Changes with heat and cold — Chemistry, 4–6 years
Heat and cold can change how a material looks or moves. Ice can melt into water, and water can freeze into ice; the material changes form but does not simply disappear.
Ice can become water
When ice gets warm, it melts and becomes liquid water. When water gets cold enough, it freezes and becomes solid ice. These are changes of state: the same water is taking a different form. The change can go backwards when the temperature changes again.
Why notice heat and cold?
People needed to know why ice disappeared on a warm day and why water became hard in a freezer. This problem helped them understand when materials would pour, freeze or melt, so they could store food and make useful things. Careful watching of these changes came before the scientific words.
Example: an ice cube
1. Put one ice cube on a plate at room temperature. 2. At first it is hard and keeps its shape. 3. After ten minutes, a small puddle appears because heat from the room is warming it. 4. After longer, the cube becomes water. 5. Put the water in a freezer and it can become ice again.
The disappearing trap
When an ice cube is gone, it is tempting to think the water has vanished too. That makes sense because the water may spread into a very thin layer or mix with other water. Look at the plate and feel for wetness, but never taste anything from an experiment. The water changed form; it did not simply stop existing.
Where it helps
Freezers keep water as ice so food and drinks can be kept cold. Ice in a glass melts and cools a drink, while an ice pack can cool a sore place with an adult’s help. Cooks also use heat to melt butter or chocolate, then let them cool and become firm again.
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