3. WATER 3.1 WATER, WATER EVERYWHERE

Transcription

1 3.1 WATER, WATER EVERYWHERE 3. WATER The picture on the right shows what the Earth looks like from space. Can you see the continent of Africa? About 70% of the surface is covered with water. Most of the water is in the oceans and seas. But there is also a lot of water in rivers and lakes, and even under the ground. The north and south poles are covered with ice, which is frozen water. The south pole is the white area at the bottom of the picture. The clouds in the sky are made of tiny drops of water. Water is the most important liquid in the world! We could not survive without water. We need water to drink and to cook our food. We use it to wash and to keep ourselves clean and healthy. Many people eat fish, which are caught in the sea or in lakes or rivers. We need water to grow our crops. In fact, all the animals and plants, on which we depend for our food, clothing and shelter, need water to survive. Modern man also uses huge amounts of water in his many different industries. Irrigated rice fields in Asia A hydroelectric damn in Africa 1. Look at the pictures. List at least seven different uses of water that are illustrated. 2. What is the difference between the water in the seas and oceans, and the water in most rivers and lakes? 3. How do we depend on animals and plants for our food, clothing and shelter? 4. Water is the most important liquid in the world. But water can also be a solid, or even a gas. (a) What different names do we give to water when it is a solid and when it is a gas? (b) Where would you expect to find water that is solid and water that is a gas? 5. Try to find out what is meant by (a) irrigation, (b) hydro-electric power. 24

2 3.2 THE THREE STATES OF WATER Water is usually a liquid, but at different temperatures it can also be a solid or a gas. Ice. When water is cooled to 0 o C, it freezes and becomes ice. This is what happens when we put water into the freezing compartment of a refrigerator. bottle of cold water Ice is water in its solid state. If the temperature rises above 0 o C, the ice melts to form liquid water again. The temperature 0 o C is called the melting point of ice. It is also called the freezing point of water. Ice is common in places where it is very cold. Even the rain may freeze and fall as snow! In high mountains there are often rivers of ice called glaciers. In the Arctic and Antarctic, the sea is often covered with ice. Melting ice Boiling water steam condensing Water vapour. When water is heated to 100 o C, it boils and changes to steam. This is what happens when water is boiled on a fire or a cooking stove. The temperature 100 o C is called the boiling point of water. Steam is water in its gas state. When a gas is formed from a liquid, we often call the gas a vapour. Steam is the vapour formed when we boil water. If the temperature falls below 100 o C, most of the steam condenses to form liquid water again. To observe condensation, hold any cold surface in the steam rising from a pan. You have observed that puddles of rainwater on the road soon disappear when the sun comes out. The water in the puddles changes into water vapour and goes up into the air. We say the water evaporates. Evaporation can occur at any temperature. Water is always evaporating from the surface of seas, lakes and rivers, so there is always water vapour in the air. THE THREE STATES OF MATTER: Solid, liquid and gas are called the three states of matter. melting H E A T I N G evaporation or boiling SOLID LIQUID VAPOUR (GAS) freezing condensation C O O L I N G The melting point of a solid is the temperature at which it melts. The boiling point of a liquid is the temperature at which it boils. 1. What is formed when (i) ice melts, (ii) water evaporates, and (iii) steam condenses? And what is (iv) a vapour? 2. Which has the higher melting point, ice or a wax candle? 3. What is the difference between evaporation and boiling? 4. There is always water vapour in the air. What do you think happens to this vapour when the air is suddenly cooled? 25

