Dissolving rocks

Is there a rock in your kettle? In many parts of Britain the answer could be "Yes" but what is the rock like and how did it get there?

An electric kettle has a metal tube near its base. This is called the element and it contains a coil of wire, which heats up when the kettle is switched on. If your kettle has a rock in it, this is where the rock will be found. It will be coating the heating element. The rock will be white and grey. Some people call it fur.

Kettles are not sold with rocky or furry elements. They acquire them over a long period of use. Every time the water is boiled, some of the limestone dissolved in the water comes out of solution and settles on the element. Limestone is unlike many other solid substances. Most substances dissolve better in hot water than cold. With limestone the opposite is true. Limestone dissolves better in cold water that it does in hot. The hottest water in the kettle is next to the heating element so this is where the limestone collects - at the place where it is least soluble.

The limestone has made a long journey to reach the kettle. This journey began long ago, when the limestone first formed. Limestone is made from the shells of sea creatures. They lived in seas as long ago as four hundred million years. When these sea creatures died, their shells collected on the seabed. Huge numbers of shells gathered and pressed down on each other until rock was formed.

Over time, all rocks in the Earth's crust move. The limestone that was once a seabed was pushed upwards to form a range of hills. It was here that the limestone began to dissolve.
When water falls on many rocks it dissolves some of their minerals and carries them away. When water falls on limestone it can dissolve large pieces of rock. If you visit limestone country, you can find evidence of dissolving rock all around you. Where large slabs of limestone have been exposed to the weather, their surfaces are carved with deep grooves. These are caused by water collecting there, dissolving the limestone, then flowing away into streams and rivers. In some places you may be able to visit a cave. There are hundreds of them in limestone country and all have been made by water dissolving limestone and carrying it away. Some caves are as large as a cathedral.

Water leaving the hills is often directed into a reservoir and then to a waterworks. Here, the water is cleaned but the dissolved limestone is not removed. It passes on in the clean water to thousands of homes.

The journey of limestone does not end in the kettle. Not all the limestone comes out of the water when it boils. Some of it passes on into drinks. One substance that is found in limestone is called calcium. If you take a cup of coffee in limestone country, some of the calcium enters your body and forms part of your bones. So the journey of limestone continues as long as you carry around its calcium in your bones.

Is water containing dissolved limestone harmful?

No. The water is rich in calcium. This mineral is needed for making strong teeth and bones. Many people also consider that the limestone in the water gives the water a pleasant taste.

Does the limestone harm a kettle?

The limestone makes the kettle less efficient. In normal conditions, when a kettle is switched on, the heat from the element passes directly to the water and warms it up straightaway. When the element is covered in fur, the heat must pass into the fur and warm the fur up. The heat then travels from the fur to the water. This means that a furry kettle has to use more energy (and electricity) to boil water than a kettle without fur.

Can the fur be removed from a kettle?

Yes, it can. The fur can be treated with vinegar. This contains a weak acid, which attacks the fur and dissolves it.

Does an acid dissolve substances in the same way as water?

No. When water dissolves something a reversible change occurs because the substance has not changed into a new substance. It is still present as solid particles and the water only needs to be removed for the solid to be made again. When an acid dissolves something, an irreversible change takes place because the solid is changed into another substance.

Does limestone in the water affect the home in other ways?

Yes, it does. Water containing dissolved limestone is called hard water. You may remember this name because if you try to make lather with soap you will have a hard time. Soap produces lather easily with water that is not hard but in hard water only a poor lather develops and solid particle called scum.

Do all rocks make hard water?

No. Only limestone and chalk. The water which runs through other kinds of rock, such as sandstone, is called soft water. It does not produce furry kettles or scum when you wash.

Can hard water be changed to soft water?

Yes, it can. Some people have a device in their home called a water softener. In this device all the substances that make water hard are removed and the water is made soft.

When you go into a cave you may see stalactites and stalagmites. How are these formed?

You find stalactites and stalagmites in caves in limestone country. Stalactites are columns of rock, which hang down from the cave ceiling, and stalagmites are columns of rock which project upward from the cave floor. They are produced from dissolved limestone. Water reaching the cave roof is rich in dissolved limestone. As it collects and waits to form drips on the cave roof, some of the dissolved limestone comes out of the water. The solid limestone forms a tiny lump on the cave roof, which is left behind when the water splashes to the floor. In time, over thousands of years, the tiny lumps build up to form a stalactite. When the water hits the floor some more limestone comes out of the water and forms a solid lump. In time the lumps form a stalactite. Over a very long time a stalactite and stalagmite may join together and form a column of limestone.