SOLIDS AND LIQUIDS

 

Solids and liquids

1. Heating and cooling

  • Heat can change solids into liquids or gases. Most solids melt into liquid when they are heated. A liquid evaporates into a gas when it is heated.
  • Removing heat (cooling) changes a substance from a gas to a liquid to a solid. A gas condenses into a liquid when it is cooled. A liquid freezes into a solid when it is cooled.

Illustration of ice lolly transforming in three stages via 'heat; from solid >liquid > gas and vice versa via 'cooling'

Solids and liquids

2. Solids

  • Solids stay in one place and can be held.
  • Solids keep their shape. They do not flow like liquids.
  • Solids always take up the same amount of space. They do not spread out like gases.
  • Solids can be cut or shaped.
  • Even though they can be poured, sugar, salt and flour are all solids. Each particle of salt, for example, keeps the same shape and volume.
  • Heating a solid can turn it into a liquid.
  • Cooling a liquid can turn it into a solid.

Examples of solids

Illustration of pile of sand and log of wood, labelled respectively

 

3. Liquids

  • Liquids can flow or be poured easily. They are not easy to hold.
  • Liquids change their shape depending on the container they are in.
  • Even when liquids change their shape, they always take up the same amount of space. Their volume stays the same.
  • Heating a liquid can turn it into a gas.
  • Cooling a liquid can turn it into a solid.
  • Heating a solid can turn it into a liquid.
  • Cooling a gas can turn it into a liquid.

Examples of liquids

Illustration of can of syrup tipped over on corner of table, pouring down onto carpet floor, labelled 'syrup'

Illustration of sky with cloud showing rain drops, labelled 'rain'

4. Gases

  • Gases are often invisible.
  • Gases do not keep their shape or always take up the same amount of space. They spread out and change their shape and volume to fill up whatever container they are in.
  • Gases can be squashed.
  • Heating a liquid can turn it into a gas.
  • Cooling a gas can turn it into a liquid.

Examples of gases

Illustration of sky and clouds to represent air, labelled 'Air'

Illustration of girl letting go of balloon, labelled 'rain'

 

Reversible and irreversible changes

1. Reversible changes

  • A reversible change is a change that can be undone or reversed.
  • A reversible change might change how a material looks or feels, but it doesn’t create new materials.
  • Melting is an example of a reversible change. For example, when chocolate is warmed until it melts, the melted chocolate can be changed back into solid chocolate by cooling.

Illustration of chocolate in normal and melted states with arrows in between labelled 'HEAT' and 'COOL'

  • Freezing is an example of a reversible change. For example, when orange juice is frozen to make ice lollies, the ice lollies can be changed back into orange
    juice again by heating.

Illustration of orange juice in normal and ice lolly states with arrows in between labelled 'HEAT' and 'COOL'

  • Boiling, evaporating and condensing are all examples of reversible changes. For example, if you could capture all the steam that is made when a kettle boils, you could turn it back to water by cooling it.
  • Dissolving is an example of a reversible change. For example, when salt is mixed with water it disappears because it dissolves in the water to make salty water. But the salt can be recovered from the salty water by boiling off the water.

2. Dissolving

  • Some substances dissolve when you mix them with water. When a substance dissolves, it looks like it disappears. But in fact it has just mixed with the water to make a transparent (see-through) liquid called a solution.
  • When you mix sugar or salt with water, they dissolve to make a transparent solution.

Illustration of funnel with salt flowing into beaker of water, creating salty water in beaker

  • When you mix sand or flour with water, they do NOT dissolve.

Illustration of funnel with sand flowing into beaker of water, creating pile of sand at bottom of beaker to demonstrate that it does not dissolve in water

  • Substances that dissolve in water are called soluble substances. Substances that do not dissolve in water are called insoluble substances.

3. Separating mixtures

  • A mixture made of solid particles of different sizes, for example sand and gravel, can be separated by sieving.

Illustration of sieve with gravel staying in sieve and sand falling through into a pile below

  • A mixture of water and an insoluble substance like sand can be separated by filtering.

Illustration of sand in filter and water dropping into beaker beneath

  • A solution of salt dissolved in water can be separated by boiling the solution until all the water has evaporated. The salt will be left behind. If the water vapour was collected, it could be cooled to form water again.

Illustration of salt at bottom of beaker with clouds of steam coming up (labelled 'Water vapour'), with red arrow underneath beaker labelled 'HEAT'

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