3 3. 3 THE WATER CYCLE Scientists have estimated that the total volume of water on Earth is more than a thousand million cubic kilometres ( km 3 ). This is such a huge volume that it is almost impossible to imagine it! About 97% of this water is in the oceans that cover more than 70% of the Earth's surface. Another 2% of our water is in the solid state! This is found in the huge ice caps around the North and South poles, and in the glaciers in our great mountain ranges. Only 1% of all our water is found on the land where we can use it, and most of that is under the ground. Water is always evaporating from the sea, and from lakes and rivers. It also evaporates from the soil and from the leaves of plants. It even evaporates from our own bodies when we breathe out and when we sweat. What happens to all this water vapour in the atmosphere? And why does everything not dry up completely? High up in the atmosphere it is quite cold. The higher we go, the colder it gets. Anyone who lives in the mountains knows that. Warm air near the surface tends to rise. The air carries water vapour with it and, as it rises, it cools. When the water vapour cools, it starts to condense. At first it forms very tiny drops that we call droplets. These droplets are too small to fall back to Earth. They remain suspended in the atmosphere as clouds. There are millions of tiny droplets in every cloud. When enough water vapour has condensed, the droplets get bigger and start falling as rain. Most of the rain falls straight back into the sea, but some falls over the land. There it soaks into the soil and drains into lakes and rivers and underground reservoirs. The rivers carry the water back to the sea. 4. Wind blows clouds Where the water is found Oceans Polar ice caps and glaciers Lakes, rivers and underground Where is all our water? Percentage 97% 2% 1% 3. Clouds form 5. Rain falls 1. Sun heats the sea 6. Rivers flow to the sea 2. Water evaporates We call this process the water cycle. The word cycle means going round and round. Because water goes round and round like this, it can be used over and over again and it never runs out. In the natural world, there are many cycles like this. The air we breathe and the food we eat are parts of cycles that we will learn about later in this book. 1. What are clouds? 2. Explain why the Earth will never run out of water. 3. A question for the mathematicians! The volume of the water on Earth is more than 10 9 km 3. How many 1 L bottles would 10 9 km 3 fill? 26

4 3.4 COOLING BY EVAPORATION Think about the water cycle. Why does so much water evaporate from the sea? It evaporates because of heat from the sun. Water absorbs heat when it evaporates. Experiment. The diagram illustrates an experiment to show that liquids really do absorb heat when they evaporate. The experiment works best on dry days when water evaporates quickly. Set up two or three thermometers as shown in the diagram. Cover the bulb of the first one with dry cotton wool. Cover the next one with cotton wool soaked in water. If you have methylated spirit or petrol (gasoline), cover the last one with cotton wool soaked in that. Read the temperature on each thermometer as soon as it is ready. Record it neatly so you will not forget it. Read each thermometer again after two or three minutes. How does the temperature of each one change? You probably observed that the temperature of the dry cotton wool did not change. But the temperature of the cotton wool soaked in water probably fell a few degrees. This is because the water absorbed heat as it evaporated. If you tried the third one, that would cool even more, because methylated spirit and petrol evaporate faster than water. You may have noticed that a liquid that evaporates fast, such as methylated spirit or petrol, feels cool if you spill it on your hand. Now you know why! It is because the evaporating liquid absorbs heat from your skin. Keeping cool. Can you guess why we sweat when we are hot? It is because the sweat cools our skin as it evaporates. In some countries, water is kept cool by storing it in an unglazed jar. A little of the water soaks through the walls of the jar and evaporates into the air. As it evaporates, it cools the water inside. The jar shown on the left comes from Ghana in West Africa. A refrigerator also works by evaporation. Study the diagram. A special liquid that evaporates easily fills the tubes you can see at the back of the refrigerator. The liquid is compressed by a pump, and then evaporates through a valve. As it evaporates, it absorbs heat from the The back of a refrigerator freezing compartment inside. The vapour returns to the pump, which compresses it back into a liquid. (The liquid gets warm from being condensed and compressed, but this happens outside the refrigerator, at the back, so the heat can escape into the air). 1. What makes water evaporate from the sea? 2. Why do you feel cool after a swim, even on a hot day? 3. (i) Why are the tubes at the back of a refrigerator exposed to the air? And (ii) why are the tubes in which the liquid evaporates placed inside the freezing compartment of the refrigerator? 27

