Changing state does not make a new substance — Physical education, 11–13 years
Melting, freezing, evaporating and condensing change how particles are arranged and move. The substance stays the same, which helps explain water, weather and reversible changes.
Three common states
In a solid, particles stay in fixed positions and only vibrate. In a liquid, they remain close but can move past one another. In a gas, they are spread out and move freely. Heating or cooling can change the arrangement without changing what the substance is.
Why states need explaining
A puddle disappears, and later water drops appear on a cold window. Without a particle model, these events can look like water vanishing and returning from nowhere. State changes solve the puzzle: water can become vapour and then liquid again, while remaining H₂O.
Ice becoming water
Take 100 g of ice at 0 °C and leave it in a warm room. Heat enters the ice, its particles vibrate more strongly, and the fixed structure breaks apart. The result is still about 100 g of H₂O, now liquid at 0 °C. No new substance has formed; only the state changed.
Vapour is still water
It is tempting to think steam is a different substance because it is invisible and seems to vanish. That is a reasonable conclusion from what our eyes show. Water vapour is still water: its particles are farther apart, and cooling can bring them together as liquid drops.
The water cycle
Sunlight warms seas, lakes and wet ground, causing some water to evaporate. Higher up, cooler air makes vapour condense into tiny drops that form clouds. The drops may fall as rain, and the cycle starts again. The water changes state, but it is not destroyed.
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