Melting and boiling points
Chemistry · Key Stage 2 · The School
Every substance changes state at its own temperature
Water melts at 0°C and boils at 100°C. Chocolate melts at about 34°C, which is why it melts in your hand and not in the cupboard. Iron melts at 1538°C. The number belongs to the substance, not to the day — it is one of the ways you can tell one material from another.
Melting and freezing are the same line
Water turns to ice at 0°C and ice turns to water at 0°C. It is one temperature read from two directions: going down it is the freezing point, going up it is the melting point. There is no separate number to learn.
The temperature stops while it changes
Heat a beaker of ice and the thermometer climbs, then sticks at 0°C while the ice melts, then climbs again. The energy is still going in, but it is being spent pulling the particles apart rather than making them move faster. That flat stretch on the graph is the change of state happening.
Imagine a class, with a teacher, warms a beaker of ice and reads a thermometer every minute. They keep the bulb in the ice and water, not against the glass, or it would measure the glass. They wait for the reading to stop moving. They put their eyes level with the top of the coloured liquid. Looking from above or below makes it seem to sit at a different mark. Then they write down what they see, not what they expect. At minute 0 the reading is minus 2°C. At minutes 1, 2, 3 and 4 it is 0°C. At minute 5 it is 3°C. The reading stayed at 0°C from minute 1 to minute 4, which is 4 minus 1, so at least 3 minutes. That flat stretch is the ice melting. The reading only climbs again once all the ice has become water. So the class records the melting point of ice as 0°C, the same number as the freezing point of water.