5 3.5 SOLUTIONS AND CRYSTALS What happens when a spoonful of salt (or sugar) is stirred in a glass of water? The salt disappears! If we taste the water, we find the salt has spread all through the liquid. We say the salt has dissolved in the water, and we call the mixture a solution of salt in water. White substances like salt and sugar make colourless solutions, which look like pure water. Coloured substances make coloured solutions. Copper sulphate, for example, is a blue solid and it makes a clear blue solution in water. Solvents and solutes. When a liquid dissolves a solid, the dissolving liquid is called the solvent, and the solid that dissolves is called the solute. When a solute dissolves in a solvent, we say the solute is soluble in the solvent. Substances that do not dissolve are said to be insoluble. For example, sugar is soluble in water but sand is insoluble. Solutes dissolve faster - (i) (ii) (iii) when crushed into a fine powder, when stirred with the solvent, when the solvent is hot. Dilute and concentrated solutions. What happens if we grind up some copper sulphate and then keep adding it, one spoonful at a time, Soluble or insoluble? What can we do to find out whether a substance is soluble in water or not? First we must grind the substance into a fine powder. Then we add a small amount of it to a glass of clean water, and stir it well. If it disappears, then it is soluble. If it settles on the bottom and will not disappear, then it is insoluble. Finally, we must check that the solution is clear and transparent. If it is cloudy, the substance has not dissolved. It is just suspended in the water. We will learn about suspensions in the next section. to a glass of water? At first the solution is only pale blue. When a solution contains only a little solute, we call it a dilute solution. As we continue adding more copper sulphate, gradually the colour becomes darker. A solution that contains a lot of solute is called a concentrated solution. As we add more and more copper sulphate, there comes a time when the last spoonful refuses to dissolve! It just sinks back to the bottom of the glass no matter how much we stir it! When a solution will dissolve no more solute, we call it a saturated solution. The illustration shows a saturated solution of copper sulphate. You can see that a little of the solute is undissolved on the bottom. Solution of copper sulphate Grinding up the solute and stirring help to speed up dissolving, but they make no difference to how much dissolves. But if we use hot water, not only does the solute dissolve more quickly, we can usually dissolve more of it too. Most solutes are more soluble in hot water than in cold water. Making crystals. Many solids form crystals. If you look closely at grains of sugar (use a hand lens if you can), you will observe that most of them have regular shapes. Crystals have flat faces, straight edges and fixed angles between the faces. They are often beautiful to look at. We can make crystals from solutions in two ways, (i) by evaporation or (ii) by cooling a hot, saturated solution. (i) Evaporation: What happens when water evaporates from a solution of salt? As the water evaporates, the solution becomes more concentrated. After some time it becomes saturated. Then, as more water evaporates, the salt is left behind as crystals. If the water is evaporated fast, by boiling the solution, the salt crystals are small. But if the solution is left to evaporate slowly in a warm, dry place, the crystals are much larger. Crystals of salt are perfect cuboids. Anyone can use this method to make salt crystals at home or at school. Some crystals 28

6 (ii) Cooling a hot saturated solution: Hot water usually dissolves more of a solute than cold water. For example, at 60 o C, 100 ml of water dissolves about 60 g of copper sulphate to make a saturated solution. At 30 o Growing a crystal C, the same volume of water can only dissolve about 30 g. If you want to make a really large crystal, you Imagine what happens when we make a saturated solution of copper sulphate at 60 o can choose a nice small one and grow it! Tie the C, crystal to the end of a fine thread and suspend it then allow it to cool to 30 o C. The cool solution in a saturated solution of the same solute. If can only hold about half the copper sulphate, possible, use the same solution from which you so the other half turns into crystals that sink obtained the small crystal. Leave it to grow for to the bottom. If the solution cools slowly, we several days in a dry, shady place. get a few, large crystals. If it cools fast, we get many, tiny crystals. We can make crystals of many different solids by making hot saturated solutions and allowing them to cool. Making crystals is a good way to obtain solids that are clean and pure. In many countries, salt is obtained by trapping sea water in ponds and allowing it to evaporate in the sun. Sugar crystals are obtained from sugar cane by crushing the cane with water and evaporating the solution. White "granulated" sugar is obtained by rapidly cooling a hot saturated solution. Different solvents. Water is a very good solvent, but it is not the only one. Methylated spirit and petrol (gasoline) are better for dissolving greasy or oily solutes. In "dry cleaning", a solvent called trichloroethane is used to dissolve stains from clothing. Separating immiscible liquids Obtaining salt by evaporating sea water Different solutes. A solute does not have to be a solid. Liquids and gases can dissolve in a solvent too. Beer and wine both contain a liquid called alcohol, dissolved in water. But petrol and cooking oil do not dissolve in water, they just float on top of it. Liquids that dissolve in each other are called miscible liquids. Those that do not dissolve in each other are called immiscible liquids. So water is miscible with alcohol, but water is immiscible with petrol or cooking oil. Liquids can also dissolve gases. Fish in the sea and in lakes can breathe only because the water dissolves oxygen gas from the air. Many popular soft drinks contain carbon dioxide gas dissolved under pressure. When the pressure is released, some of the dissolved gas escapes making the drink "fizz"! 1. What are (i) a solvent, (ii) a solute, (iii) a dilute solution, (iv) a saturated solution, (v) a crystal? 2. What is the difference between dissolving and melting? 3. Name (i) 2 gases which dissolve in water, (ii) 2 immiscible liquids. How would you separate 2 immiscible liquids? 4. Try to explain why a crystal grows in a saturated solution. 29

