Changes of State
Science: KS3, topic 2.1.5
Changes of State
A material doesn’t stay as a certain state forever. Materials are continuously changing between all three states depending on temperature and pressure. This is known as a reversible physical change.
Melting
- Melting is a change from solid to liquid.
- Melting happens when the solid particles get some extra energy, which makes them vibrate faster and faster until they have enough energy to break away from their fixed position.
- Then the particles can move freely over each other, and have turned into liquid particles
- Heating something will give solid particles enough energy for them to adopt a liquid particle arrangement.
Boiling
- Boiling is a change from liquid to gas.
- Boiling happens when liquid particles are heated, making them move faster and faster.
- Eventually, the liquid particles will be moving very quickly and in random directions, without being connected to any other particles – they have changed into the gas particle arrangement.
- When you boil the kettle the steam that comes out the top is water vapour (water as a gas), because the liquid water has been heated up enough to boil.
- Water boils at 100oC.
Evaporation
- Evaporation is also a change from liquid to gas.
- The difference between evaporation and boiling is that evaporation can happen at any temperature.
- When you get out the shower, the water on the doors eventually 'disappears' because it evaporates.
- It doesn't boil, but it does turn into a gas.
- This is because different particles in a liquid have different amounts of energy.
- Some particles will gain enough energy to break free and will become a gas.
- A liquid boils when every particle has this much energy.
Condensation
- Condensation is a change of state from a gas to liquid.
- Condensation happens when you cool down a gas and reduce the energy of the particles.
- The particles slow down and clump together, taking the arrangement of a liquid.
Freezing
- Freezing is when a liquid turns into a solid.
- E.g. when water freezes to ice, which happens at 0oC.
- To freeze a liquid, you have to cool it down, which makes the particles travel slower and slower until they are just vibrating in a fixed position and are all very close together.
- The particles will then look like the particle arrangement in a solid and the substance has frozen.
Test yourself on Changes of State
Which of these is a change from a gas to a liquid?
A sample of water is cooled from 20°C down to -5°C. At what temperature does the water turn into a solid?
Water can turn into ice, and ice can turn back into water. What type of change is this an example of?