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Before this: Physical and chemical changes, Chemical formulas

Conservation of mass

In a chemical reaction, atoms are rearranged. They are not created or destroyed. So the total mass of the reactants (the starting substances) equals the total mass of the products (the substances formed).

This is the law of conservation of mass.

A chemical equation describes a reaction. Reactants are on the left, products are on the right, and the arrow means “produces”.

2 H2 + O2 → 2 H2O

Worked example: burning magnesium

24.3 g of magnesium burns and forms 40.3 g of magnesium oxide. How much oxygen reacted?

mass of reactants = mass of products

24.3 g + mass of oxygen = 40.3 g

mass of oxygen = 40.3 g - 24.3 g = 16.0 g

Common mistake

Concluding that mass was lost when a reaction gives off a gas. A burning log leaves light ash because most of the products, carbon dioxide and water vapor, escape into the air. In a sealed container the total mass does not change.

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Watch Atoms are rearranged, not created
  1. Magnesium burns in oxygen and forms magnesium oxide.
  2. Follow the magnesium: 2 atoms before the reaction, 2 after.
  3. Follow the oxygen: 2 atoms before, 2 after.
  4. No atom was created or destroyed. The same atoms, so the same total mass.

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Practice

1. 12.0 g of carbon reacts completely with 32.0 g of oxygen. What mass of carbon dioxide forms?

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2. 100.0 g of calcium carbonate breaks down into 56.0 g of calcium oxide and carbon dioxide. What mass of carbon dioxide forms?

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3. During a chemical reaction, atoms are:

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4. Steel wool is burned in open air and its mass increases. Why?

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