Heated, cooled, mixed: the mass story
When matter is heated, cooled or mixed, its particles are still there. They only move in a different way or mix with other particles. So the mass stays the same, as long as nothing enters or leaves the container. If 60 g of ice melts in a sealed bag, we get 60 g of water. If water freezes in a sealed bag, the mass is the same again. But an open pot of boiling water gets lighter, because water vapour escapes into the air. The water did not disappear, because the vapour is still in the room and still has mass. A scientific explanation has three parts. The claim says what happens to the mass. The evidence gives the measurements. The reasoning uses the particle idea to say why the evidence supports the claim.
| Change | Mass before (g) | Mass after (g) | Container |
|---|---|---|---|
| ice melts | 60 | 60 | sealed bag |
| water freezes | 200 | 200 | sealed bag |
| salt dissolves in water | 115 | 115 | closed jar |
| water boils | 500 | 460 | open pot |
| Part | What it says |
|---|---|
| Claim | The mass of ice stays the same when it melts in a sealed bag. |
| Evidence | 60 g before and 60 g after. |
| Reasoning | The same particles are still in the bag; they only moved differently. |
Ice in a bag (a grown-up helps). Put one small ice cube in a zipped bag, squeeze the air out and seal it. Weigh it on a kitchen scale and write the mass. Let the ice melt, dry the outside of the bag and weigh it again. Then put the same amount of water in an open saucer in a sunny window, weigh it now and again the next day. Explain both results.