Lesson 4 of 8
Conservation of mass
Goal: Explain why mass is conserved in a reaction.
In a chemical reaction the total mass stays the same. This is called conservation of mass.
- Total mass of the reactants = total mass of the products. If 10 g of reactants react completely, they make 10 g of products.
- Why? No atoms are made or destroyed. The same atoms are there at the end, joined in a new way.
- The mass can seem to fall when a product is a gas that escapes into the air. Count the gas as well and nothing is missing.
- The mass can seem to rise when a solid joins with a gas from the air. Burning magnesium gains the mass of the oxygen it joins with.
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Key words in this lesson
Chemical reaction A change in which a new substance is made
Reactant A substance you start with in a reaction
Product A new substance made in a reaction
Conservation of mass The total mass is the same before and after a reaction