Filtration – IGCSE Chemistry Definition

Filtration is one of the most fundamental separation techniques. It appears in salt preparation (removing excess base), precipitate collection, and water treatment. Paper 6 often tests the practical setup, and Paper 4 tests when and why filtration is used.

The 0620 definition

Filtration is the separation of an insoluble solid from a liquid by passing the mixture through a filter (usually filter paper in a funnel).

The method

1. Fold filter paper into a cone shape and place in a filter funnel
2. Place the funnel in a conical flask or beaker
3. Pour the mixture into the filter paper
4. The liquid passes through the tiny pores in the filter paper — this is the filtrate
5. The insoluble solid is trapped on the filter paper — this is the residue

Key terms

Term	Definition
Residue	The insoluble solid left on the filter paper
Filtrate	The liquid that passes through the filter paper

Where filtration is used

1. Salt preparation (Method 1)

After adding excess insoluble base to acid:

• Filter to remove excess solid base (residue)
• The filtrate contains the dissolved salt → crystallise

2. Collecting a precipitate

After a precipitation reaction:

• Filter to collect the insoluble salt (residue)
• Wash with distilled water to remove soluble impurities
• Dry the precipitate

3. Water treatment

Sand filtration removes small insoluble particles from potable water.

What filtration cannot separate

Filtration only separates insoluble solids from liquids. It cannot separate:

• Dissolved solutes from solvents (use distillation or crystallisation)
• Two miscible liquids (use fractional distillation)
• Dissolved dyes or pigments (use chromatography)

Worked exam question

A mixture contains sand and salt dissolved in water. Describe how to obtain pure, dry sand and pure, dry salt. [4]

Mark scheme

Filter the mixture [1]; the sand is the residue on the filter paper — wash and dry it [1]; the filtrate contains dissolved salt [1]; evaporate/crystallise the filtrate to obtain salt crystals [1]

Common exam mistakes

• Confusing residue (solid on the paper) with filtrate (liquid that goes through) — remember: residue remains, filtrate flows.
• Trying to use filtration to separate dissolved substances — filter paper cannot trap dissolved particles.
• Forgetting to wash the residue — impurities from the solution remain on the solid unless washed.