Latent Heat
7 Minutes
Latent heat explained for students: learn how heat changes states of matter, melting points, and why ice expands. A short science lesson in English.
- Particle movement and attraction in solids, liquids, and gases
- Melting point, latent heat of fusion, and solidification
- Why melted wax shrinks as it cools but freezing water expands
What this lesson teaches
Latent heat is the heat energy absorbed or released by a substance during a change of state that does not cause a change in temperature. Instead of making a substance hotter or colder, this energy works entirely to overcome or strengthen the force of attraction between its particles.
Particles across the three states
Matter is made of tiny particles held together by a force of attraction. In a solid, the particles are held tightly in place and can only vibrate where they are. In a liquid, the particles have enough freedom to move around, yet they still maintain a steady pull toward one another. In a gas, the gaps between particles are very large, which leaves the force of attraction very weak and allows the particles to roam freely.
Melting point and latent heat of fusion
When a solid receives heat, its particles vibrate faster until they reach the melting point, where they begin to loosen their grip on each other. The temperature does not rise while the solid melts, which can be seen when ice stays between 0.6 and 0.8 degrees Celsius in a beaker until all of it has melted. The heat supplied during this stage does not raise the temperature because it is used to reduce the attraction between particles. This absorbed thermal energy is called the latent heat of fusion.
Latent heat of solidification
When a liquid cools and turns back into a solid, the process happens at the exact same temperature as melting. During this change, heat energy is released into the surroundings so that stronger attractive forces can form between the particles. This released energy is known as the latent heat of solidification. Once the liquid has fully turned into a solid, the temperature begins to drop again.
Why freezing water behaves differently
- Most substances shrink in volume when changing from a liquid to a solid because the settling particles take up less space, just as neatly folded clothes take up less room than scattered clothes.
- Molten wax demonstrates this regular shrinkage because it reduces in volume and contracts inside a container as it cools down and solidifies.
- Water is an unusual exception that expands when it freezes because hydrogen bonds cause the gaps between water particles to increase in the solid state.
- A closed bottle filled to the brim with water will bulge or break in a freezer because the expanding ice requires extra space.