Mechanical Separation: Sieving and Filtration
Mechanical separation methods split a mixture by the size of its particles; sieving separates solid pieces of different sizes, and filtration separates an insoluble solid from a liquid or gas using a porous material. In brief: a kitchen strainer sieves pasta from water, and a coffee filter removes ground coffee from the brewed liquid.
These two techniques need no heating and no chemicals, so they are among the simplest ways to separate mixtures. They work only when the components differ in particle size. This article explains both methods, describes the equipment and steps, compares filtering types in a table, works through an example separation and notes limits and mistakes.
What is sieving?
A sieve is a mesh or perforated screen with holes of a defined size. When a dry mixture is shaken over the sieve, smaller particles fall through, and larger particles stay on top. Sieving is used for solids that are dry and differ in size, such as flour and lumps, gravel and sand, or soil and stones.
Several sieves with different hole sizes can be stacked, with the coarsest on top. This arrangement separates a sample into several size groups and is used to analyze soil and building materials.
What is filtration?
Filtration separates an insoluble solid from a liquid. The mixture is poured through a filter, which has pores that allow the liquid to pass but trap solid particles. The solid left on the filter is called the residue, and the liquid that passes through is called the filtrate.
Filtration works for suspensions such as muddy water or sand in water. It does not separate dissolved substances: if salt is dissolved in water, the salt passes through the filter paper together with the water, because its particles are far smaller than the pores.
How do you filter a mixture in the laboratory?
- Fold a circular piece of filter paper in half, then in half again, and open it into a cone.
- Place the cone in a funnel and put the funnel in a clean flask or beaker.
- Moisten the paper with a little solvent so that it sticks to the funnel wall.
- Pour the mixture slowly down a glass rod into the paper, keeping the liquid level below the top edge of the paper.
- Rinse the original container with a small amount of solvent and pour it through to collect all the solid.
- Collect the filtrate in the flask and allow the residue to dry.
Types of filtration
| Method | How it works | Typical use |
|---|---|---|
| Gravity filtration | Liquid flows through paper under its own weight | Routine laboratory filtering |
| Vacuum filtration | Suction pulls liquid through a Büchner funnel | Fast filtering of large amounts of solid |
| Sand filter | Layers of sand and gravel trap particles | Water treatment, swimming pools |
| Membrane filter | Very fine pores block tiny particles | Sterilizing liquids, water purification |
| Air filter | Fibers trap dust and pollen | Air conditioners, face masks |
Worked example: separating a three-part mixture
A sample contains small gravel (about 5 mm), sand (about 0.5 mm) and salt dissolved in water. The task is to separate all three.
- Remove the gravel. Pour the mixture through a sieve with 2 mm holes. The gravel stays on the sieve, and the rest passes through.
- Remove the sand. Pour the liquid that passed through into filter paper. The sand stays as the residue, and the salt water forms the filtrate.
- Recover the salt. Filtration cannot remove the dissolved salt, so evaporate the water from the filtrate to leave solid salt.
The sample mass can be used to check the result. Suppose the starting mixture had 20 g of gravel, 30 g of sand and 10 g of salt. After drying, the recovered masses of 19.6 g, 29.5 g and 9.8 g give recoveries of 98%, 98.3% and 98%. Small losses occur from material that sticks to equipment.
Where are sieving and filtration used?
- Kitchens: flour sifters, pasta strainers and tea strainers.
- Construction: sieves sort sand and gravel for concrete.
- Water treatment: sand filters remove particles from drinking water.
- Medicine: kidney dialysis machines filter waste from blood.
- Vehicles: oil and air filters keep engines clean.
How have filters developed?
People have strained liquids through cloth and sand since ancient times. In 1804 John Gibb built one of the first large sand filters for a public water supply in Paisley, Scotland, and sand filtration spread through European cities in the 1800s. Modern membrane technology developed in the 20th century and allows the removal of particles much smaller than a bacterium.
What are the limits of these methods?
Sieving and filtration depend on size. They cannot separate dissolved substances from a solvent or particles that are the same size. Very fine particles can clog the filter, which slows down the process, and some particles pass through if the pores are too large. For separating a solute from a solvent, methods such as evaporation or distillation are used.
Which mistakes are common?
Mistake 1: Trying to filter out dissolved salt or sugar
Dissolved particles pass through filter paper. Only undissolved solids are removed.
Mistake 2: Overfilling the filter paper
If the liquid rises above the paper’s edge, solid can flow around it and contaminate the filtrate.
Mistake 3: Confusing residue and filtrate
The residue is the solid left on the paper, and the filtrate is the liquid that passes through.
Mistake 4: Sieving damp material
Wet material clumps and blocks the holes. Dry the sample first if you want accurate results.