Changes of stateIB MYP Chemistry: Revision notes
Section 1
The six changes of state
A substance changes state when its particles gain or lose enough energy. There are six changes of state.
- Melting: solid to liquid
- Freezing: liquid to solid
- Boiling: liquid to gas, throughout the liquid, at the boiling point
- Evaporation: liquid to gas, at the surface only, at any temperature below the boiling point
- Condensation: gas to liquid
- Sublimation: solid straight to gas, with no liquid stage (for example dry ice and iodine)
The temperature at which a pure solid melts is its melting point. This is the same temperature as its freezing point.
Do not mix up evaporation and boiling. Boiling happens at one temperature with bubbles throughout the liquid. Evaporation happens only at the surface and at many temperatures.
Section 2
What happens to the particles
Changes of state are physical changes. The particles are not changed. Only their energy, spacing and arrangement change.
- Melting and boiling (heating): particles gain energy, overcome the forces between them and move further apart
- Freezing and condensing (cooling): particles lose energy, slow down and the forces pull them closer together
A hot, fast-moving particle at the surface of a liquid can escape as a gas. This is why puddles dry up even when it is not boiling.
Section 3
Heating curves
A heating curve shows how temperature changes with time as a substance is heated at a steady rate.
For pure water starting as ice at −10 °C:
- The temperature rises as the solid ice warms up
- It stays at 0 °C while the ice melts
- It rises as the liquid water warms up
- It stays at 100 °C while the water boils
- It rises again as the steam warms up
The flat sections are where the state is changing. A cooling curve is the same in reverse, with flat sections at the condensing and freezing points.
Describe a curve in order: rising, flat, rising, flat, rising. Quote the temperature at each flat section.
Section 4
Why the temperature stays constant
During a change of state the temperature does not change, even though energy is still being supplied.
- When melting or boiling, the energy is used to overcome the forces between the particles. It is not used to speed the particles up, so the kinetic energy and temperature stay the same.
- When freezing or condensing, new attractions form between particles and energy is released. This balances the heat lost to the surroundings, so the temperature stays the same until the change is complete.
Once the whole sample has changed state, the temperature can change again.
Section 5
Worked example: a cooling curve
Molten wax at 80 °C is allowed to cool. The temperature falls to 54 °C, stays at 54 °C for six minutes, then falls to 30 °C.
- The first fall: the liquid wax loses kinetic energy, so its particles slow down.
- The flat section at 54 °C: this is the freezing point of the wax. The liquid is turning into a solid.
- The last fall: the wax is now solid and its particles vibrate less as it cools.
The wax melts at 54 °C when heated, so melting point and freezing point are the same.
Must Know
- Melting, freezing, boiling, evaporation, condensation and sublimation are the six changes of state
- Boiling happens throughout the liquid at the boiling point, but evaporation happens at the surface at any temperature below it
- Changes of state are physical changes
- Heating and cooling curves have flat sections at the changes of state
- The temperature stays constant because the energy changes the forces between particles, not their kinetic energy
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Changes of state
- A student in Reykjavik heats a beaker of crushed pure ice at −10 °C with a gentle flame, stirring and recording the temperature every minute. The temperature rises, then stays constant for several minutes while the ice melts, even though heating continues. It then rises again until the water boils.Explain why the temperature stays constant while the ice is melting, even though the flame is still heating the beaker.2 marks
- A delivery company in Dubai packs ice cream in insulated boxes with blocks of dry ice (solid carbon dioxide). The dry ice changes straight from a solid to a gas, leaving no liquid behind. When a box is taken out into the warm, humid air, droplets of water form on the cold outside of the box.Describe, in terms of particles, what happens when dry ice sublimes.2 marks
- Students in Accra heat solid stearic acid in a test tube in a hot water bath until it is completely liquid at 80 °C. They then stand the test tube in a beaker of cool water and record its temperature every minute for 13 minutes. The temperature falls from 80 °C to 69 °C by minute 3, stays at 69 °C until minute 8 while the liquid turns into a solid, and then falls to 55 °C by minute 13.Describe the pattern in the temperature results.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).