Cool Science for Children


Water

Introduction

Water is the most important substance on Earth. Without it, there would be no plants, no animals, no life. The human brain is approximately 85% water, and if you lost 10% of the water in your body you would not be able to walk. A loss of 20% would be the end of you.

As we are constantly losing water we need to take in more to replace it. Although there is a huge amount of water on Earth, 97% of it is in the oceans and far too salty to drink. Of the fresh water, most is frozen in the polar ice caps. Even so, there is plenty of water around (in most places anyway) in the form of rivers and lakes, both above and below ground. As we use this water, it is replaced by natural processes - part of the water cycle.

Water has many unusual properties, and is a special compound because many substances dissolve in it. This means all the special molecules that make up you and me and all other life can meet together in solution and react.

The Water Cycle

1) Most of the water in water cycle is in the oceans and seas of the world. The sun's energy, which drives the water cycle, heats the water, which evaporates (becomes a gas).

2) This also happens to water on land, in rivers and lakes, and from the leaves of plants, when it is called transpiration.

3) The sun also heats the surface of the planet, which in turn heats the air near the ground. This causes the air to rise, taking the evaporated water with it. As the air rises, it cools, and can no longer hold as much water. The water condenses back to a liquid, forming droplets, which form clouds.

4) When the drops reach a critical size, they fall from the sky as rain (or more generally, precipitation, which includes rain, snow, hail, and all other water falling out of the sky).

5) The water falls on the oceans, or on the land, where it runs together in rivers and percolates through the rock on it's way back to the sea. As it travels itis used by us and all the other animals and plants. The cycle is never ending, going round and round, driven by the sun's energy.

The water that comes out of your taps at home is very different from the water found in rivers and lakes. Natural water is cloudy (due to colloidal particles) and contains bacteria. The water companies remove both of these during the treatment process.

Solid particles are removed by a process called coagulation. This involves the addition of aluminum or iron ions, which attract the small negatively charged particles so that they comes together to form a precipitate. This is then filtered from the water using filters of fine sand (and sometimes activated carbon). The use of aluminum ions also leads to an increase in the acidity of the water (a drop in pH). To counteract this a base (or alkali), such as sodium hydroxide is added.

Disinfection of the water is achieved by adding chlorine. Chlorine is very soluble in water so is simply dissolved in the supply. Often, ammonia is added to the water after chlorination to form chloramines. These are less effective than free chlorine in killing bacteria, but have the added advantage of persisting longer in the pipes that supply homes and businesses. This is called a residual disinfectant. Another chemical that is sometimes used is sulphur dioxide. This removes excess chlorine, allowing the dosing to be carefully controlled.

The quality of water is very carefully measured by water companies and also several regulating bodies. Water is analyzed for metal ions (such as cadmium, lead and copper), bacteria, chlorine, and also the turbidity (cloudiness) and color.

Evaporation, Condensation and Precipitation
Observe the water cycle in the privacy of your own kitchen

Transpiration
Leaking plants? Surely not!

Evaporation, Condensation and Precipitation (or 'your very own water cycle')  These processes are the most important in the water cycle. They can be easily demonstrated by the following experiment:

Apparatus

One mug (of the drinking vessel variety)
Some hot water, straight from the kettle
A metal spoon

Instructions

Fill the mug with the freshly boiled water and hold the metal spoon above the liquid.
Water droplets should form on the spoon, get larger and finally drop back into the liquid.

Conclusions

What you have observed (what do you mean you haven't done it yet? It only takes five minutes, go and do it!) is the evaporation of water from the hot liquid, it's condensation on the surface of the spoon, and it's precipitation back down.

The evaporation is the transition of the water from liquid to gas. You can't see the gas, the steam you see is water droplets suspended in the air, much like clouds. The condensation is the transition back to liquid again as the vapor loses it's heat to the spoon. The precipitation is simply the way in which the drops deposit.

Transpiration

This process also plays a part in the water cycle, although it is less important than evaporation. It is, however, very important to plants.

Apparatus

One pot plant with green leaves
One clear plastic bag
One large elastic band (or string would probably do)

Instructions

Put the plastic bag over the top of the pot plant, attach tightly with the elastic band, then leave for two days. 

Conclusions

The water droplets on the inside of the bag come from a process called transpiration, that is, lost from the leaves of the plant. How could you prove that this is true and that it does not come from the evaporation of water from the soil?

Water is lost by the plant because it's green leaves have pores, or small holes, in them called stomata. These are there to allow gases into, and out of, the leaves so the plant can photo synthesis (the process whereby it makes its food).

Unfortunately, these pores also leak water. And this allows us to demonstrate that they are there without using a microscope to look at the leaves.

I think I am also right in saying that transpiration is one of the ways in which plants "pull" water up through their roots.

By Hari Kochat

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