

Tags

NGSS.MS-PS1-5

Which of the following may indicate that a chemical reaction has occured?

• release of energy as light

• a color change

• gas bubble formation

• all of these

Tags

NGSS.MS-PS1-2

Which of the following is a product in the reaction described by the word equation: iron + copper(II) sulfate --> iron(II) sulfate + copper

• iron(II) sulfate

• copper

• iron

• two of these are correct

Answer explanation

Iron(II) sulfate and copper are both products of this chemical reaction. Products are substances produced by the chemical reaction that are pointed to by the arrow.

Tags

NGSS.MS-PS1-2

What does CH4 represent in the balanced chemical equation: C + 2 H2 --> CH4

• 1 atom CH4

• 5 atoms CH4

• 1 molecule CH4

• 1 g CH4

Answer explanation

Balanced chemical equations can be interpreted in terms of molecules of a compound or moles of a compound.

Tags

NGSS.MS-PS1-1

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

• 4

• 15

• 18

• 33

Answer explanation

Add all the coefficients together to determine the total number of molecules described in the chemical equation.

Tags

NGSS.MS-PS1-5

• 1, 1, 1, 1

• 1, 2, 2, 1

• 2, 1, 2, 1

• 2, 1, 1, 2

Tags

NGSS.MS-PS1-5

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

Which of the following is a product that can be formed by double displacement reactions?

• gas

• precipitate

• water

• all of these