Filtration

Jump to

Key points

  • Filtration is used to separate an solid from a pure liquid or a .
  • Filtration usually involves a circle of folded to make a cone and placed into a filter funnel.
  • The filtrate is the liquid which passes through the filter paper and the residue is the solid left on the filter paper.

A sieve can be used to separate small stones from fine sand. Why does sand pass through a sieve but stones do not?

Sand passes through a sieve because the sand particles are small enough to go through the holes in the sieve. The stones are too large to fit through.

Video

Watch this video about and solutions. It explains how is used to separate a mixture.

Why did the water molecules pass through the tiny holes in the filter paper, but the sand particles did not?

The water molecules are so small that they pass through the tiny holes in the filter paper. The sand particles are much larger, so they cannot fit through.

When is filtration used?

If a substance does not dissolve in a solvent, it is called insoluble. For example, sand does not dissolve in water, meaning it is insoluble.

Filtration is the process of separating solids from liquids using .

The process can be used to separate an insoluble solid, for example stone or sand grains from a liquid. The liquid could be a pure liquid, for example water, or it could be a , for example, salty water.

When a mixture of sand and water is filtered:

  • the sand stays behind in the filter paper, it becomes the .
  • the water passes through the filter paper, it becomes the .

The slideshow below demonstrates how filtration works.

One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper

One beaker contains a mixture of solid and liquid, the other contains a funnel with filter paper.

1 of 3

How does filtration work?

  1. The filter paper has lots of tiny holes in it, which are far too small to see.
  2. The of the liquid including any dissolved will pass through the holes in the filter paper.
  3. If the liquid contains an solid made of large particles, these larger particles cannot pass through the holes in the filter paper.
The mixture is poured slowly into the filter paper

Why can’t filtration be used to separate salt from sea water?

Salt dissolves in water, so the solution of salt water passes through the filter paper. The particles of salt in the sea water are small enough to pass through the holes in the filter paper.

Working scientifically

Drawing scientific apparatus

There are photos and graphics of apparatus on this page including:

  • Filter paper
  • Conical flask
  • Funnel

You could practise drawing scientific diagrams of the apparatus you can see in the photographs.

Keep your diagram simple and draw a 2-dimensional ‘cut-through’ style of diagram. To help, use a sharp pencil and a ruler for your straight lines, and don’t forget to include labels to make your diagram clear.

Find out more about drawing scientific apparatus.

Test your knowledge