KS2 Filtering Experiment

What is a filter?

What would you do if your paddling pool was full of stones, leaves and bugs? You could empty it out and start again or you could filter out the debris. Filters are used to separate mixtures. For example a fishing net is a type of filter, it traps the fish, but lets the water out as water is small enough to pass through the holes, but the fish is too big. When you boil vegetables and then sieve them to remove the water, that is also filtration. The sieve filters out the vegetables separating them from the water.

Can you think of any more examples of a filter?

KS2 Filtering experiment

Soluble or insoluble

Filtration only works when the solid you want to separate is insoluble, which means it doesn’t dissolve in water. Imagine a cup of tea with sugar added, the sugar dissolves in the hot tea, it is no longer a solid particle. If you pass the tea through a filter, the sugar would not be left behind, but if you added sand to your tea, which is insoluble ( doesn’t dissolve ) this would be left behind in the filter. We can filter sand.

Filtration Activity


  • Funnels
  • Filter Paper – available here or you can use kitchen roll or coffee filters.
  • Beaker
  • Warm water
  • Sugar
  • Sand


  1. Pour about 300 ml of warm water into a beaker.
  2. Add about a tablespoon of caster sugar and stir until it dissolves completely.
  3. Place filter paper inside a funnel and place the funnel over a beaker.
  4. Slowly pour your sugar solution into the funnel, observing what happens.
  5. Repeat with fresh water and sand.


The sugar dissolved in the water passes through the filter into the beaker below.

The sand gets trapped by the filter with just water passing through into the beaker below.

Primary Science - Filtering Experiment

Why does this happen?

The insoluble sand is too big to pass through the holes in the filter paper so remains on the surface, the dissolved sugar has become a liquid so can pass through the filter into the beaker below.

Extension Ideas

Does the the sugar still dissolve if the water is cold?

Can you add mud or dirt to water and filter it until it’s clean? Do you need to filter it more than once?

DO NOT drink the water, it might look clean, but our home filters are not small enough to filter micro-organisms out of the water which could make you poorly.

How many filters can you find in your home?

Try using sand or stones as a filter, which work the best?

Dissolving – if a material has dissolved, it will combine with a liquid.

Did you know?

Filters are used as part of the process which cleans our drinking water before it reaches our homes. The filters used are so small that they can remove bacteria from the water!!

Can you use your new knowledge to think which kind of particles cannot be removed from water by even the smallest filter?

Suitable for Key stage 2 Science

Properties and Changes of Materials

This post was written by Emma of Science Sparks


Emma is a busy Mum to three who is passionate about science education. You can find Emma’s experiments and activities over at Science Sparks which is full  of fun, creative and engaging science based activities for children of all ages, perfect for home or school. Find out more at Science Sparks www.science-sparks.com
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