CBSE Class 7 Science Advanced Chemistry Water Notes Set 01

Water

Introduction

Our planet stands apart from every other known world because it is crowded with plants and animals, and water is the reason. Water is plentiful, and it is the one resource that keeps life going here.

Life began in water. Scientists believe that, millions of years ago, minerals dissolved in sea water reacted with one another under sunlight, and the first single-celled creatures such as bacteria and fungi appeared. Water is still needed for photosynthesis, the process that feeds the living world, and it forms the main part of every living cell, so it takes part in all the activities of life.

Distribution of Water on the Earth

About 97% of the earth's water is saline water found in oceans and seas. Only 3% is fresh water. Of this fresh water, 70% is surface water and 30% is underground water. The surface water is shared by rivers (87%), lakes (2%) and swamps (11%).

Physical Properties of Water

1. Nature: pure water is a liquid with no colour, no smell and no taste.
2. Freezing point: at normal atmospheric pressure (76 cm of mercury) water turns to ice at 0°C.
3. Boiling point: at the same pressure water boils and turns to steam at 100°C. The boiling point goes up when the outside pressure goes up and comes down when the pressure falls.
4. Density and anomalous expansion: most liquids expand when heated, so their density keeps falling as the temperature rises. Water behaves differently in the range 0°C to 4°C. Ice is lighter than water. When ice melts at 0°C the density rises, and it keeps rising as the water is warmed until 4°C. Beyond 4°C water follows the usual rule and its density falls on heating. So water is densest at 4°C, where its density is 1 g/cc. Between 4°C and 0°C water expands on cooling, while all other liquids shrink on cooling. This odd behaviour is called the anomalous expansion of water.

5. Specific heat: the heat needed to raise the temperature of 1 g of a substance by 1°C is its specific heat capacity. For water it is 1 cal g⁻¹ °C⁻¹, which is the highest value among all substances.
6. Latent heat: when a substance changes its state, heat goes in to loosen the forces holding its particles together, and the temperature does not change during the change of state. The heat needed to turn 1 g of ice at 0°C into water at 0°C is the latent heat of fusion, 80 cal/g. The heat needed to turn 1 g of water at 100°C into steam at 100°C is the latent heat of vaporization, 540 cal/g. Water also turns slowly to vapour at any temperature between 0°C and 100°C. This is evaporation, and it becomes faster as the temperature goes up.
7. Conductivity: pure water hardly conducts electricity. Dissolved salts make ordinary water a good conductor.
8. Solvent property: water dissolves a very large number of substances, so it is called the universal solvent.

Forms of Water

Water covers about 70% to 75% of the earth's surface. Although we usually think of liquid water, it also occurs as a solid and as a gas. In the Arctic, in Antarctica and on high mountains the very low temperature keeps water as snow and ice. In the air it is present as water vapour, and when this vapour condenses it forms clouds, fog and mist. The liquid state is the most common one. Liquid water occurs as surface water and as underground water.

Sources of Water

1. Surface Water

Water lying on the earth's surface in oceans, seas, rivers, springs and similar places is surface water.

Oceans and seas. These are the biggest stores of water, holding about 97% of all the water on the earth. This water has a lot of dissolved salt (about 2.5% is common salt) and so it cannot be drunk or used for most purposes. Such water is called saline water.

Desalination. Taking the dissolved salts out of sea water is called desalination. It is done by reverse osmosis. Sea water is kept in an inner vessel and pure water in an outer vessel, with a semipermeable membrane in between. A piston presses on the sea water with a pressure higher than its osmotic pressure. Pure water is then forced through the membrane into the outer vessel and the salts stay behind.

Rivers and lakes. Rain falling on high ground runs downhill and forms rivers, and melting snow and ice add to them. A river flows a long way over land and picks up impurities and soluble salts. Since it has far fewer dissolved salts than sea water, it is treated as fresh water and is used for many everyday needs.

Rain water. Rain is the main natural supply of water, and because it comes straight from the atmosphere it is the purest natural water. The first showers wash dust and germs out of the air, but the later rain is free of impurities.

2. Underground Water

Part of the rain flows into rivers and lakes, while another part soaks into the soil and moves down. Rain and some river water seep through the soil and collect in the gaps and cracks deep below the ground. This seeping is called infiltration, and the water collected is underground water.

