Suspensions, spinning and clumping

A suspension is a heterogeneous mixture where solid bits do not dissolve but float about in the liquid. The bits are bigger than in a solution and you can see them: sand, sawdust or tea leaves in water.

Leave muddy water to stand and the heavier mud settles, but the water may still look cloudy. Filtering through cloth or filter paper catches only the bigger bits. So we need other methods.

Centrifugation: spin it. Spin the mixture very fast in tubes. The tubes swing out flat, and the spinning pushes the heavier particles outwards, to the bottom of each tube; the lighter liquid stays on top. You feel the same outward push in the spinning game *phugadi (Marathi) or kikli* (Punjabi).

Centrifuges separate blood into red cells and plasma, and are used in industry. The paperfuge is a hand-powered spinner made of card and string. With no electricity at all, it spins blood samples to test for malaria and anaemia in remote villages.

Coagulation: make it clump. Stir a little powdered alum (fitkari) into muddy water. It works like glue, sticking the tiny particles together into bigger clumps. The alum is the coagulant. Then:

• the heavy clumps sink: sedimentation;
• pour off the clear water on top: decantation;
• or catch the clumps on a filter: filtration.

Paneer is coagulation too. Add lemon juice or vinegar to hot milk and the milk proteins clump together.