Boiling

Have you waited for a kettle to boil? Moments before, you filled it with water, plugged it in and switched it on. Now you are probably staring out of a window waiting to make a hot drink. It seems that nothing is happening in the kettle for ages. Then a small rumbling noise can be heard. As time goes on the rumbling gets louder and louder. Tiny wispy clouds appear around the kettle's spout. They are soon pushed away and much larger cloud forms. It billows up into the kitchen air. Curiously the cloud is not connected directly to the spout. There is a space of clear air between them. Before you can make any more observations the thermostat on the kettle switches it off with a click and the boiling water is ready to make your drink.

Although you could not see them, changes were taking place in the water as it came to the boil. First, heat from the element in the bottom of the kettle warmed the water. This made it rise and cooler water rushed in to take its place. This movement of water is called convection. It spreads out heat through the water. All the time the convection currents are in action the temperature of the water is rising. Eventually there comes a time when the temperature stops rising even though the element is still heating up the water. Water is made from tiny particles you cannot see. They slide round each other. This makes the water flow. The particles have energy to move. When water is heated the particles receive more energy and move faster. Particles at the water surface move so fast that they escape from the liquid in a process called evaporation. The water particles that escape form a gas called water vapour. This gas mixes with the air.

As water gets hotter and hotter some particles inside the liquid start moving so fast that they form a gas too. This hot gas is called steam. The particles in the liquid move so fast that they stop sliding on each other and completely separate. They bounce around like the balls in a lottery show but they are trapped by the liquid water around them. A spherical hollow forms in the water. It is full of rapidly moving water particles, which have formed steam. We call these watery spheres bubbles. As the steam is much lighter in weight than the water around it, the bubbles float upwards. When they reach the surface the bubbles pop and the steam is released into the air. The hot steam pushes out the air in the top of the kettle then rushes out of the spout. As soon as it reaches cooler air some of the gas particles in the steam cool down. They join together to form water droplets. These droplets make the first wispy clouds around the kettle spout. As the water continues to boil more steam is produced. It rushes out of the spout and gets further into the air before it turns into a cloud. When it does, the clouds billow upwards into the kitchen air. We usually call these clouds steam but we are wrong to do so. If you want to see steam look at the top of the kettle spout or the bubbles in boiling water.

What is the temperature at which water boils?
It is 100°C. The temperature at which a liquid boils is called its boiling point.

Why doesn't the temperature of boiling water get any higher?
At 100°C the particles of water separate and become a gas. Water ceases to exist as a liquid above 100°C. It only exists as a gas.

Why do you seem to have to heat the water a long time before it boils?
Heat is a form of energy. The particles in the liquid use energy to move around. When the particles are changing from the liquid state to the gas or gaseous state they need to soak up a large amount of energy. They need this energy to pull themselves away from each other and to move about rapidly in the gas. It takes time to soak up this energy and this is why it feels there is nothing happening for long periods when you are boiling water. However, inside the liquid water the particles are picking up speed to form bubbles.

So if the cloud above a boiling kettle is not steam - what is it?
It is simply a cloud of very tiny water droplets. Each one has formed by a process called condensation. Steam can only exist at temperatures above 100°C. When it rushes out of the kettle spout it enters the air in the kitchen. This is much lower than 100°C so the steam can no longer exist. The rapidly moving particles in the steam come together and slide over each other. They form water again. The water forms around dust particles in the air.

Why can you see the cloud of water droplets and not the steam?
The water droplets are tiny balls of water. When light shines on them they reflect it in a way that a mirror reflects light. As there are millions of droplets in a cloud they reflect the light to each other and scatter it. This scattering of the light makes the whole cloud look white. Steam is a colourless gas. Light simply passes straight though it. This means that you cannot see it.

How did steam engines work?
You may have seen steam engines at a railway museum. They used to pull trains in Britain up until the middle of the twentieth century. They are still in use in some poorer countries today. When a volume of water changes to steam it expands rapidly to fill a space seventeen hundred times larger. As the steam expands it pushes hard on everything around it. In a steam engine, water is heated in a boiler and the steam is directed along pipes to pistons by the engine's wheels. The expanding steam pushes so strongly that it moves the piston and turns the wheel. The piston is enclosed in a cylinder and the steam is piped in a complicated way so that it can push the piston backwards and forwards to keep the wheels turning.

Were steam engines used for anything else besides pulling trains?
Yes. They were first used for pumping water out of mines. They were used in textile mills to operate machinery such as looms. The first car had a steam engine and the first motor cycle had a steam engine too. The steam engine was under the saddle, which made it rather uncomfortable for the motorcyclist. Steam-driven motor cycles were not a success.

In some places steam shoots out of the ground. Why is this?
The steam and hot water shoot out of the ground in some places where a volcano has been in the past. Although the volcano is extinct, hot rocks remain deep underground. Water collects in spaces above the hot rocks and receives heat from them. The heat makes the water boil. Steam builds up in the spaces. When a certain amount of steam builds up, it pushes on the hot water and sends it shooting up to the surface as a fountain. These hot water chambers which make fountains in this way are called geysers.