Famine in Recent Times Human environments: Trade and development issues 166

Convection occurs when heat is transferred by the movement of a liquid or a gas. A liquid or a gas is lighter if is warm and heavier if is cool. If a liquid or gas has a warm part and a cold part, the warm part naturally moves upwards and the cold part naturally moves downwards. This movement is known as a convection current. A convection current is created when we boil a saucepan of water on the cooker. The water at the bottom of the saucepan is hottest because it is closest to the cooker ring, so it moves up to the surface. At the same time the water at the surface is cooler, so it moves to the bottom. A convection current also causes warm air (a gas) to rise. This is why it is often warmer upstairs in a house than downstairs. Convection currents in the Earth’s atmosphere contribute to weather systems, moving warm or cool air around the globe.

What do convection currents in water look like?

Equipment: red and blue food colouring, freezer, ice cube tray, transparent plastic container of room-temperature water, small bottle of hand-hot water

Action:

● Make a prediction about what happens when an ice cube is added to room-temperature water, and hand-hot water is added to roomtemperature water. ● Fill an ice cube tray with water. Add a couple of drops of blue food colouring for each ice cube before placing the tray in the freezer. ● Once the ice cubes are frozen, add a few drops of red food colouring to the bottle of hand-hot water. ● Place the ice cubes in the container of room-temperature water. Record your observations on your worksheet.

Why do you think this happened? ● Open the bottle of hand-hot water, hold your thumb over the opening, gently lay it on its side in the container of room-temperature water, and remove your thumb. Record your observations on your worksheet.

Why do you think this happened?