Water Class 9 ICSE — Simplified Chapter 6

Water class 9 ICSE chemistry explains water as a compound and a universal solvent, its physical and chemical properties, solubility, and the causes and treatment of water pollution. This page walks through Simplified Chapter 6 section by section. You will find every definition, reaction, and experiment stated in plain language, with formulas and worked examples, so you can follow along with the chapter open.

Download Water Water Pollution Class 9 ICSE Chemistry Chapter 6 PDF

Download the Water Water Pollution Class 9 ICSE Chemistry chapter PDF from the Simplified textbook to revise offline or print for practice. It is the full chapter as printed in the book — 23 pages, 50 sections, 80 questions.

What is inside Chapter 6

23 pages · 50 sections · 80 exercise questions

• Questions: 15 questions
• Additional Questions: 65 questions

1 Water

The chapter opens with the scope set by the Class 9 ICSE syllabus. It names water as a compound and a universal solvent, and lists the chief physical properties: density, boiling point, and melting point. It also names an experiment to show that the water we drink contains dissolved solids and dissolved gases. You can verify the exact scope on the official CISCE site when you revise.

The chemical properties covered are the action of cold water on sodium and calcium, hot water on magnesium, and steam on iron, including the reversibility of the iron–steam reaction. The syllabus also lists saturated solutions, the qualitative effect of temperature on solubility, and the causes and treatment of water pollution. Practise the observations for each metal and the balanced equations, since the syllabus says students must form the reactivity series from them.

2 A. INTRODUCTION

W. Ostwald stated that almost all natural chemical processes take place among solutions in water. Water is therefore called the elixir of life. Natural water is water found in its natural state. Treated water is water that has received some form of treatment.

Water is vital for plant and animal growth. The human body needs it for nearly every cell function. Industry depends on it for agriculture, transport, and hydro and thermoelectric power generation.

Water occurs in the free state in three forms:

• Solid: ice, snow, frost.
• Liquid: river, lake, sea, and spring water; well water and soil moisture; dew.
• Gaseous: water vapour, clouds, mist, and fog.

Water also occurs in the combined state. It is present in all living matter, in hydrated salts such as \mathrm{MgCl}_2\cdot 6\mathrm{H}_2\mathrm{O}, and in certain minerals. Water covers roughly 75% of the earth’s surface and makes up nearly 70% of human body weight. Green vegetables hold 80-90% water, milk 80-85%, and dry cereals only 3-5%. The other Class 9 ICSE chemistry book chapters build on these same ideas.

3 THE WATER CYCLE

Nature maintains the water cycle by circulating water from the earth’s surface into the atmosphere through evaporation, then back to the surface as rainwater. This cycle strongly influences the climate of the land near large water bodies.

4 WATER – Is Considered a Compound and not an element.

Henry Cavendish first synthesised water by igniting two volumes of hydrogen with one volume of oxygen. This showed water is not an element. A. Lavoisier later confirmed that a molecule of water contains two hydrogen atoms and one oxygen atom.

Water satisfies the three properties of compounds. The table maps each property to water.

5 NATURAL & TREATED WATER

Natural water is rain, spring, well, river, lake, or sea water. Treated water has received some form of treatment. Distilled water is the purest form, prepared by condensing steam. It is soft, free from solutes, and used for qualitative analysis. Pipe-borne water is filtered and chemically treated; it is free from bacteria but keeps essential mineral solutes. Chlorinated water is used in swimming pools to kill germs.

The physical properties of water are:

• Colourless, transparent clear liquid; odourless; tasteless, though a slight taste comes from dissolved gases and minerals.
• Boiling point 100°C at 760 mm Hg; freezing point 0°C at 760 mm Hg.
• Maximum density 1 g/cm³ at 4°C.
• Bad conductor of heat and electricity; high dielectric constant.
• Universal solvent.

Water exists in all three states. It freezes below 0°C into ice, boils at 100°C into steam at normal pressure, and condenses back to liquid.

6 BOILING AND FREEZING POINTS OF WATER ARE AFFECTED BY – CHANGE IN PRESSURE AND BY ADDITION OF SOLUTES

Two factors shift the boiling and freezing points of water. Increased pressure on the surface raises the boiling point and lowers the freezing point below 0°C. Dissolved impurities also raise the boiling point and lower the freezing point.

Students often mix up the direction of these changes. The rule to remember: anything that makes boiling harder raises the boiling point, and anything that makes freezing harder lowers the freezing point. Impurities and pressure both act this way.

