What are sieving, sedimentation, decantation and filtration?

Sieving

Before Valli’s Mami kneads wheat flour for meethi puri, she sieves it. The fine flour passes through the holes of the sieve, and the bigger particles, such as bran and small stones, stay on top. This is sieving.

Sieving is used when the parts of a solid–solid mixture have different sizes. It only works if the holes are bigger than one part and smaller than the other: if the holes were larger than everything, all of it would fall through. You can also see sieves at construction sites, where they separate pebbles and stones from sand.

Sedimentation and decantation

In Puducherry, Dada shows the children how to remove tea leaves without a strainer. He leaves the vessel of tea undisturbed for a while, and the tea leaves settle at the bottom. Then he tilts the vessel and pours the tea gently into a cup.

• The settling of a heavier insoluble part at the bottom of a liquid is sedimentation.
• Removing the liquid by tilting the vessel is decantation.

Decantation is also used when rice and pulses are washed, and to separate oil and water, which do not mix: the oil forms a layer on top and can be poured off. But decantation is not complete, so a few tea leaves still reach the cup.

Filtration

With a tea strainer, all the tea leaves are held back. Separating the leaves from the tea this way is filtration. A strainer has fairly big holes, so it cannot clean muddy water. A piece of cloth has much smaller pores between its threads and works better; people have used cloth this way since ancient times. For water that is still muddy, filter paper, with very fine pores, is used.

To filter muddy water in the laboratory, fold a filter paper into a cone, place it in a funnel over a conical flask, and pour the muddy water in. The mud stays on the paper as the residue, and clear water collects in the flask as the filtrate.

Other materials, such as cotton, charcoal and sand, can also be used as filters. The choice depends on the size of the particles to be removed: the pore size of the filter is what matters. Tea bags show this history well: they were first made of silk, then of gauze or muslin, and today mostly of filter paper.

Filtration around us

• A fishing net lets the water drain out through its mesh and keeps the fish, a bit like a filter.
• Our nose has hairs that trap dust from the air we breathe.
• A mask stops tiny droplets and dust from passing through.

What these methods cannot do

Sieving, sedimentation, decantation and filtration separate parts that do not dissolve. Salt dissolved in water passes straight through any filter. To get it back, the water has to be removed by evaporation. And clear-looking filtered water may still have germs, which is why drinking water from a pond should also be boiled.

Choosing the method

Mixture	Method	Why
Bran in flour, pebbles in sand	Sieving	Different particle sizes
Tea leaves in tea, oil and water	Sedimentation and decantation	The heavier part settles, or the liquids form layers
Mud in water, chalk powder in water	Filtration	Insoluble solid held back by the filter

Grains go through threshing and winnowing before they ever reach the sieve.