Hot liquids rise, cold liquids sink
The oceans cover a large part of our planet and they are also very deep. You can imagine how deep they are in the following way. Look up into the sky when there is plenty of blue sky and some cloud. Look for small fluffy clouds high in the sky. If you were at the bottom of an ocean, those clouds would be on the surface - about five kilometres above your head! In some places the oceans are even deeper.

The waters of the oceans receive most of their heat from the Sun. It only reaches into the upper waters. They become warm and lighter in weight than the cold water below. This means that the warm waters stay where they are - at the surface - and do not move into the colder waters by convection.

For many years, scientists have been inventing machines to explore the cold water of the deep oceans. About twenty five years ago a small submarine called Alvin was designed and built to carry scientists into the deep. As it is completely dark on the ocean floor, Alvin was equipped with powerful searchlights. When the scientists reached the ocean floor they cruised above its surface with the searchlights on. After a while they were astonished to see tall chimneys sticking out of the ocean floor with what appeared to be black smoke rising from them.

By studying the chimneys, the black smoke and the movement of the water, scientists were able to explain what they had found. Below the ocean floor is a region of hot rock that is not far away from a volcano. Cold water flows by these rocks and becomes hot. This hot water rises through the rocks that make the ocean floor and dissolves some of the minerals in them. When the hot water reaches the surface of the ocean floor, it meets cold water and loses some heat. This causes some of the dissolved minerals in the water to turn back into solids again and form a chimney. Another substance that dissolves in the hot water turns into little black solid particles. These particles travel in the rising hot water and look like smoke.

As the hot water rises above the chimney, it loses its heat. When it is at the same temperature as the surrounding cold water it stops rising. In time it may sink back into the ocean floor to be heated once more. It may then flow back up through the chimneys that scientists now call black smokers.

Why did the hot water rise above the black smoke?
Water, like all substances, is made from very tiny particles which can only be seen by very powerful electron microscopes. When the water receives heat, the particles move further apart from each other. This makes the water lighter in weight than the surrounding cold water. When any substance is lighter in weight than the water around it, the substance floats. Think of a cork. It is much lighter in weight than water. If you hold a cork under water then let it go, it pops up quickly. In a similar way, the hot water from the black smoke rises up into the ocean.

Does the hot water reach the surface?
No, because the ocean is too deep. It does not take long for heat to flow out of the hot water and into the cold water. When the hot water has lost its heat it stops rising and never reaches the surface.

Why does the water stop rising?
It stops rising because as it loses heat, its particles come closer together again. When the water is cooled down, its particles are as close together as the particles in the cold water around it. This means that the water is now the same weight as the cold water and cannot float above it.

Does cold water ever sink in the oceans?
Yes, it does. Wherever there are differences in temperature in the water there will be rising and sinking of the water. The movements of the currents in the deep ocean are very complicated because they are generated by currents on the surface. The currents on the surface are generated by the way the earth turns and the winds on the ocean surface. However there is one particular place where cold water sinks. This occurs in the Antarctic Ocean. In winter, ice, called pack ice, forms on the surface of the ocean. Under the pack ice the very cold water sinks very deep and enters the lower parts of the Atlantic, Indian and Pacific Oceans.

Does hot water rise in a kettle?
Yes. That is why the heating element in an electric kettle is near the bottom. When the element gets hot, some of its heat warms the water around it. The warm water rises and cold water from other parts of the kettle move round the element. This cold water is warmed and rises too. In the meantime, the first water to be warmed has risen and cooled as it reached the surface. It now sinks back to be warmed again. As the water rises and sinks in the kettle, heat spreads throughout the kettle. In a short time the water is so hot that it boils and the kettle switches itself off.

Does hot water rise in a pan?
Yes, it does. It behaves in just the same way as the water in the kettle but it receives heat from the hot plate or gas burner beneath the pan. Oil or melted fat used in deep fat frying or fondue cooking is heated up in just the same way as the water in a kettle.