Atoms rearrange
In a chemical reaction, atoms are rearranged. No atoms are created or destroyed. The number of each type of atom stays the same, even though the substances can change.
Lesson 3.1 · AQA Combined Science
Conservation of mass
Total mass stays the same
Total mass of reactants = total mass of products. Reactants are the starting substances; products are the substances formed. Include all the substances, including any gases.
Find a missing mass
12 g of carbon reacts completely with 32 g of oxygen to make carbon dioxide.
12 + 32 = 44 g of carbon dioxide.
For a missing reactant: if the product is 44 g and carbon is 12 g, oxygen mass = 44 − 12 = 32 g.
Choose the system you are weighing
In a closed system, matter cannot enter or leave. The mass of the container and all its contents stays the same during a reaction. A gas still has mass, even when you cannot see it. In an open container, a gas can enter or escape; we return to this in Lesson 3.6.
Remember
No atoms are created or destroyed. Count all reactants and products, including gases.
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