What You Need to Know

Key Concept Diagram

A balanced equation has the same number of each type of atom on both sides

Only coefficients (numbers in front of formulas) can be changed, not subscripts

The law of conservation of mass states that matter is neither created nor destroyed

Word equations describe reactions in words; symbol equations use chemical formulas

Key Vocabulary

Reactant

A substance that enters into and is altered in a chemical reaction (left side)

Product

A substance formed as a result of a chemical reaction (right side)

Coefficient

A number placed in front of a formula to balance an equation

Conservation of mass

The principle that total mass is unchanged in a chemical reaction

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

In the equation H2 + O2 -> H2O, why is this equation not balanced?

There are too many hydrogen atoms on the left Hydrogen and oxygen cannot react Water has the wrong formula There are 2 oxygen atoms on the left but only 1 on the right

Question 2

To balance an equation, you are allowed to change:

The coefficients in front of chemical formulas The products of the reaction The subscripts in chemical formulas The type of atoms in the reaction

Question 3

Why must chemical equations be balanced?

To make them look neat To reflect conservation of mass - atoms are not created or destroyed in reactions Because unbalanced equations cannot be written To show that reactions are reversible

Balancing Chemical Equations