Water ICSE Class 9: Solutions, Solubility, Hydrated Salts and Hard Water

What Water is about

Weightage: CISCE publishes no chapter weightage for ICSE Chemistry. Water is topic 3 of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2028 (Class IX)

Key concepts

Solutions, colloids and suspensions

A solution is a homogeneous mixture of a solute dissolved in a solvent. Water dissolves so much because its molecules are polar and attract ions and other polar molecules.

Property	True solution	Colloid	Suspension
Particle size	smallest (below about 1 nm)	intermediate (about 1-1000 nm)	large (above about 1000 nm)
Appearance	clear, transparent	translucent, looks uniform	opaque, cloudy
Settles on standing?	no	no	yes
Passes through filter paper?	yes	yes	no (residue left)
Scatters a light beam (Tyndall effect)?	no	yes	yes (may be opaque)
Examples	salt or sugar in water, copper sulphate solution	milk, starch solution, ink, fog	chalk in water, muddy water, sand in water

A solution can be unsaturated (can dissolve more solute at that temperature), saturated (can dissolve no more at that temperature) or supersaturated (holds more solute than a saturated solution at that temperature; it is unstable, and a seed crystal or a shake makes the excess crystallise out).

Worked example

A beam of torch light is passed through (a) salt solution, (b) milk diluted with water. In which is the path of the beam visible, and why?

Try it first, then show the solutionSolution

1. Salt solution is a true solution: particles are too small to scatter light.
2. Diluted milk is a colloid: its particles scatter light (Tyndall effect).

Answer: Visible in diluted milk only.

Solubility and solubility curves

Solubility of a solute at a given temperature is the maximum mass in grams that dissolves in 100 g of water to form a saturated solution.

$\text{Solubility}=\frac{\text{mass of solute}}{\text{mass of water}}\times 100$

A solubility curve plots solubility (y-axis) against temperature (x-axis):

• Potassium nitrate: rises steeply as temperature rises.
• Sodium chloride: almost flat, rising only slightly.
• Calcium sulphate: rises slightly and then falls at higher temperature.

Uses: read the solubility at any temperature; find how much solute crystallises when a hot saturated solution is cooled (salts with steep curves, like potassium nitrate, are purified by crystallisation); choose the salt that is most soluble at a temperature.

Gases behave the opposite way: they are less soluble in warmer water, which is why aquatic life suffers in warm water.

Worked example

12 g of a salt saturates 30 g of water at 30 °C. Find its solubility at 30 °C.

Try it first, then show the solutionSolution

1. Solubility = (12 ÷ 30) × 100
2. = 40 g per 100 g water

Answer: 40 g per 100 g of water at 30 °C.

Water of crystallisation: hydrated and anhydrous substances

Water of crystallisation is the fixed number of water molecules that are chemically combined in a crystal of a salt and give it its crystal shape (and often colour).

Hydrated substances contain water of crystallisation:

• Blue vitriol, $CuS{O}_4\cdot 5H_{2}O$ (blue)
• Green vitriol, $FeS{O}_4\cdot 7H_{2}O$ (green)
• Washing soda, $N{a}_{2}C{O}_3\cdot 10H_{2}O$
• Glauber's salt, $N{a}_{2}S{O}_4\cdot 10H_{2}O$

Anhydrous substances crystallise without water: NaNO₃, KCl, PbCl₂, K₂SO₄.

On heating, a hydrate loses its water and its crystal shape (and colour):
$CuS{O}_4\cdot 5H_{2}O\to CuS{O}_4+5H_{2}O$ (blue → white). Adding water back turns white anhydrous copper sulphate blue again: a test for water.

Worked example

Calculate the mass of water lost when 24.95 g of blue vitriol is heated until it is anhydrous. (Cu = 63.5, S = 32, O = 16, H = 1)

Try it first, then show the solutionSolution

1. Mr of CuSO₄·5H₂O = 249.5; water = 90.
2. Water lost = 24.95 × 90/249.5 = 9.0 g.

Answer: 9.0 g (15.95 g of white CuSO₄ remains).

