Convection in liquids

Do liquids also rise when heated, like air? Activity 7.3 half-fills a 500 mL beaker with water and, using a straw, places a grain of potassium permanganate at the centre of the base. The purple colour shows you where the water moves. A candle is put right below the centre of the base.

As the water heats, a streak of colour moves up the middle and then down the sides.

Why? The water at the bottom gets heated first. It expands, becomes lighter, and rises. The water at the sides is cooler and heavier, so it sinks to take its place. That water is then heated in turn and rises too.

This cycle goes round and round until all the water is hot. The whole body of water is heated by the actual movement of water particles. This kind of heat transfer is called convection, and the loop of rising and sinking water is sometimes called a convection current.

So water, like air, gets heated by convection. In convection, heat travels with the particles of a liquid or gas as they move from one place to another. That is the big difference from conduction, where the particles stay put.

You use convection at home. An air conditioner is fitted high up, near the ceiling: it cools the warm air that has risen, the cooled air sinks, and the room fills with cool air. A room heater is kept low, near the floor: it warms the cool air down there, which rises, and the whole room warms up.