7 3. 6 SUSPENSIONS AND FILTRATION Separating solids from liquids. Water from rivers, ponds and wells is sometimes dirty. If we look carefully, we may see tiny solid particles suspended in it. When we leave dirty water to stand, sand and mud slowly sink to the bottom. After some time the water may become completely clear. Then we can pour it carefully into a clean container without disturbing the sediment. Separating clear water from a sediment in this way is called decanting (or decantation). A quicker way to obtain a small volume of clear water is to filter it. Scientists use circular filter papers for this. The filter paper is folded into quarters, opened into a cone, and placed in a filter funnel. The dirty water is then poured into the cone of paper. The water passes through the filter paper, but the suspended particles are trapped. Filtering can be used to separate any liquid from solid particles suspended in it. The liquid that passes through the filter paper is called the filtrate and the solid that remains in the filter paper is called the residue. You should note that filtration does not remove any solute that is dissolved in the liquid. Solute and solvent together pass straight through filter paper. Decantation Filtration Separating salt and sand How would you separate a mixture of salt and sand? Easy! First stir up the mixture with water. The salt dissolves. Then filter. The sand remains as a residue on the filter paper. The salt is in the filtrate. So evaporate the water from the filtrate to obtain the salt. Filter paper under microscope To see how filter paper works, tear off a small piece and look at it under a microscope. Like any paper, it is made of tiny fibres jammed together. The clear liquid can find its way between the fibres, but the solid particles get stuck. Suspensions and colloids. When particles of a solid are suspended in a liquid, we call the mixture a suspension. Muddy water is a suspension of sand and clay in water. When the water is still, most of the particles settle to the bottom quite quickly, but the tiniest particles of clay may take a long time. If they are very small they may never settle, so the water always looks a bit cloudy. Even when we filter the water, it still remains cloudy. The solid particles are so small that they pass through the filter paper. A suspension that never settles, and cannot be separated by filtration, is called a colloid. You can make a cloudy colloidal suspension of starch at home by boiling a little flour with water. This colloid makes a good glue for sticking paper. Another wellknown colloid is black ink. Suspensions are quite common and may involve different combinations of solids, liquids and gases. Milk is a colloidal suspension of solids and immiscible liquids in water. Colloidal suspensions of immiscible liquids are called emulsions. Toothpaste and many household paints are emulsions. They contain millions of tiny droplets of oily liquids suspended in water. Smoke is a suspension of solid particles in the air. Mist, fog and clouds are suspensions of tiny droplets of water in the air. We use aerosol cans to spray tiny droplets of insecticides or pleasant smelling scents which remain suspended in the air for a long time. The foam we get when we use soap is a suspension of air bubbles in a watery solution. Ice cream consists of very tiny air bubbles suspended in a very cold flavoured cream. Coloured plastics are suspensions of solids in solids - the plastic has tiny particles of colour embedded in it. 1. What are (i) decantation, (ii) filtration, (iii) a suspension (iv) a colloid, and (v) an emulsion? 3. How would you prepare a suspension of chalk in water? 2. Name six colloids that are mentioned on this page. 4. Give examples of suspensions of (i) a liquid in a gas, (ii) a gas in a liquid, and (iii) a solid in a gas. 30

