Boiling and melting run on latent heat
- The idea
- Predict
- See it
- Why
- Check
The idea
“Evaporation and steam take so much energy because water has a high specific heat.”
You learn early that water has a high specific heat, so it's tempting to credit it whenever water soaks up a lot of energy.
The short version
The mistake: Evaporation and steam take so much energy because water has a high specific heat.
What's actually true: Specific heat covers temperature change. A change of state takes in or gives off latent heat as hydrogen bonds break or re-form.
- Specific heat is the energy to warm 1 g by 1 °C, about 4.2 J/g·°C for liquid water. Hydrogen bonds make it high.
- Heat of vaporization is the energy to turn 1 g of liquid into gas, about 2,260 J/g for water at 100 °C. It breaks hydrogen bonds without warming anything.
- Condensing gives that energy back, about 2,260 J per gram of steam. Melting works the same way. Ice takes in about 334 J per gram at 0 °C, its heat of fusion.
On the exam: If water only warms or cools, it's specific heat. If it changes state, it's heat of vaporization or fusion.