While going down, the water first crosses the zone of aeration, where the soil is not fully soaked. Deeper down, in the recharge area, more of the spaces get filled until the zone of saturation is reached. At the top of this zone the pressure of the ground water equals atmospheric pressure, and that level is the water table. Below the water table lies a layer of permeable rock or loose material that holds water and from which water can be drawn out through a well. This layer is the aquifer.

Underground water is filtered by the soil layers while it seeps, so it has no suspended impurities. It can be pumped out with hand pumps and tube wells, and this is well water; it contains dissolved impurities. How deep a well must be depends on the place.

Ground water that moves through cracks and fissures may come out at the surface where the aquifer meets the ground, flowing out under pressure. This is spring water. It has no suspended impurities but has dissolved salts and minerals. Because it carries rare minerals, it is used for medicinal purposes.

Uses of Water

1. It is a raw material of photosynthesis, by which plants make food for themselves and for animals.
2. It is the medium in which the chemical reactions of living things take place.
3. It carries nutrients and minerals to the cells of living organisms.
4. It helps in the exchange of gases through the blood during respiration.
5. It is needed for seeds to germinate.
6. It is needed for plants to grow.
7. It helps to regulate body temperature.

Water Scarcity

When rainfall is very low or a drought sets in, water becomes scarce. People get less water to drink, and plants suffer badly: they wilt, dry up and finally die. Rivers dry up in summer, so dams are built to store water. This stored water is used in farming and to produce hydroelectric power.

Water Cycle

Rain is the chief source of water. Water vapour is added to the air by many processes. This vapour is lighter than air, so it rises, and in the cold upper air it condenses into tiny droplets that hang in the air. Droplets joining dust particles form bigger groups called micelles, which we see as clouds. When the clouds hold too much water, it falls as rain. The rain fills rivers, lakes and other surface water bodies and also recharges the ground water. This natural process in which water goes to the air and comes down again as rain is the water cycle. It keeps a natural balance in the ecosystem.

In the figure, water vapour is added to the air by transpiration from plants, breathing of animals, evaporation from water bodies and burning of fuels. The vapour forms clouds, and rain falls both on the ground (feeding the underground water table) and on water bodies.

Depletion of Water Table

Most of the earth's water is saline and cannot be used for drinking. Only river water and underground water are available as sources, and only a very small share of all water on the earth is of this kind. Rivers are limited and many dry up in summer, so people have to depend on ground water.

Ground water is drawn out with hand pumps and tube wells, and it is refilled when rain seeps into the ground. If we take out more than gets refilled, the water table sinks. This lowering is called depletion of the water table, and it happens most when rainfall is poor.

Causes of Depletion

1. Little rain or drought: as long as the water drawn out matches the water refilled by rain, nothing goes wrong. In a long dry spell the use continues at the same rate while refilling almost stops, so the table falls.
2. Deforestation: the roots of big forest trees hold the soil together, slow down the run-off of rain and let the water soak in. Cutting forests lets the rain rush into large water bodies, so less seeps down. Trees also give out water vapour that forms clouds and rain, so fewer trees mean less rain.
3. Growing population: more people need more buildings, which covers the ground and reduces the area through which water can seep in, and they also need more water.
4. Industrialization: every industry uses water in many ways. When the number of industries grows, the use of water rises far beyond the rate at which the ground water is refilled.
5. More farming: where rainfall is uncertain, canal irrigation is unreliable, so farmers depend on ground water. More farming means a higher demand for it.

Over a long time these factors lower the ground water level very much and lead to a serious shortage of water. Water is precious, and so everyone, citizens and authorities alike, must use it carefully and take steps to prevent the fall of the water table.

Water Management

Usable fresh water is limited and the water cycle is affected by natural factors, so water must be managed with care. Water management has two main parts.

1. Preventing wastage: supply water through pipes that do not leak, and cut down careless use in daily life.
2. Rain water harvesting: collecting rain water and using it to recharge the ground water, for example by fitting harvesting systems in houses.

