Changes of state — Physics, 7–10 years
The same substance can be a solid, liquid or gas. Heating and cooling can change how its particles move, so ice can melt into water and water can become invisible vapour.
One substance, three forms
A solid keeps its shape, a liquid flows and takes the shape of its container, and a gas spreads through the space around it. They can all be the same substance. Water, ice and water vapour are three forms of H₂O.
Why do states change?
This idea helps explain puddles disappearing, frost forming and food being frozen or cooked. Heating gives particles more movement; cooling takes some movement away. When the movement reaches a different range, the substance behaves as a solid, liquid or gas.
From ice to vapour
Put an ice cube at 0 °C in a warm room. First, it absorbs heat and melts, changing from solid to liquid water while staying close to 0 °C. Next, heating the water to 100 °C makes it boil. Some liquid then changes into water vapour, a gas that spreads through the air.
Invisible does not mean gone
After a puddle dries, it can seem that the water has vanished. That is a reasonable thought because our eyes cannot see the water vapour mixed with air. The water has changed into a gas and spread out; it has not been destroyed.
Freezing and boiling in daily life
Freezers slow particles until water becomes ice, which helps preserve food. Kettles and pans add heat so water boils and cooks food. Clouds form when invisible water vapour cools into tiny liquid drops or ice crystals, so this idea also helps explain weather.
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