← Separation Techniques

Lesson 3 of 7 · Textbook 4.2

Filtration

A sieve with very small holes. Everything about filtration follows from one fact: the holes are a particular size.

The property being used

Definition Filtration is the process of separating substances of different particle sizes using a sieve.

A sieve has small holes called pores. Particles smaller than the pores pass through; particles bigger than the pores are retained. In the laboratory the sieve is a piece of filter paper in a filter funnel.

Two words you will be asked to name The solid left on the filter paper is the residue.
The liquid that passes through is the filtrate.

Filtration

mixture filter paper filtrate
Residue (on the paper)—
Filtrate (through the paper)—

Run salt solution through and watch nothing happen. That is the most examinable fact in this lesson.
Filtration cannot separate a solution Salt dissolves in water to form salt solution, and all of the salt solution passes through the filter paper. The dissolved particles are as small as the water particles, so the pores cannot stop them. To get salt out of salt solution you must use evaporation — the next lesson.

Rule of thumb: filtration works on insoluble solids only. If it has dissolved, filtering is pointless.

Filtration around you

WhereWhat acts as the sieve, and what it traps
Tea strainerThe mesh holes let tea through but retain the tea leaves.
Siltation pondLayers of sand and rocks of different sizes are laid as filters. Groundwater flows through the holes between them, so clean water reaches the pond.
Your noseThe hairs in your nostrils and mucus trap large solid particles in the air you breathe, preventing them from reaching your lungs.
WaterworksRainwater from local catchment and imported water are chemically treated, filtered and disinfected to make tap water.

Check yourself