Sedimentation and decantation

In Puducherry, Malli and Valli visit Dada and Dadi and their old friend Balan. It is Dadi's tea time and Dada makes the tea. How will he remove the tea leaves? "With a strainer," says Dada, "but even without one we can remove most of them."

Dada leaves the pan undisturbed for a while, then gently pours the tea into a cup. Valli sees why it works: the tea leaves settle at the bottom.

• Sedimentation: a heavier part that does not dissolve (an insoluble part) settling at the bottom of a liquid.
• Decantation: pouring off the liquid by gently tilting the vessel.

The pan must be left still: keep stirring or shaking it and the solid never settles.

But Dada says he may still get a few tea leaves in his mouth. Decantation does not separate all the solid, so it is not a complete method of separation. It is also used when we wash rice and pulses: we pour the water off and keep the grains.

Oil does not mix with water; it forms a separate layer when left alone. Because it sits in its own layer, decantation can separate oil from water.