What is in dirty water?

Imagine you are standing on a hillside in the pouring rain. The water is soaking into the ground. When you move a little further down the hillside you find a place where the water is gurgling out of the ground. This place is a spring. The water swills over the rocks and flows into a stream. The heavy rain has filled the stream and as the water moves along, it nudges the turf on the banks and carries some of the soil away. If you were to take a jar and dip it in the stream, how would the water appear?

When a jar of stream water is allowed to settle for a few moments you can see it is divided into three parts. On the top are floating solids such as grass stems and leaves. In the main body of the water are particles of clay and at the bottom of the jar is a layer of grit and sand. The water also contains some other substances. They cannot be seen because they are dissolved.

The materials that you can see have one thing in common. They do not dissolve in the water. When something does not dissolve in a liquid, it is said to be insoluble. The three materials are separated from each other because of their weight. The grass and leaves are lighter than water and so float on top of it. The sand and grit are heavier than water and so sink beneath it. The clay particles are just heavier than water too but because of their small size they take a long time to push downwards through the water. The clay particles form a suspension because they appear to be suspended in the water. If you were to leave the jar for a day or two, most of the clay would slowly sink and form a layer on top of the sand and grit.

The three different materials in the soil water can be separated from it in three different ways. The leaves and grass can be skimmed off the top by using a sieve. The holes in the sieve are large enough to let the water and its clay suspension pass through but they are too small to let the grass and leaves pass through. As a result, by passing the sieve across the surface a few times, all the leaves and grass can be removed.

The material which has sunk through the water and collected at the bottom of the jar forms a sediment. The water can be separated from the sediment by a process called decanting. In this process the water is poured slowly and carefully out of the jar into a second jar. When this is done the sediment remains behind in the first jar.

Finally the clay particles suspended in the water can be removed by filtration. In this process water is poured through a filter paper into a third jar. If you look at filter paper with a magnifying glass you can see that it has tiny holes in it. These holes are still large enough to let water pass through them but they are too small for clay particles to pass through them. This means that when the water and its clay suspension are poured through a filter paper only the water passes through. The clay particles remain in the filter paper.
When you leave the rain-soaked hillside and return to the town below you turn on a tap to make a drink. This water may have come from the hillside too, but it is clear. This is because at the water treatment works any material suspended in the water is allowed to settle, and also the water is separated from its sediments. Then it is filtered to remove anything that remains suspended.

Are dissolved substances removed when water is filtered?

No. The particles of dissolved substances are spread out between the particles of water. When the water passes through a filter paper it takes the dissolved particles with it because the particles of water and the dissolved particles are small enough to pass through the holes in the filter paper.

What kinds of things can dissolve in the rainwater?

As the water falls through the air carbon dioxide can dissolve in it. When the water flows through the soil any fertiliser can dissolve in it. As the water flows through the rocks under the soil it can dissolve some of the minerals in the water. When the water splashes over the rocks in the stream oxygen from the air can dissolve in it.

When water leaves the waterworks is it pure?

It is pure enough to drink safely but it is not completely pure. If something is pure it contains no other substances. Tap water contains small amounts of dissolved minerals and it also contains a dissolved gas called chlorine. This is added to kill harmful microbes which the water may have picked up from the soil.

Is bottled water pure?

It is pure enough to drink safely but, like tap water, it is not completely pure. If you look on the side of the water bottle you will be able to find out about the minerals it contains. Indeed, some waters are bottled and sold because people like their taste and claim the minerals in the water keep them well.

Is there such a thing as pure water?

Yes, there is. It is called distilled water. In the distillation process the water is boiled and its vapour is collected. Substances which are dissolved in the water cannot travel with it when it turns into a vapour so they are left behind. When the water vapour is allowed to condense on a clean surface distilled water is produced. Nothing is dissolved in it.