Filtration

Malli hands Dada the tea strainer. Dada pours the tea through it, and all the tea leaves collect in the strainer. This way of separating tea leaves from tea is called filtration.

Could a strainer clean muddy water? Dada suggests a piece of cloth. A cloth has very small holes (pores) between its woven threads, so it can act as a filter; people did this in ancient times too. If the water is still muddy, we need a filter with even smaller pores: filter paper, which has very fine pores.

In the book's activity, filter paper is folded into a cone, placed in a funnel on a conical flask, and muddy water is poured in:

• the mud stays on the filter paper: the residue;
• the clear water that collects in the flask: the filtrate.

So filtration separates insoluble solids (ones that do not dissolve) from a liquid. If there is a lot of solid, the filter paper can get clogged and the liquid passes through very slowly.

On a boat ride with the fisherman Otukkam, Valli watches water drain out through the fishing net while the fish stay in. That is a lot like filtration! Cotton, charcoal and sand can also be used as filters. Which filter to use depends on the size of the particles you want to remove. The most important thing in filtration is the size of the pores.