Change of State
Substances melt, boil, freeze or condense when energy is added or removed.
About this topic
Substances change state when energy is added or removed: melting, boiling and sublimation absorb energy, while freezing, condensation and deposition release it. During a change, temperature stays constant because the energy goes into breaking or forming bonds, not speeding particles up.
This energy is called latent heat: Q = mL, with L_f for fusion and L_v for vaporisation. For water, L_f is about 334 kJ/kg and L_v about 2260 kJ/kg, which is why a burn from steam is much worse than one from boiling water, and why sweating cools you. A heating curve shows slopes where temperature rises and flat plateaus where the state changes.
Worked example: melting 0.5 kg of ice at 0 °C takes 0.5 × 334,000 = 167,000 J, with no temperature rise at all. Boiling 0.5 kg of water at 100 °C into steam needs 0.5 × 2,260,000 = 1,130,000 J, which is over six times more.
Applications include refrigerators, air conditioners, ice in drinks, cooking and the sweating of animals. Dry ice sublimes straight from solid to gas. A common error is to apply Q = mcΔT during a change of state; during a plateau the right equation is Q = mL. Another is to think boiling and evaporation are the same, though evaporation happens at any temperature, only from the surface.
Key ideas
- Temperature stays constant during a phase change
- Latent heat: Q = mL
- Evaporation cools
Quick check
Use cases: where Change of State helps
- Homework: review the key idea behind Change of State before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Change of State without notes.
- Visual learning: play with the interactive lab to see how Change of State changes when you adjust its inputs.
- Teaching a friend: explain Change of State in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Change of State?
Substances melt, boil, freeze or condense when energy is added or removed.
What will I learn in Change of State?
Temperature stays constant during a phase change. Latent heat: Q = mL. Evaporation cools.
Is Change of State part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Change of State lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Change of State?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.