Thermal Expansion: As the temperature of a substance increases, its particles move faster and spread farther a substance to expand When a substance cools.its particles slow down and the substance Questions: 7. What happens to the size of a substance when it is heated? __ 8. Why do substances expand when heated and contract when cooled? __

## Step 1<br />The first question asks about the change in size of a substance when it is heated. According to the given context, when a substance is heated, its particles move faster and spread farther apart. This causes the substance to expand.<br /><br />## Step 2<br />The second question asks why substances expand when heated and contract when cooled. The explanation provided in the context is that as the temperature of a substance increases, its particles move faster and spread farther apart. This causes the substance to expand. Conversely, when a substance cools, its particles slow down and come closer together, causing the substance to contract.