Change of State and Latent Heat
Simple Explanation
When a substance changes state (like melting or boiling), it absorbs or releases energy called latent heat — but crucially, this happens at a CONSTANT temperature, since the energy goes into breaking or forming intermolecular bonds rather than increasing the kinetic energy (and therefore temperature) of the particles.
Why Do We Need It?
Understanding that temperature stays constant during a state change (despite continued heating) explains a distinctive feature of heating curves and sets up the two specific latent heat concepts that follow.
Why Does This Work?
During a state change, added heat energy is used entirely to overcome the intermolecular forces holding particles in their current state (e.g. breaking the rigid bonds of a solid to allow liquid flow) — since this energy goes into potential energy (bond-breaking), not kinetic energy, the particles' average speed (and therefore temperature) does not change until the state change is complete.
Real-Life Example
Why ice stays at 0°C while melting
A glass of ice water stays at almost exactly 0°C the entire time the ice is melting, even while sitting in a warm room.
All the heat energy flowing into the ice-water mixture goes into breaking the bonds holding the ice in its solid structure (melting it), not into raising the temperature — only once all the ice has melted does the temperature begin to rise above 0°C.
Practice
Why does temperature stay constant while a substance is changing state (e.g. melting)?
MediumCommon mistake
Assuming continued heating always means rising temperature — during a phase change specifically, temperature plateaus at the melting or boiling point until the entire state change is complete.
Quick Review
- Latent heat: energy absorbed/released during a state change, at constant temperature.
- Energy goes into breaking/forming intermolecular bonds, not raising kinetic energy.
- Temperature plateaus during melting/boiling until the state change is fully complete.