8 3. 7 EVAPORATION AND DISTILLATION What is the difference between evaporation and boiling? A liquid can evaporate at any temperature, but boiling occurs only at one particular temperature called the boiling point. Evaporation is slow at low temperatures, but fast at the boiling point. What other things affect the rate at which evaporation occurs? Factors that affect the rate of evaporation. When you hang out your clothes to dry, what can you do to help the water evaporate quickly? There are three things you can do: (i) You can hang the clothes in a warm place. As we just saw, temperature is one factor that affects the rate of evaporation. The higher the temperature, the faster the clothes dry. (ii) You can spread the clothes out to expose the biggest possible area to the air. The area exposed is a factor that affects the rate of evaporation. The larger the area exposed, the faster the clothes dry. Three simple experiments For each experiment, you will need two equally wet pieces of paper. In each case (i) find out which one dries first, and (ii) decide which factor made the water evaporate faster. 1. Spread out one piece of paper in a warm place and one in a cool place. Both places must be equally windy. 2. Fold one piece of paper into quarters, but spread out the other one. Place them side-by-side in the same warm place. 3. Spread out one piece of paper in a calm place and one in a windy place. Both places must be equally warm. (iii) You can hang the clothes in a place where the air is moving. The wind is the third factor that affects the rate of evaporation. The stronger the wind, the faster the clothes dry. Distillation. How can we obtain really pure water? If the water looks dirty we can filter it. But even then, there may be something dissolved in the water. Remember, a solute passes through the filter paper with the solvent. A good way to find out if a solute is present is to evaporate the water. You can try this with the water you use at home or at school. Leave a clean plate full of water out in the sun to evaporate. When the water has completely evaporated, look closely at the plate. Has any solute been left on the plate? In fact most of the water we use has some minerals from the earth dissolved in it. One way to obtain really pure water is to distil it. The impure water is boiled in a flask. Only the water evaporates into steam. Any solute or suspension stays in the flask. The steam passes through a tube called a condenser which has cold water flowing over it. As it cools, the steam condenses into pure water. This process of evaporation and condensation is called distillation. Even sea water, which contains a lot of salt, can be fully purified by distillation. Pure water obtained in this way is called distilled water. 1. What are the three factors which speed up evaporation? What does the word factor mean? 3. What is distillation? How does a Liebig condenser work? 2. In each of the experiments described above, why must the two pieces of paper be equally wet? 4. A mixture of two miscible liquids with different boiling points can be separated by distillation. Try to explain how you think this works. Why must the temperature be carefully controlled? 31

9 3. 8 FLOATING, SINKING AND DENSITY We all know that sticks float and stones sink. Some people say that light things float and heavy things sink, but that cannot be true. A huge tree trunk floats and a small stone sinks! The real difference between materials like wood (which float), and materials like stone and metal (which sink), is that wood is much lighter when we compare objects that are the same size. When we think about the mass of something, as compared to its size, we are thinking about its density. Density. The density of any material is the mass of one unit volume of the material. We normally measure mass in grams and volume in cubic centimetres, so a common unit of density is grams per cubic centimetre, g/cm 3 (or g cm -3 ). The SI unit of density is kg/m 3, but this is rarely used. To find the density of an object, we first measure its mass in grams and its volume in cubic centimetres. Then we calculate the density of the object by dividing its mass by its volume. Density = mass volume. Mass (in grams) Density = g/cm 3 Volume (in cm 3 ) Example 1: A small block of wood has a length of 5 cm, a breadth of 2 cm and a height of 2 cm. The mass of the block is 10 g. What is the density of the wood? The mass of the block is 10 g The volume of the block is = 20 cm 3 The density of the wood is = 0.5 g/cm 3 Example 2: A steel teaspoon has a mass of 16 g. When it is dropped into a measuring cylinder of water, the level of the water goes up from 70 cm 3 to 72 cm 3. Find the density of the steel. The mass of the spoon is 16 g The volume of the spoon is = 2 cm 3 The density of the steel is 16 2 = 8 g/cm 3 Floating and sinking. The table shows the densities of some common materials. Notice that the density of water is exactly 1.0 g/cm 3. Now look at the density of things that float and the density of things that sink. Try to make a generalisation about floating and sinking in water. You should be able to see that: Things that float in water have a density of less than 1 g/cm 3, and things that sink in water have a density of more than 1 g/cm 3. Material Density g/cm 3 Floats or sinks Air Aluminium Cooking oil Copper Glass Iron Lead Mercury Petrol Ice Stone (silica) Water Wood (hard) Wood (soft) floats sinks floats sinks sinks sinks sinks sinks floats floats (just) sinks - floats floats A ship made of iron floats because it is full of air. If we divide the mass of the ship by its volume, including all the space that is filled with air, the average density of the ship is less than 1 g/cm (i) What is density? (ii) In what units is density measured? (iii) How can we calculate density? 2. What is the density of water? What rule tells us whether something will float in water or not? 3. Why does an empty cup float, but a cup full of water sink? cm 3 of a plastic called PVC has a mass of 12 g; 10 cm 3 of another plastic called polythene has a mass of 9 g. Do these two plastics float or sink? 32

