Reading
| Teacher : Now, we are going to do an experiment with water and salt. First, pour some water into two plastic cups. Fill the cups to about half way. Now, add some salt to one of the cups. Stir the salt into the water with a spoon. | |||
| Boy 1 : Do you mean like this? [water being stirred by a spoon] | |||
| Teacher : Good. As you stir the salt, it will disappear. Are you following me? | |||
| Boy 2 : Yes. I can’t see it at all. | |||
| Teacher : Keep adding more salt until it doesn’t disappear any more. Do you understand what I mean? You’ll need quite a lot of salt – about a third of a cup. Now, put both cups in the freezer for three hours. [fade] | |||
| Teacher : OK, let’s see what’s happened to the water in each cup. | |||
| Boy 1 : Only one cup of water has become ice! The other cup hasn’t changed at all. | |||
| Teacher : Yes, that’s because when salt is added to water, it doesn’t freeze at the same temperature. Water with salt in it has to be much colder to freeze. Is that clear to you? | |||
| Boy 2 : Yes, I see what you mean. | |||
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Boy 1 : Can I just check that I have understood correctly? Is that why
the sea isn’t usually frozen even in the winter, because the water contains salt? |
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| Teacher : That’s right. | |||
| Boy 1 : I understand! How interesting! |
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