7 WATER EXHIBITS – ANOMALOUS EXPANSION

Water contracts as it cools, but below 4°C it expands until it freezes at 0°C. So water at 4°C has maximum density and minimum volume. Ice is less dense than warmer water, so it floats. That is why a pond freezes from the top down, and marine life can survive beneath the ice in winter.

9 WATER IS – A UNIVERSAL SOLVENT

Water is a polar covalent compound with a high dielectric constant. It can break or reduce the electrostatic force holding the ions of an electrovalent compound. The ions become mobile, and the compound dissolves rapidly. That is why water dissolves inorganic compounds, many organic compounds, and a large number of gases.

Gases fall into three classes by solubility:

• Highly soluble and ionising: NH₃, HCl.
• Fairly soluble: CO₂, SO₂, Cl₂.
• Sparingly soluble: O₂, H₂, N₂, CO.

Two experiments show what tap water holds. In the first, a watch glass of tap water sits over boiling water. Evaporation leaves concentric rings of solid material. These are dissolved salts, minerals, and impurities. Rain water and distilled water leave no rings. In the second, boiling tap water in a flask drives out dissolved gas into an inverted tube.

The boiled-off air holds 30-35% oxygen and 65-66% nitrogen, against 21% and 78-79% in ordinary air. Carbon dioxide rises from 0.03% to 0.6%. Oxygen is more soluble in water than nitrogen, so dissolved air is richer in oxygen.

Dissolved salts give water its taste, supply minerals the body needs, and support plant growth. Dissolved oxygen supports respiration of aquatic plants and animals. Dissolved carbon dioxide drives photosynthesis in aquatic plants:

6\mathrm{CO}_2 + 12\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\mathrm{H}_2\mathrm{O} + 6\mathrm{O}_2

It also helps form shells in marine animals:

\mathrm{CaCO}_3 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2 \rightarrow \mathrm{Ca}(\mathrm{HCO}_3)_2

11 REACTIONS OF COLD WATER WITH METALS – Potassium, Sodium, Calcium

Potassium, sodium, and calcium react with cold water and give hydrogen plus an alkali solution that turns red litmus blue. Their behaviour differs in detail, as the table shows.

Vigour decreases from potassium to calcium. Magnesium shows no cold-water reaction but reacts slowly with boiling water, forming MgO and hydrogen, which pops with a lighted taper. Aluminium needs steam at high temperature; its oxide coating breaks and gives 2\mathrm{Al} + 3\mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Al}_2\mathrm{O}_3 + 3\mathrm{H}_2. Zinc in the red-hot state gives \mathrm{Zn} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ZnO} + \mathrm{H}_2. Iron in the red-hot state gives a reversible reaction:

3\mathrm{Fe} + 4\mathrm{H}_2\mathrm{O} \rightleftharpoons \mathrm{Fe}_3\mathrm{O}_4 + 4\mathrm{H}_2

The reactivity series runs K, Na, Ca, Mg, Al, Zn, Fe, Pb, H, Cu, Hg, Ag, Pt, Au. A mnemonic for the order: Please Send Cats Monkeys And Zebras Into Lovely Happy Cages — Hungry Goats Prefer Apples.

Water also reacts with non-metals and oxides. Red-hot coke gives water gas: \mathrm{C} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{CO} + \mathrm{H}_2. Chlorine gives chlorine water: \mathrm{Cl}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{HCl} + \mathrm{HClO}. Non-metallic oxides give acids: \mathrm{SO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{SO}_3, \mathrm{SO}_3 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{SO}_4, \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{H}_2\mathrm{CO}_3, and 2\mathrm{NO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{HNO}_2 + \mathrm{HNO}_3.

Metallic oxides give bases: \mathrm{K}_2\mathrm{O} + \mathrm{H}_2\mathrm{O} \rightarrow 2\mathrm{KOH} and \mathrm{CaO} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2.

12 E. TESTS FOR WATER

Physical tests fix the boiling point at 100°C and the freezing point at 0°C, both at 760 mm Hg. Chemical tests use anhydrous salts that change colour when water is added.

Suppose you have an unknown white powder. Add a few drops of water. If it turns blue, water is present, since white anhydrous copper sulphate hydrates to blue copper sulphate.

21 SATURATED SOLUTION

Three terms define a solution. The solvent is the dissolving medium, generally water. The solute is the substance that dissolves. A solution is a homogeneous mixture whose composition can change with the relative amounts of its components.

