Language in use

THE WATER CYCLE

However much water we consume, the total amount of water in the world never changes. This is because virtually all the world`s water is in the oceans, locked up in the polar sheets or involved in a continuous cycle, called the hydrological cycle, that is forever circulating water between the sea and the sky. We get most of our water by tapping into this cycle. When your bath water runs away down the plug hole, it eventually ends up in rivers, lakes or the sea. From here it evaporates in the heat of the sun to fill the lowest layers of the atmosphere with invisible water vapour. A little of this vapour may be carried aloft by rising air currents until it cools enough to condense into clouds of water droplets and ice. Once these grow large enough, they fall as rain trapped to fill reservoirs and supply taps. So the cycle begins all over again.

Rivers return water to the sea.

The Sun`s heat evaporates water from both land and sea, turning it into invisible water vapour. Water vapour rising into cold air condenses into tiny drops of water. If the air is very cold it may turn into ice.

Millions of water droplets form a cloud.

Droplets inside the cloud join together to form bigger drops. When they are large and heavy enough they fall as rain or snow.

Some rain runs off the land and into streams and rivers.

Rivers may be damned to fill reservoirs.

Some water seeps away under ground through porous rock to feed rivers from springs.

Water is a truly remarkable substance. It is the most common compound on Earth, occuring as anything from tiny dewdrops to vast oceans. It is the basis of all living processes. And its special nature means that it plays a part in a vast number of chemical reactions from rusting to nuclear power production.

One of the unusual things about water is that it commonly occurs in all three phases of matter - ___________. Few other substances are ever naturally found in more than two states.

About 2 per cent of the world`s water is solid - either frozen briefly as snow and frost, or more or less permanently in ___________. Most of the rest is liquid - filling oceans, seas, and lakes, flowing in rivers and streams or trickling through the ground (groundwater). Nearly trree quarters of the world`s surface is covered in water.

Water vapour

Barely one thousandth of a per cent of the world`s water is gaseous, __________. But water vapour in the atmosphere plays a very important part in our weather. It is the presence of water vapour that distinquishes the "troposphere", the lowest layer of the atmosphere, from the atmosphere above. _____________ and in which we live, breathe and move.

The amount of water vapour in the atmosphere is known as the "humidity". At any one temperature, air can only hold a certain amount of water vapour. Water constantly evaporates from seas, lakes and the surfaces of the plants, adding to the vapour in the air. When the air can take up no more water vapour, it is said to be "saturated". Once it is saturated, evaporation stops and droplets of water condense from the air, forming clouds or dew, and eventually falling to the ground as reain or snow.

Water as a chemical

In every water molecule, there are two hydrogen atoms and one oxygen atom. The atoms are said to be bonded together "covalently". This means that the hydrogen and oxygen atoms "share" electrons. Hydrogen atoms have only one electron, so each hydrogen atom donates its electron to  the oxygen atom and receives one elctron from the oxygen in return.

Water as a solvent

So many substances dissolve well in water that pure water occurs only rarely in nature. Sea water, _______, of which most is sodium chloride- the ordinary salt you use on your food.

Salts get into the sea from the rivers and groundwater that run into it, for even fresh water is very far from pure. Fresh water typically contains a range of dissolved substances including chlorides, carbonates and sulphates of metals such as sodium, magnesium, calcium, and iron.

When water is hard, it is because of the magnesium and calcium compounds dissolved in it. Rainwater is fairly pure, but ______ such as carbon dioxide and sulphur dioxide.

It is because water is so good at dissolving things that it is vital for life. Life is thought to have begun in the oceans, which are actually very complex solutions. And all living organisms use "aqueous" (water-based) solutions, like those in the blood and digestive juices, to carry out biological processes. Of course, this is also why water pollution can be so dangerous. Pesticides, for example, can easily dissolve in water and be taken up by living organisms, from plants to human beings.

Reading comprehension  exercises

1. Were you correct with the statements about the water cycle in the "pre-reading task"? Did you get answers for all your questions?

2. The following parts of the sentences were removed from the text. Try to put them back. Where do they go?

a/ glaciers and the polar  ice sheet

b/ the troposphere is the layer in which the world`s weather occours

c/ that is, as a solid, a liquid and a gas

d/ that is, water vapour

e/ even rainwater tends to contain dissolved gases

f/ for instance, is about 3.5 per cent salts

Vocabulary part

3. Find words in the text that are in obvious sense the opposite of the terms below:

discharge

temporarily

start

coldness

evident

dropping

4. Match up the following words and definitions:

Discussion part

5. Try to explain the following:

a/ how the water cycle works

b/ why water vapour in our atmosphere plays an important part in our weather

6. Talk about substances that dissolve well in water.

7. What do you expect to happen to the water cycle in the  foreseeable future, anvd why?