Other helpful measures are the following.
1. Drip irrigation: in farming, narrow tubes carry water straight to the base of each plant, so very little water is used.
2. Carrying irrigation water in closed pipes instead of open canals lowers the loss by evaporation.
3. In places with scanty rain, cetyl alcohol is sprayed over the water of large reservoirs. It forms a film that slows evaporation.
4. Growing some plants under plastic covers saves water, because the vapour condenses on the inside of the cover and drips back into the soil.

Hard and Soft Water

Ground water stays in the lower layers of the earth's crust for a long time and dissolves many salts. Depending on the salts, water is of two types.

Soft water easily forms lather with soap. Hard water does not form lather with soap and instead forms a precipitate called scum.

Causes of Hardness

Water that contains bicarbonates, chlorides and sulphates of calcium and magnesium does not form much lather. These salts make water hard.

Disadvantages of Hard Water

1. It is not good for drinking.
2. It gives very little lather, so it is poor for washing clothes, cleaning and bathing.
3. In boilers it leaves a hard scale, which wastes heat and fuel in industries.
4. It is not good for dyeing, since the dye does not fix well on the cloth.

Types of Hardness and Removal of Hardness

Bicarbonates get into water because carbon dioxide dissolves in rain water. As this water flows over rocks, it reacts with calcium carbonate and magnesium carbonate in the rocks and forms the soluble bicarbonates. These bicarbonates make the water hard, and the hardness they cause is called temporary hardness.

\(\ce{CaCO3 + H2O + CO2 -> Ca(HCO3)2}\)
\(\ce{MgCO3 + H2O + CO2 -> Mg(HCO3)2}\)

Removal of Temporary Hardness

1. Boiling. On boiling, the soluble bicarbonates change to insoluble carbonates, which can be filtered off. Since plain boiling removes this hardness, it is called temporary hardness, and such water is temporary hard water.

\(\ce{Ca(HCO3)2 -> CaCO3 + H2O + CO2}\) (on heating)
\(\ce{Mg(HCO3)2 -> MgCO3 + H2O + CO2}\) (on heating)

2. Clark's method. Slaked lime, \(\ce{Ca(OH)2}\), is added to the water as a solid or as a solution. It turns the soluble bicarbonates into insoluble carbonates, and filtering gives soft water.

\(\ce{Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 + 2H2O}\) (the calcium carbonate settles as a precipitate)
\(\ce{Mg(HCO3)2 + Ca(OH)2 -> CaCO3 + MgCO3 + 2H2O}\) (both carbonates settle as precipitates)

Water may also dissolve chlorides and sulphates of calcium and magnesium. These do not go away on boiling, so the hardness they cause is called permanent hardness.

Removal of Permanent Hardness

1. Adding washing soda (sodium carbonate). This removes both temporary and permanent hardness. The bicarbonates, sulphates and chlorides of calcium and magnesium undergo double decomposition with sodium carbonate. The insoluble carbonates formed settle down and are filtered off.

\(\ce{Ca(HCO3)2 + Na2CO3 -> CaCO3 + 2NaHCO3}\)
\(\ce{MgSO4 + Na2CO3 -> MgCO3 + Na2SO4}\)
\(\ce{CaCl2 + Na2CO3 -> CaCO3 + 2NaCl}\) (in each reaction the carbonate of calcium or magnesium is the precipitate)

2. Permutit process. Permutit is hydrated sodium aluminium silicate, \(\ce{Na2Al2Si2O8.xH2O}\). Hard water is passed through a cylinder packed with permutit. The calcium and magnesium ions of the water are exchanged for sodium ions of the permutit. Sodium ions do not form a precipitate with soap and do not stop lather, so the water becomes soft.

After long use, the permutit holds \(\ce{Ca^2+}\) and \(\ce{Mg^2+}\) in place of \(\ce{Na+}\) and is said to be exhausted. It can be regenerated by passing a sodium chloride solution through it.

3. Ion exchange method. Here the ions that cause hardness are swapped for other ions. The apparatus has two cylindrical tubes: A is the cation exchanger and B is the anion exchanger. They are filled with porous solids called ion exchange resins, which do not dissolve in water but swell in it and hold ions that can be exchanged. A resin that exchanges cations is a cation exchange resin, and one that exchanges anions is an anion exchange resin. An H-cation exchanger replaces \(\ce{Ca^2+}\) or \(\ce{Mg^2+}\) by \(\ce{H+}\), and a Na-cation exchanger replaces them by \(\ce{Na+}\).

