Melting, boiling, latent heat and evaporation
Watch the whole lecture free
Make a free account to watch every lecture to the end: 2 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
That was the teaching part. The rest of this lecture is free with an account: 2 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
No password needed now: we email you a link to set one. Already have an account? Sign in, then come back to this page.
The teaching part (2 min) is free to watch. A free account opens the whole lecture: 2 worked examples, 4 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- A puzzle with ice
- Changes of state
- A heating curve
- What happens on the flat part
Read the transcript of the teaching part
Last time
Before we start: last time, in States of matter, gas pressure and Boyle's law, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
A puzzle with ice
Heat a glass of ice with a steady heater. The ice melts, but for a while the thermometer does not move. The heater is still giving energy, yet the temperature stays at zero. Where is the energy going? That is the story of latent heat.
Changes of state
A solid turns into a liquid by melting. A liquid turns into a solid by freezing, or solidifying. A liquid turns into a gas by boiling or evaporating. A gas turns into a liquid by condensing. The temperature at which a solid melts is the melting point. For pure water it is zero degrees Celsius. The temperature at which a liquid boils is the boiling point. For pure water it is one hundred degrees Celsius.
A heating curve
This curve shows a solid being heated steadily until it melts, then boils. First the temperature climbs as the solid warms. Then it goes flat: the substance is melting, and the temperature does not change. It climbs again as the liquid warms, and goes flat a second time while it boils. The flat parts are where energy changes the state.
What happens on the flat part
On the flat part, the energy supplied is not making the particles move faster. Instead it is used to break the forces holding the particles together, or to pull them far apart. So the potential energy of the particles increases, but the temperature, which tracks the kinetic energy, stays constant. Energy that changes the state without changing the temperature is called latent heat.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Melting, boiling and evaporation · Evaporation · Melting, boiling and latent heat.
The whole chapter: Thermal Properties and Temperature, Thermal Properties and Temperature.
Chapter 2.2: Thermal properties and temperature
- Thermal expansion 6 min
- Specific heat capacity 6 min
- Melting, boiling, latent heat and evaporation 6 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.