What happens to the temperature of a liquid while it is boiling?

It stays the same. The energy goes into changing the state.

For water, which is bigger?

the specific latent heat of vaporisation. Turning a liquid into a gas means separating the molecules completely.

While a solid melts, what does the energy supplied increase?

the potential energy of its molecules. The molecules move further apart, but no faster.

Specific latent heat · Cambridge A Level Physics revision

In plain words

Keep heating ice at 0 °C and, strangely, it doesn't get any warmer. It just melts. All the energy is going into pulling the molecules apart rather than speeding them up. The energy needed to change the state of each kilogram is called the specific latent heat.

Three things to know

The specific latent heat L is the energy per unit mass needed to change the state of a substance at constant temperature. ΔE = LΔm. Units: J/kg.
Latent heat of fusion is for melting or freezing. Latent heat of vaporisation is for boiling or condensing, and it is much bigger, because the molecules have to be separated completely.
During a change of state the temperature stays the same. The energy increases the potential energy of the molecules, not their kinetic energy.

Worked example

Find the energy needed to turn 0.50 kg of ice at 0 °C into water at 20 °C. (Specific latent heat of fusion of ice = 3.3 × 10⁵ J/kg; specific heat capacity of water = 4200 J kg⁻¹ K⁻¹.)

To melt it: 0.50 × 3.3 × 10⁵ = 165 000 J.
To warm the water: 0.50 × 4200 × 20 = 42 000 J.
Total = 207 000 J, about 2.1 × 10⁵ J.

Tips and tricks

Treat changing state (use L) and changing temperature (use c) as separate stages, and add the energies.
To measure L for boiling: boil the liquid with a heater of known power and measure the mass that boils away in a known time. L = Pt ÷ Δm.

5 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Specific latent heat: 5 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What happens to the temperature of a liquid while it is boiling?
- It stays the same. (the answer)
- It rises steadily.
- It falls.
- It rises and then falls.
The energy goes into changing the state.
What are the units of specific latent heat?
- J/kg (the answer)
- J kg⁻¹ K⁻¹
- J/K
- J kg
Energy per unit mass.
For water, which is bigger?
- the specific latent heat of vaporisation (the answer)
- the specific latent heat of fusion
- they are the same
- it depends on the mass
Turning a liquid into a gas means separating the molecules completely.
How much energy is needed to melt 2.0 kg of ice at 0 °C? (L = 3.3 × 10⁵ J/kg.)
- 1.7 × 10⁵ J
- 3.3 × 10⁵ J
- 6.6 × 10⁵ J (the answer)
- 6.6 × 10⁶ J
ΔE = LΔm.
While a solid melts, what does the energy supplied increase?
- the potential energy of its molecules (the answer)
- the kinetic energy of its molecules
- the temperature
- the mass
The molecules move further apart, but no faster.

5 questions

Tap an answer and you’ll see straight away whether it’s right, and why.

Worksheet

3 questions, 7 marks. Write your answers on paper, then check them.

Specific latent heat

Cambridge International AS & A Level Physics 9702 · 7 marks · papermunch.org

Name ______________________________ Date ______________

Find the energy needed to boil away 0.20 kg of water at 100 °C. (L = 2.3 × 10⁶ J/kg.)[2]
Show answerHide answer
4.6 × 10⁵ J.
I got this one
A 60 W heater melts 0.036 kg of ice at 0 °C in 200 s. Find the specific latent heat of fusion of ice.[3]
Show answerHide answer
3.3 × 10⁵ J/kg. 12 000 J ÷ 0.036 kg.
I got this one
Explain why the temperature of a solid stays the same while it melts, even though it is being heated.[2]
Show answerHide answer
The energy supplied is used to break the bonds between molecules, increasing their potential energy. Their kinetic energy does not change, so the temperature does not change.
I got this one

Answers: Specific latent heat

1. 4.6 × 10⁵ J.
2. 3.3 × 10⁵ J/kg. 12 000 J ÷ 0.036 kg.
3. The energy supplied is used to break the bonds between molecules, increasing their potential energy. Their kinetic energy does not change, so the temperature does not change.