10 3.9 WATER PRESSURE What happens if there is a small hole in a bucket of water? Water leaks from the bucket of course! A jet of water shoots out from the hole. The force that pushes the jet of water is called water pressure. You may have noticed that, as the water level in the bucket goes down, the jet of water slows down too. In a few minutes it is only a trickle, and it stops when the water is level with the hole. The water pressure obviously gets less as the water level goes down. We can do a simple experiment to check how water pressure depends on the depth of the water. All we need is an old plastic bottle. With a pointed object such as a nail, make three holes, all the same size. One hole must be near the bottom of the bottle, one half way up, and one quite near the top. Cover all the holes with tape (or with three fingers). Fill the bottle with water, and take it outside. Uncover all three holes at the same time and observe what happens. As the diagram shows, the pressure depends on the depth of the water. The jet of water is weakest where the water is shallow near the top of the bottle. It is strongest where the water is deepest at the bottom of the bottle. If we make holes all round the bottom of the bottle, we observe that the jets of water shoot out equally in all directions. This tells us that, at the same depth, the pressure is equal in all directions. We can sum up with a generalisation: Water pressure increases with the depth of the water and acts equally in all directions. Measuring pressure with a water manometer. A water manometer is an open, U-shaped tube, half filled with water. The water rises to the same level on both sides of the U. The tube is made of glass or plastic so that the water levels can be seen. A scale of cm and mm allows us to measure any changes in the water levels. A flexible plastic or rubber tube is attached to one side of the U. water If we blow gently into the plastic or rubber tube, the water level goes down on one side of the U and up on the other side. The difference between the levels measures the pressure of the air in your lungs in cm of water. A water manometer can only measure quite small pressures, so be careful not to blow too hard! A water manometer The diagram shows a manometer being used to measure pressures at different depths in a bucket of water. This is another experiment which confirms that the pressure increases with the depth. If we point the manometer tube in different directions, we can also confirm that, at the same depth, the pressure acts equally in all directions. This includes upwards and downwards as well as sideways. 1. (i) What is water pressure, and (ii) in what direction does it push? (iii) How does water pressure change with depth? 2. (i) Describe how to use a water manometer. (ii) In what units does this manometer measure pressure? (iii) What is the pressure shown by this manometer. 33

11 3.10 FLOWING WATER A characteristic of all liquids is that they flow. When rain falls, the water flows downhill over the ground, and through the earth, until it reaches a river. The river also flows downhill until it reaches a lake or the sea. There it spreads out and the water creates a level surface. There is a well-known saying that "water always finds its own level". This means that water always flows until it reaches the lowest level it can find. And it keeps flowing and spreading out until no part of the surface is higher than any other part. If we pour water into different containers, joined together by pipes, the water fills all the containers to the same level. The diagram (right) shows a piece of apparatus found in some school laboratories. Notice that the water level is the same in all the containers; it is not affected by their shape or size. A simple experiment with water levels Try this outside to avoid making a mess! Cut the bases off two plastic bottles. Now the bottles can be used as funnels. Join the necks of the bottles (funnels) with a piece of hose pipe. Water is poured into one of the funnels (bottles) until it is nearly full. When the funnel (bottle) on one side is raised, the water flows across to the other side. Notice that the water level is always the same on both sides. The siphon. A siphon is a tube used for taking water (or any liquid) from a higher level to a lower level in a particular way. Look at the diagram below. When the tube is filled with water, water flows from the high container to the low one. An easy way to fill the tube, is to suck at the lower end. When the water enters your mouth, pinch the end of the tube between your fingers to stop air entering. Now put the end of the tube back into the lower container and release it. The siphon will start working. Water will flow from the high container to the low one. In a siphon, the water actually flows uphill to start with! But this only happens when the tube is full of water. And it only happens when the outlet end of the tube is lower than the water level in the high container. A siphon is useful for making water flow steadily from a container that has no tap. Sometimes truck drivers use a siphon when they have run out of petrol. They may be able to siphon petrol from the tank of another vehicle. 1. We say that water always finds its own level. Explain, in your own words, what this means. 2. When does water flow uphill (without a pump)? 3. Why is it very dangerous to play with petrol? 4. Describe how you would decant clean water from a large container to a small one, using a siphon. SAFETY NOTE: Do not try to siphon petrol from the tank of a vehicle yourself. The owner may be very angry, and petrol is very dangerous. It is poisonous if swallowed and it very easily explodes into flame! 34