A dilute solution holds a relatively small amount of solute in a given mass. A concentrated solution holds a relatively large amount. A true solution is clear, transparent, and homogeneous. Its particles are molecular in size, pass through filter paper, cannot be seen under a microscope, and do not settle. The solute is recovered by physical means, so a true solution is a mixture, not a compound.

An unsaturated solution can dissolve more solute at a given temperature. A saturated solution cannot. A supersaturated solution holds more solute at a given temperature than a saturated one. It is prepared by cooling a hot saturated solution. One exception: sodium sulphate decahydrate \mathrm{Na}_2\mathrm{SO}_4\cdot 10\mathrm{H}_2\mathrm{O} can cool without crystallising, provided no undissolved solid is present.

A saturated solution turns unsaturated by heating it slowly or by adding more solvent. Water is not the only solvent. Benzene dissolves rubber, turpentine oil dissolves paints and paraffin wax, carbon disulphide dissolves sulphur and phosphorus, petrol dissolves grease and rubber, acetone dissolves cellulose acetate, alcohol dissolves iodine and naphthalene, oxalic acid dissolves rust, and methylated spirit dissolves chlorophyll. These find use in dry cleaning, perfumes, chlorophyll extraction, and tincture of iodine.

23 DEFINITION – Solubility

Solubility at a given temperature is the maximum grams of solute that will saturate 100 g of solvent at that temperature. The formula is:

\text{Solubility} = \frac{\text{weight of solute (g)} \times 100}{\text{weight of solvent (g)}}

or, using the solution weight:

\frac{\text{weight of solute}}{\text{weight of solution} - \text{weight of solute}} \times 100

To find the solubility of KCl, prepare a saturated solution in about 100 ml of distilled water. Weigh a dry evaporating dish (M g), add the saturated solution and reweigh (M₁ g), then heat to dryness and reweigh (M₂ g). Solubility at t°C equals \frac{M_2 - M}{M_1 - M_2} \times 100.

Here is a worked pattern with original numbers. Take 12 g of potassium nitrate that saturates 50 g of water at 40°C. Solubility = \frac{12}{50} \times 100 = 24 g per 100 g water. Now cool a hot saturated solution holding 60 g of KNO₃ in 100 g of water at 90°C down to 30°C, where only 20 g stays dissolved. Cooling deposits 60 - 20 = 40 g of crystals.

A solubility curve plots solubility against temperature. It lets you read the solubility at any temperature, compare two solutes, and calculate how much crystallises when a hot saturated solution cools. Pressure and temperature effects on gas solubility connect directly to the study of gas laws.

Temperature affects solids differently. Solubility increases with temperature for KNO₃, KClO₃, NaNO₃, CuSO₄, and NH₄Cl. It increases slightly for NaCl, KCl, and Ca(OH)₂ below 70°C. It decreases for CaSO₄ and Ca(OH)₂ above 70°C. \mathrm{Na}_2\mathrm{SO}_4\cdot 10\mathrm{H}_2\mathrm{O} shows a sharp break at 36°C, hydrous below and anhydrous above. KNO₃ has the highest solubility at 100°C. Solid solubility is independent of pressure.

Gases behave differently. Increased pressure raises gas solubility — a soda bottle holds CO₂ under pressure, and gas bubbles out when the bottle opens. Increased temperature lowers gas solubility, so boiling water loses its taste as dissolved gases escape.

33 Crystals –

Crystals are homogeneous solids arranged symmetrically, with sharp edges meeting at definite angles and a regular shape. Water of crystallisation is the definite number of water molecules that enter loose chemical combination when a substance crystallises from a hot saturated solution.

Hydrated substances carry this water: washing soda \mathrm{Na}_2\mathrm{CO}_3\cdot \mathrm{H}_2\mathrm{O}, gypsum \mathrm{CaSO}_4\cdot 2\mathrm{H}_2\mathrm{O}, blue vitriol \mathrm{CuSO}_4\cdot 5\mathrm{H}_2\mathrm{O}, Epsom salt \mathrm{MgSO}_4\cdot 7\mathrm{H}_2\mathrm{O}, green vitriol \mathrm{FeSO}_4\cdot 7\mathrm{H}_2\mathrm{O}, and Glauber’s salt \mathrm{Na}_2\mathrm{SO}_4\cdot 10\mathrm{H}_2\mathrm{O}. Anhydrous substances such as KNO₃, KCl, and K₂SO₄ carry none.

Drying or desiccating agents remove moisture from other substances. Most hygroscopic substances serve this role: fused CaCl₂, fused P₂O₅, concentrated H₂SO₄, and quicklime.

