UNIT 3 – CHEMICAL WORLD Changes of state


When a solid is heated, its particles gain energy and move apart. The particles break free from their tightly packed positions and the solid changes to a liquid. When a solid changes to a liquid it is called melting.


When a liquid is heated, the particles gain even more energy and break free from each other. The liquid turns into a gas. When a liquid turns into a gas it is called boiling.


These changes are called physical changes. In a physical change, no new substance is formed, there is no change in mass and the reaction can be easily reversed.


Melting Freezing Evaporation/Boiling Condensation


Energy in Energy out


Energy in Energy out


Solid


:Fig. 14.9 Changes of state Melting


What happens when an ice cube melts? The heat causes the water particles that make up the ice to vibrate faster. The solid ice cube changes state to become liquid water. The temperature a substance melts at is called its melting point. The melting point of ice is 0 °C.


Freezing


Cooling a liquid takes energy from its particles so they can only vibrate, not move around. The liquid changes state into a solid. When a liquid changes into a solid it is called freezing. The freezing point of water is 0 °C.


Evaporation and boiling


Both evaporation and boiling cause a change of state from a liquid to a gas. However, these changes take place slightly differently.


Some of the particles in a liquid have more energy than others. The particles that gain the correct amount of energy will leave the surface, spread out and change state from a liquid to a gas. This is called evaporation. The liquid does not bubble, and evaporation happens at all temperatures.


Evaporation Boiling


Liquid


Gas


)Fig. 14.10 The differences between boiling and evaporation


140 Elements of Science


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