12 3.11 WATER IN LIVING THINGS All living things need water to survive. About 70% of the mass of your body is water. Your blood, which carries materials to every part of your body, is mostly water. So are the liquids in your gut that dissolve your food. Almost all the complicated processes that support life in every part of your body, take place in solutions in water. As you can see from the diagram, all living organisms contain a lot of water. The percentage of water in some living organisms We can check that living organisms contain water by a simple experiment. Put any animal or plant material (such as meat, leaves or fruit) in a test tube. Warm the tube gently in a small flame. You will observe water vapour leaving the material and drops of liquid water condensing at the mouth of the test tube. If we want to check that the liquid really is water, we can condense some in another test tube as shown in the diagram. The box on the right of the diagram explains how we can test the liquid. Testing for water A simple test for water uses paper that has been soaked in a solution of cobalt chloride and then dried. Cobalt chloride is blue when it is dry, but goes pink with water. So any liquid that changes cobalt chloride paper from blue to pink must contain water. Collecting water from animal or plant materials Living things are part of the water cycle. Every living organism takes in and gives out water. Plants take in water through their roots. The water passes up the stem of the plant as sap, and evaporates as water vapour through their leaves. Human beings usually need two or three litres of water every day. We take in water when we drink, but everything we eat contains a lot of water too. We also make water inside our bodies during respiration. We lose water in our urine and in our sweat. We also lose water vapour in our breath every time we breathe out. 1. Give the percentage of water in (i) tomatoes, (ii) corn, (iii) chicken meat? 2. Why does your body need water? State 3 ways in which your body obtains water, and 3 ways in which you lose it. 3. Does soil contain water? How can you check? How can you find the percentage of water in soil? 35 In humid regions, blue cobalt chloride paper slowly goes pink because of the water vapour in the atmosphere. In such regions, the paper must be dried before use. When we have plenty of liquid, measuring the boiling point is another good test. If the liquid is water, it boils at 100 o C.

13 3.12 THE TECHNOLOGY OF WATER SUPPLY The water we use in our homes comes from lakes, rivers or wells, or sometimes from rain that we collect. In the past, water had to be collected in buckets, and this still happens in many regions. But now, more and more of us obtain water simply by turning on a tap. Three important parts of a modern water supply system are reservoirs, pipes, and taps. Reservoirs are places where water is stored before it goes to homes and factories. Reservoirs must be situated above the places they supply. The pressure of the water coming out of a tap depends on the height of the water Water is very heavy 1 cm 3 of water has a mass of 1 g 1 L = 1000 cm 3 and has a mass of 1000 g = 1 kg 1 m 3 = 1000 L and has a mass of 1000 kg = 1 tonne level above the tap. Reservoirs may be natural lakes, or lakes created by damming rivers, or large water tanks. Dams need to be very strong to hold back the huge mass of water in the lakes behind them. A dam must be thicker at the bottom than the top because the water pressure increases with depth. Water tanks are often used as reservoirs around villages and towns. They may be filled from larger reservoirs in the hills or by pumping underground water from wells or bore holes. They must be situated on high ground nearby or on specially built towers. The towers have to be very strong because water is so heavy. A cross section through a dam Pipes carry water from one place to another. The pipes that carry water from local reservoirs to peoples' homes are usually made of galvanised iron or black plastic. Galvanised iron is covered with a layer of zinc metal to stop it from rusting. T-joints are needed to connect branches that go off at right angles. L-joints are needed for sharp bends, especially in galvanised iron pipes, which are not very flexible. The pipes are screwed into the joints. Wider plastic pipes are used for drains to L-joints and T-joints carry away waste water. Drainpipes need T-joints and L-joints too, but they are often glued, not screwed. Taps control the flow of water from the end of a pipe. The water flows through a hole surrounded by a flat valve seat. When the tap is closed, a rubber washer on a valve disk seals the hole and the water cannot flow. The tap is opened by turning the handle anticlockwise. This raises the valve disk and the washer and allows the water to flow. Turning the handle clockwise, screws the valve disk down onto the valve seat, and the washer seals the hole again. Taps should be closed gently. Screwing down too hard soon destroys the rubber washer and may damage the valve seat. When a tap leaks it probably needs a new washer. It is easy to take a tap apart with a spanner, and replace the washer - but don't forget to stop the flow of water to the tap first! If the valve seat has been damaged, a new tap may be needed. A domestic water tap 1. What does the water pressure in a tap depend on? 3. What is a T-joint and how are pipes fixed to it? 2. Why are local water tanks often placed on high towers? 4. How would you try to fix a tap that starts dripping? 36

