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Melting and freezing

Heat and matter · level 2, lesson 2 of 6

A question to start

Put an ice cube in your hand and it turns to water. Put water in the freezer and it turns to ice. What is going on?

The idea

When a solid gets warm enough, it melts and becomes a liquid. When a liquid gets cold enough, it freezes and becomes a solid.

melting (warming up)freezing (cooling down)

Each solid has its own melting point, the temperature where it melts. Water's is 0 °C: ice melts above it, and water freezes below it.

Solid Melts at about
Ice 0 °C
Butter 32 °C
Chocolate 30 to 35 °C, just under body heat
Candle wax 60 °C
Iron 1,540 °C

That is why chocolate melts in your hand but iron needs a furnace.

Worked examples

1. An ice lolly. On a hot day it is warmer than 0 °C outside, so it melts and drips.

2. Ice cubes. Water in a tray in a −18 °C freezer is colder than 0 °C, so it freezes.

3. Chocolate. It melts in a warm pocket. Put it back in the fridge and it sets solid again.

Try it

  1. What does a solid become when it melts?
  2. At what temperature does water freeze?
  3. Why does chocolate melt in your hand?
  4. Can melted chocolate turn back into solid chocolate?
Answers
  1. A liquid.
  2. 0 °C.
  3. Your hand is warmer than chocolate's melting point.
  4. Yes, if it cools down below its melting point.

Common mistake

"Melting makes something disappear." The ice is still there as water. Nothing is lost; it has only changed state.

Where it leads