When matter changes state, such as from a solid to a liquid (melting) or from a liquid to a gas (vaporization), thermal energy is absorbed or removed from the substance without a change in temperature. This removal or addition of thermal energy is known as "latent heat."
Here's how it works:
1. **Melting (Solid to Liquid)**:
- When you apply heat to a solid (like ice), it gains thermal energy and starts to increase in temperature.
- However, when the temperature reaches the melting point (0°C for water), the solid doesn't get hotter. Instead, it starts absorbing thermal energy without a temperature change.
- This absorbed thermal energy causes the solid to change its state from a solid (ice) to a liquid (water) without a change in temperature.
- This energy absorption is called the "latent heat of fusion."
2. **Vaporization (Liquid to Gas)**:
- When you heat a liquid (like water), it again gains thermal energy and starts to increase in temperature.
- Once it reaches the boiling point (100°C for water), the liquid doesn't get hotter; instead, it absorbs more thermal energy.
- This absorbed energy causes the liquid to change its state from a liquid (water) to a gas (water vapor) without a change in temperature.
- This energy absorption is called the "latent heat of vaporization."
The key point is that during these phase changes (melting and vaporization), the thermal energy is used to break the intermolecular forces holding the particles together in their current state, rather than increasing the temperature. It's why, for example, ice can stay at 0°C while absorbing heat to turn into water, and why boiling water can stay at 100°C while absorbing heat to turn into steam.
Conversely, when matter changes state in the opposite direction (condensation from gas to liquid or freezing from liquid to solid), thermal energy is released in the form of latent heat as the substance loses energy to form stronger intermolecular forces in the denser state.