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Change of state
Change of state
Substances can change state, usually when they are heated or cooled. For example, liquid water turns into steam when it is heated enough, and it turns into ice when it is cooled enough.
The closeness, arrangement and motion of the particles in a substance change when it changes state. Simple diagrams of particles in a solid, liquid and a gas are shown like this:
Gaining energy
Gaining energy
The table summarises what happens to the particles in a substance when it gains energy, and it melts or boils (ie changes state):
Evaporation happens below the boiling point of a liquid. When the liquid reaches its boiling point, evaporation happens very quickly and the liquid boils.
Losing energy
Losing energy
The table summarises what happens to the particles in a substance when it loses energy, and it freezes or condenses (ie changes state):
Conservation of mass
Conservation of mass
The particles in a substance stay the same when it changes state - only their closeness, arrangement or motion change. This means that the mass of the substance stays the same. For example, 10 g of water boils to form 10 g of steam, or freezes to form 10 g of ice. This is called conservation of mass.
Changing state
Changing state
A substance must absorb heat energy so that it can melt or boil. The temperature of the substance does not change during melting, boiling or freezing, even though energy is still being transferred.
A heating curve is a graph showing the temperature of a substance plotted against the amount of energy it has absorbed. You may also see a cooling curve, which is obtained when a substance cools down and changes state.
A heating curve for ice
Notice that the temperature stays the same during a change of state, melting or boiling, even though heat energy is still being absorbed.
The temperature also stays the same while a liquid freezes, even though heat energy is still being released to the surroundings.
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