Hard water goes first through tube A, where all positive ions are replaced by \(\ce{H+}\) (or \(\ce{Na+}\), depending on the resin). It then goes through tube B, where all negative ions are replaced by \(\ce{OH-}\) from the anion resin. The water leaving the apparatus contains only pure water and is called demineralized water. When the resins run out of \(\ce{H+}\) and \(\ce{OH-}\), they are regenerated by passing HCl solution (or NaCl solution) through A and NaOH solution through B.

\(\ce{MgCl2 + 2H-Resin -> Mg-Resin + 2HCl}\)
\(\ce{RNH3OH + Cl- -> RNH3Cl + OH-}\)
\(\ce{H+ + OH- -> H2O}\)

Once hardness is removed, the water is fit for daily use and for industry. Besides the salts that dissolve naturally, human activities add many other substances to water, and these are called pollutants.

Worked Examples

Example 1. State and explain the anomalous expansion of water.
Most liquids get less dense as they are warmed, because they expand. Water does not do so between 0°C and 4°C. Ice is less dense than water. When ice melts at 0°C, the density of the water goes up, and it goes on rising until 4°C. After 4°C the density falls with a rise in temperature, as in other liquids. Therefore water has its maximum density of 1 g/cc at 4°C. From 4°C down to 0°C water expands while all other liquids contract. This reverse of the normal pattern is the anomalous expansion of water.

Example 2. What is infiltration? What is its role in water management?
A part of the rain goes into rivers and lakes and another part soaks into the soil and reaches the lower layers. Rain and some river water seep down and fill the empty spaces and cracks deep underground. This process is infiltration. It recharges the water table, so it helps in managing the water supply.

Example 3. The electronic configurations of metals X and Y are 2, 8, 2 and 2, 8, 8, 2. What is the effect of their hydrogen carbonates and sulphates in water?
X (2, 8, 2) is magnesium and Y (2, 8, 8, 2) is calcium. Hydrogen carbonates (bicarbonates) of calcium and magnesium make water temporarily hard, and the sulphates of these metals make it permanently hard.

Example 4. In the permutit process, why is a concentrated solution of the solid compound formed from NaOH and HCl added?
The process uses sodium permutit, \(\ce{Na2Al2Si2O8.xH2O}\). It exchanges its \(\ce{Na+}\) ions for \(\ce{Ca^2+}\) and \(\ce{Mg^2+}\) of the hard water. After long use most of its sodium ions are gone and it stops working. NaOH reacts with HCl to give sodium chloride, and a concentrated solution of sodium chloride is passed through to regenerate the permutit.

Example 5. A container is filled to the brim with water and has ice cubes floating in it. Will the water overflow when the ice melts?
No. The floating ice already pushes aside a volume of water equal to the volume of the water that the ice will turn into. So the level stays the same and the container stays full to the brim.

Example 6. Why is water called the universal solvent?
Water can dissolve most substances, so it is called the universal solvent.

Example 7. What is the advantage of the continuous water still over Liebig's condenser?
The continuous still saves fuel, because the water to be boiled is already warmed by the condensing steam.

Example 8. Why is distilled water not suitable for drinking?
It has no mineral salts and no dissolved gases. Used for drinking, it would draw nutrients out of the body fluids and carry them away, causing a shortage of vital nutrients. It also tastes flat and bland, so it is not fit for drinking.

Water Pollution

Table 5.1 Causes and Control of Water Pollution

Potable Water or Drinking Water

The authorities take many steps to control water pollution. A well-known programme is the Ganga Action Plan, which requires industries by law to set up water treatment plants. Even then, water from natural sources still carries some suspended impurities and germs. It may be fine for other jobs, but it cannot be used for drinking, so it has to be purified first. Purification has two stages: removing suspended impurities and removing microorganisms.

