1. IC5.2.4 Explanation and ratio examples © Oxford University Press Explanation and ratio examples

2. IC5.2.4 Explanation and ratio examples © Oxford University Press  Mass is conserved during a chemical reaction.  The total mass of reactants before the reaction is the same as the total mass of products formed during the reaction.  This is because no atoms are created or destroyed during the reaction. They are only rearranged to form new substances.

3. IC5.2.4 Explanation and ratio examples © Oxford University Press The idea of conservation of mass can be used  to work out the mass of a substance released to the surroundings in a chemical reaction, or  to work out the mass of a substance gained from the surroundings.

4. IC5.2.4 Explanation and ratio examples © Oxford University Press In a thermal decomposition reaction, a single substance breaks down or decomposes to form two or more other substances.

5. IC5.2.4 Explanation and ratio examples © Oxford University Press For example: CaCO 3  CaO + CO 2 Carbon dioxide is a gas, so it escapes into the air during the reaction. calcium carbonate calcium oxide carbon dioxide  +

6. IC5.2.4 Explanation and ratio examples © Oxford University Press CaCO 3  CaO + CO 2 100 g  56 g + 44 g If 100 g of calcium carbonate is decomposed, 56 g of calcium oxide is left behind. The difference in mass is the mass of carbon dioxide released, in this case (100 – 56) = 44 g.

7. IC5.2.4 Explanation and ratio examples © Oxford University Press The masses are directly proportional to each other. Can you work out how much calcium oxide and carbon dioxide would be made by 200 g of calcium carbonate? CaCO 3  CaO + CO 2 100 g  56 g + 44 g 200 g  ? + ? 200 g  112 g + 88 g

8. IC5.2.4 Explanation and ratio examples © Oxford University Press CaCO 3  CaO + CO 2 100 g  56 g + 44 g Suppose you wanted to make 33 g of carbon dioxide. What mass of calcium carbonate would you need? Here is how you would work it out. mass of calcium carbonate needed = 100/44 × 33 g = 75 g

9. IC5.2.4 Explanation and ratio examples © Oxford University Press magnesium + oxygen  magnesium oxide 2Mg + O 2  2MgO In some reactions, a substance is gained during the reaction. For example, oxygen is gained from the air when magnesium is heated strongly:

10. IC5.2.4 Explanation and ratio examples © Oxford University Press If 48 g of magnesium is heated, 80 g of magnesium oxide forms. The difference in mass is the mass of oxygen gained, in this case (80 – 48) = 32 g: 2Mg + O 2  2MgO 48 g + 32 g  80 g

11. IC5.2.4 Explanation and ratio examples © Oxford University Press Again, the masses are directly proportional to each other. Suppose you wanted to absorb 4 g of oxygen. What mass of magnesium would you need? mass of magnesium needed = 48/32 × 4 g = 6 g magnesium + oxygen  magnesium oxide 2Mg + O 2  2MgO