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Particles and changes of state
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Heating a substance either raises its temperature or changes its state. The particle model explains both.
Particles in solids, liquids and gases
Solid: particles close together in a regular arrangement, vibrating about fixed positions.
Liquid: particles close together but randomly arranged, able to move past each other.
Gas: particles far apart, moving quickly and randomly in all directions.
Liquid: particles close together but randomly arranged, able to move past each other.
Gas: particles far apart, moving quickly and randomly in all directions.
Heating changes the energy stored
Heating a system increases the energy stored in it. This either raises its temperature or changes its state.
Melting: particles gain enough energy to break free of their fixed positions.
Boiling: happens throughout the liquid at the boiling point; bubbles of gas form.
Evaporation: the fastest particles escape from the surface of a liquid at any temperature; the liquid left behind is cooler.
During a change of state the temperature stays constant, because the energy is used to overcome the forces between particles.
Melting: particles gain enough energy to break free of their fixed positions.
Boiling: happens throughout the liquid at the boiling point; bubbles of gas form.
Evaporation: the fastest particles escape from the surface of a liquid at any temperature; the liquid left behind is cooler.
During a change of state the temperature stays constant, because the energy is used to overcome the forces between particles.
Temperature-time graph practical
Heat crushed ice in a beaker, stirring, and record the temperature every 30 seconds until the water is warm.
The graph rises, then has a flat section at 0 °C while the ice melts, then rises again.
The flat section shows the constant temperature during a change of state.
The graph rises, then has a flat section at 0 °C while the ice melts, then rises again.
The flat section shows the constant temperature during a change of state.
On a cooling curve for a liquid, the temperature stays at 54 °C for 3 minutes. Explain this.
- The substance is changing state from liquid to solid (freezing).
- Energy is released as bonds form between particles, so the temperature stays constant.
Answer: It is freezing at 54 °C; the temperature is constant during a change of state.
Solids: regular, vibrating. Liquids: random, moving past each other. Gases: far apart, fast and random. Heating raises temperature or changes state. Temperature stays constant during melting and boiling. Evaporation is from the surface at any temperature and cools the liquid.
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