43 The Term – Water pollution

Pollution is an undesirable change in the natural environment brought about by chemical, physical, or biological factors. Pollutants are foreign substances introduced in quantities that harm human health and the environment. Pollutants may be chemical (gases, heavy metals, compounds, wastes), physical (heat, noise, radiation), or biological (micro-organisms).

Pollutants also classify by degradation. Non-biodegradable ones like plastics, glass, heavy metal compounds, and pesticides do not break down; biodegradable ones like sewage do. Primary pollutants such as CO, SO₂, and NO come directly from a source, while secondary pollutants like CO₂, SO₃, and NO₂ form from them.

Water pollution is an undesirable change in the chemical, physical, and biological conditions of water caused by foreign substances. The causes and their impacts are below.

Control measures for offshore drilling include a full environmental assessment before drilling, biodegradable drilling fluids with low aquatic toxicity, removing oil by skimming or filtering, dispersing oil into small droplets, and removing oil clumps with coagulating agents.

46 B. TREATMENT OF WASTE WATER – Basic Water Treatment Plant

A sewer lateral pipe collects sewage from a household. A larger sewer main pipe on the main street carries it, and the waste water then goes to a water treatment plant. Six processes clean it.

1. Screen — removes large wastes such as leaves, rocks, and twigs; algal blooms may be treated with KMnO₄ solution.
2. Coagulation — coagulants such as alum settle suspended impurities before filtration.
3. Sedimentation — solid suspended particles settle in the tank.
4. Biological treatment — water-borne microbes metabolise organic matter; aerobic in the presence of oxygen, anaerobic without.
5. Filtration — sand and gravel filters remove remaining pollutants.
6. Disinfection — liquid chlorine, ozone, or bleaching powder kills remaining pathogens.

48 C. INDUSTRIAL WASTE WATER TREATMENT USING VARIOUS PROCESSES

Industrial effluent treatment uses physical processes (screening, sedimentation, flotation, filtration), chemical processes (coagulation, disinfection, precipitation), and biological processes (anaerobic and aerobic treatment).

Modified methods handle specific effluents. An oil-water separator cleans refinery waste: oil collects at the top, cleansed water in the middle, and sludge at the bottom. Acids and alkalies are neutralised under controlled conditions. Solvents, paints, and pesticides are treated by distillation and adsorption. Biodegradable organics from food and agriculture are handled by the activated sludge process and biological trickling filters.

What to practise from Water Class 9 ICSE

Water class 9 ICSE chemistry closes the chapter with an equation worksheet of 20 reactions to complete and balance, then two review blocks. Practise them in this order.

• Equation Worksheet: 20 reactions to complete and balance
• Questions: 15 questions, including past board questions from 1984 to 2010
• Additional Questions: 65 questions

Past board questions repeat certain topics year after year.

Water Class 9 ICSE frequently asked questions

Why is water class 9 ICSE considered a compound and not an element?

Cavendish synthesised water by igniting two volumes of hydrogen with one volume of oxygen, and Lavoisier confirmed the molecule has two hydrogen atoms and one oxygen atom. Water has a fixed composition by weight, its properties differ from hydrogen and oxygen, and its components separate only by chemical means such as electrolysis.

What is the difference between a saturated and a supersaturated solution?

A saturated solution holds the maximum solute it can dissolve at a given temperature, so it cannot take more. A supersaturated solution holds more solute than a saturated one at that temperature, prepared by cooling a hot saturated solution.

How do you test for water using anhydrous copper sulphate?

Add the suspected water to white anhydrous copper sulphate. It turns blue as the hydrate \mathrm{CuSO}_4\cdot 5\mathrm{H}_2\mathrm{O} forms. Cobalt chloride is the second test: it changes from blue to pink.

Why does dissolved air in water contain more oxygen than ordinary air?

Oxygen is more soluble in water than nitrogen. Boiled-off air holds 30-35% oxygen, while ordinary air holds only about 21%.

How do household detergents cause eutrophication?

Detergents contain phosphates, mainly sodium triphosphate. Added nutrients cause algal bloom, which reduces oxygen, light, and heat penetration and deoxygenates the water, killing marine organisms.

Reference: ICSE Class 9 Chemistry textbook material, chapter 6: Water Water Pollution — CISCE-aligned.

More from this book

• Chapter 1: Matter and Its Composition Law of Conservation of Mass
• Chapter 3: Elements Compounds and Mixtures