Is the mass of the cake different than the mass of the ingredients?

Does mass change during a chemical reaction?

Topic 5: Conservation of Mass

In this lesson you will learn

❑ what the Law of Conservation of Mass is

❑ how to balance chemical equations so they obey the Law of Conservation of Mass

Experiment 1

Experiment 2

The Law of Conservation of Mass

• The Law of Conservation of Mass states that matter cannot be created or destroyed• If you begin with 10 g of reactants, you will end with 10 g of

products.

• What is the total mass of the reactants? ______

• What is the total mass of the products? ______

Oxygen

Practice

• What will be the mass of water?

15 g 20 g 10 g ?

Count the number of atoms of each element in the reactants and products. Does this chemical equation obey the law?

Balancing Chemical Equations

• To write chemical equations that obey the Law of Conservation of Mass, we must balance them so that the atoms are equal on both sides.

• To do this, we add copies of reactants and products as needed until the number each element is the same.

EXAMPLE 1

____ Li(s) + _____O2(g)→ ____ Li2O(g)

Count atoms:

Draw:

EXAMPLE 2

____Fe2O3(s)→ ____ Fe(s) + ___O2(g)

Count atoms:

Draw: