The Effects of Heat on Matter


1. Matter exists in three states, solids, liquids and gases.


2. The physical state of matter can be changed by heating or cooling (changing the temperature) the matter.


Changes of state

1. All substances on Earth can exist as solid, liquid or gas.


2. Substances can change from one state to another by cooling or heating.


3. When a substance changes from the solid state to the liquid state, it is said to melt. The temperature at which a solid melts is called the melting point.


4. The change of a substance from a liquid to a solid is called freezing. The temperature at which a substance freezes is called its freezing point.


5. The freezing point of water is also the melting point of ice. Both are at 0°C.


6. When a liquid changes to the gaseous state, the liquid is said to evaporate.


7. The reverse process, the change from the gaseous to the liquid state, is called condensation. Condensation does not take place at a fixed temperature.


8. The evaporation process takes place at any temperature. It only happens at the surface of the liquid.


9. When a liquid is heated to a certain temperature, it changes into a gas. This is called boiling.


10. The temperature at which a liquid boils is called its boiling point.


11. Some substances change directly from the solid state to the vapour state. This is known as sublimation.


12. Solid carbon dioxide (dry ice) and iodine crystals are exaples of substances which sublime when heated.


13. A change of state requires heat energy. Figure below shows how the change in states of matter involves absorption and release of heat.



Daily observations which show a change in state of matter

Some examples of daily applications which show a change in state of matter.

1. Ice melts when it is exposed to air or mixed with water or drinks. It is used to make cold drinks or to help things be kept cool.



2. Water or liquid solidify when the temperature is lowered. This process is used to make ice cubes, ice-cream and jelly.



3. Water changes to vapour under the hot sun and evaporates. This process is used to dry things like wet clothes, salted fish and meat. Salt is obtained from sea water by evaporation.



4. Petrol is obtained from the refinery through distillation of crude oil.
Crude oil is heated and petroleum changes from liquid to vapour (gas). Condensation changes the petrol from vapour state to liquid state.



5. Moth balls change from solid to gas (sublimation). The vapour is used as a repellent of moths when storing clothes.