State changes

Substances can exist as a solid, liquid or gas. Converting from one state to another usually involves heating or cooling.

Solids melt to become liquid, and liquids freeze to become solid. Liquids evaporate or boil to become gas, and gases condense to become liquid.
  • Heat must be supplied to a substance for it to melt, evaporate or boil. For example, you need to heat ice to melt it, and you need to heat water to make steam.
  • Heat must be removed from a substance to condense or freeze it. In other words, the substance must be cooled down.

Under certain conditions, some solids turn straight into a gas when heated. This process is called sublimation. A good example is solid carbon dioxide, also called ‘dry ice’. At atmospheric pressure, it turns straight into gaseous carbon dioxide.

Liquid carbon dioxide can only exist under high pressure, such as in fire extinguishers. Iodine also sublimes - it turns directly from shiny purple-black crystals to a purple vapour when warmed up.

Changing the pressure

A gas will also liquefy (turn into a liquid) if its pressure is increased enough. This is because the particles are moved close enough for bonds to form between the particles.

Gas cylinders used for camping stoves and barbecues contain liquefied petroleum gas (LPG) under high pressure. As soon as the pressure is released, the liquid turns back to a gas.

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