14 3.13 WATER IN THE HOME How pure is domestic water? The water we use at home is called domestic water. As we have observed, domestic water usually comes from lakes, rivers and wells or bore holes. It is sometimes stored in reservoirs and supplied through a system of pipes and taps. This water is not pure like distilled water. It may be contaminated by suspended solids, by solutes that are dissolved in it, and by tiny living organisms! We can easily remove suspensions from domestic water by decantation or filtration. In a modern water supply system, the water is filtered at a water works before it is stored in local reservoirs. Most domestic water contains a small amount of dissolved solutes. These come from the rocks and soil over which the water has flowed. Most of these solutes are harmless and do not need to be removed. If you fill a small plate with domestic water and leave it to evaporate in the sun, you will probably find that a small residue of solute remains. Unfortunately, substances that we use in modern agriculture and industry often contaminate water. Some of these substances are poisonous. If they dissolve in lakes or rivers, they make the water unfit for domestic use. The water we obtain from lakes, rivers and wells sometimes contains tiny living organisms. These are too small to be seen by the naked eye and are called microorganisms. Some of the larger microorganisms can be seen using a school microscope. Instructions for doing this are given in the box on the right. The smallest of the micro-organisms are called bacteria and Observing micro-organisms in water Collect rain water in a small jar and add a little dry grass and soil. Leave the jar to stand for one or two days. Your teacher will place a drop of the water on a microscope slide and set up the microscope for you. You should be able to see living micro-organisms moving around! viruses. Some of these are harmless but some cause diseases. They are so small that they can only be seen with a very powerful microscope. They are not removed by ordinary filtration. In a modern water works, the micro-organisms that remain after filtration are killed by treating the water with a gas called chlorine. They can also be killed by boiling the water for several minutes. Using domestic water. We need domestic water for drinking and preparing our food, and for keeping ourselves, our clothes and our surroundings clean. A simple water closet toilet (WC) The water we use for drinking and preparing food should be free from micro-organisms. If it has not been treated with chlorine at the town water works, it should be boiled for several minutes before it is used. This will help us to avoid diseases caused by bacteria in the water. Washing tables and dishes, and ourselves and our clothes, with soap and water, also helps to keep us healthy. It washes away the dirt and food particles that bacteria like to live and grow on. We can also use disinfectants to kill these bacteria. Keeping clean to avoid disease is called hygiene. Another important use of water in modern homes is in the water closet toilet. This removes our wastes in a hygienic way. A water seal keeps the smells in and the flies out! The handle (1) is pulled up. This opens the flush valve (2). Water pours into the toilet bowl (3). It flushes everything down the waste pipe (4). The float (5) falls, opening the inlet valve (6) to refill the tank. The handle falls closing the flush valve. As the tank fills, the float closes the inlet valve. 1. What is contamination? Describe three main groups of impurities that may contaminate water. How is water purified at a water works? 2. What are bacteria? List three ways of killing bacteria. 3. What is hygiene? List the advantages of a water closet toilet. 37