1. Removal of Suspended Impurities

(a) Sedimentation. River water is pumped into settling tanks and left undisturbed. The bigger suspended particles sink to the bottom.
(b) Adding chemicals. Sedimentation leaves the water only partly clear. It is sent to a mixing chamber, where alum and lime are added and the water is stirred by large mechanical blades. It then goes to a sedimentation tank. Alum speeds up the settling of suspended impurities, and some bacteria are trapped and carried down with them.
(c) Filtration through sand and gravel. The clear water is passed through a sand and gravel filter, which removes the last traces of suspended matter.

2. Removal of Microorganisms

Bleaching powder is added. It releases chlorine, which kills the harmful bacteria. The water is then kept in storage tanks.

The water now has no suspended impurities and no harmful bacteria. It is fit to drink and is called potable water. It still has dissolved gases such as \(\ce{O2}\) and \(\ce{CO2}\) and some dissolved salts, mainly common salt. These salts are needed for the body's activities and give the water its taste, so this water is well suited for human use.

Distilled Water

Most chemical reactions in a laboratory take place in water, and many tests need pure water. For this, the dissolved salts must be removed, and this is done by distillation.

Distillation Using Liebig's Condenser

The water is boiled in a distillation flask. The steam passes through Liebig's condenser, where cooling turns it back to liquid, and the liquid collects in the receiver flask. The dissolved salts remain in the distillation flask.

Distillation Using a Continuous Water Still

Here the incoming water is first warmed by the condensing steam, and only then is it boiled in the still. The benefit is that less fuel is needed, since the water has already been heated.

Uses of Distilled Water

1. It is used for chemical work in laboratories.
2. It tastes bland and flat, and is regarded as tasteless, because the taste and freshness of water come from its salts and dissolved gases.
3. It cannot replace potable water, even though it is pure, because it has no mineral salts and no dissolved gases. It draws nutrients from the body fluids and removes them, leading to a lack of essential nutrients.
4. Sea water can be distilled, and a little common salt added to the distillate makes it fit for drinking.

Volumetric Composition of Water by Electrolysis

Decomposition is a chemical change. It can be carried out using energy such as light or electricity. Breaking down a substance by passing electricity through it is called electrolysis.

Principle. Pure water conducts electricity very poorly, so a little sulphuric acid is added to it. This acidulated water, when electricity is passed, splits into hydrogen and oxygen. The volume of hydrogen is twice that of oxygen (ratio 2 : 1).

\(\ce{2H2O -> 2H2 + O2}\) (on passing electricity)

Procedure. Two test tubes are filled with acidulated water, with no air bubbles, and turned upside down over two iron nails. The nails are connected to the two terminals of a battery and act as electrodes. The nail joined to the positive terminal is the anode and the one joined to the negative terminal is the cathode.

After electricity has been passed for a while, a gas collects in each tube, pushing the solution down. The fall in the solution level shows the volume of gas formed. The gases are tested with a burning splinter. In test tube A the splinter burns very brightly, so the gas is oxygen. In test tube B the splinter is put out with a pop sound and the gas burns with a pale blue flame, so it is hydrogen. The volumes of oxygen and hydrogen are in the ratio 1 : 2.

The same ratio is found when water is made from its elements: hydrogen and oxygen combine in the volume ratio 2 : 1.

\(\ce{2H2 + O2 -> 2H2O}\)

Points to Remember

1. Rain water is the purest of all natural sources of water.
2. Distilled water is a pure form of water, but we use potable water for drinking.
3. Water has the highest specific heat of any known substance, 1 cal g⁻¹ °C⁻¹. Its latent heat of fusion is 80 cal/g and its latent heat of vaporization is 540 cal/g.
4. Water shows anomalous expansion, and its density is maximum (1 g/cc) at 4°C.
5. The boiling point increases as the external pressure increases.
6. Calcium and magnesium salts prevent lather with soap, and water containing them is hard water.
7. Hardness is either temporary or permanent. Temporary hardness is removed by boiling or Clark's method. Permanent hardness is removed by washing soda, the permutit process or the ion exchange method.
8. Water is polluted by the excreta of living beings, industries, farming, thermal power plants and so on.
9. Pollution is controlled by converting human waste and cow dung into biogas, using natural manure and pesticides, and by drip irrigation.
10. Hydrogen and oxygen are present in water in the volume ratio 2 : 1.