Efflorescence, deliquescence and hygroscopy

• Efflorescence: a hydrated crystal loses some or all of its water of crystallisation on exposure to dry air and turns to powder. Example: washing soda $N{a}_{2}C{O}_3\cdot 10H_{2}O\to N{a}_{2}C{O}_3\cdot H_{2}O+9H_{2}O$ (crystals become a white powder; mass decreases). Glauber's salt also effloresces.
• Deliquescence: a substance absorbs so much moisture from the air that it dissolves in it to form a solution. Examples: ferric chloride (FeCl₃), calcium chloride, magnesium chloride, sodium hydroxide. Mass increases; the solid turns liquid. (Common salt becomes damp in the rainy season because of magnesium and calcium chloride impurities.)
• Hygroscopy: a substance absorbs moisture from the air without dissolving in it (without changing its state). Examples: quicklime (CaO), concentrated sulphuric acid, silica gel. Mass increases.

Store deliquescent and hygroscopic substances in airtight bottles.

Worked example

28.6 g of washing soda crystals are left in dry air until they become the monohydrate. Find the loss in mass. (Na = 23, C = 12, O = 16, H = 1)

Try it first, then show the solutionSolution

1. $N{a}_{2}C{O}_3\cdot 10H_{2}O\to N{a}_{2}C{O}_3\cdot H_{2}O+9H_{2}O$
2. Mr: decahydrate 286; nine waters = 162.
3. Loss = 28.6 × 162/286 = 16.2 g.

Answer: 16.2 g lost (12.4 g of monohydrate remains).

Drying agents and dehydrating agents

• A drying agent removes moisture (free water) from a substance, usually a gas, without reacting with the substance being dried. Examples: quicklime (for ammonia), concentrated sulphuric acid (for neutral and acidic gases such as Cl₂, HCl), fused calcium chloride.
• A dehydrating agent removes the elements of water (hydrogen and oxygen in the ratio 2 : 1) from a compound, or its water of crystallisation. Example: concentrated sulphuric acid chars sugar: $C_{12}{H}_{22}{O}_{11}\to 12C+11H_{2}O$ (white sugar becomes a black, spongy mass of carbon), and turns blue vitriol white.

All dehydrating agents can dry, but not all drying agents can dehydrate (quicklime cannot char sugar).

Worked example

Why is concentrated sulphuric acid called both a drying agent and a dehydrating agent?

Try it first, then show the solutionSolution

1. It absorbs moisture from gases such as chlorine: drying.
2. It removes hydrogen and oxygen as water from sugar, leaving carbon: dehydrating.

Answer: It removes free water and also the elements of water.

Hard and soft water

Soft water forms lather easily with soap. Hard water does not lather easily; it forms a scum (insoluble curd) because calcium and magnesium ions react with soap:

$2C_{17}{H}_{35}COONa+CaC{l}_2\to (C_{17}{H}_{35}COO{)}_{2}Ca+2NaCl$

Causes: rainwater dissolves carbon dioxide and, flowing over rocks, dissolves calcium and magnesium salts.

	Temporary hardness	Permanent hardness
Cause	bicarbonates of Ca and Mg	chlorides and sulphates of Ca and Mg
Removed by boiling?	yes	no
Removed by washing soda?	yes	yes

Soft water: advantages: good lather, no scale in kettles and boilers, saves soap. Disadvantages: lacks minerals such as calcium needed for bones and teeth; can dissolve lead from old pipes.
Hard water: advantages: supplies calcium and magnesium, has a pleasant taste, does not dissolve lead. Disadvantages: wastes soap; forms scale (fur) in kettles and boilers, wasting fuel and risking bursting; unsuitable for dyeing and many industries.

Worked example

Why does boiling soften some hard water but not all?

Try it first, then show the solutionSolution

1. Boiling decomposes calcium bicarbonate into insoluble calcium carbonate.
2. Calcium chloride and sulphate are not changed by boiling, so they stay dissolved.

Answer: Boiling removes temporary (bicarbonate) hardness only.

Removing hardness and making water potable

By boiling (temporary hardness):
$Ca(HC{O}_3{)}_2\to CaC{O}_3+H_{2}O+C{O}_2$
$Mg(HC{O}_3{)}_2\to MgC{O}_3+H_{2}O+C{O}_2$
The insoluble carbonate is filtered off.

By washing soda (both kinds): calcium and magnesium are precipitated as carbonates.
$CaC{l}_2+N{a}_{2}C{O}_3\to CaC{O}_3+2NaCl$
$MgS{O}_4+N{a}_{2}C{O}_3\to MgC{O}_3+N{a}_{2}S{O}_4$
$Ca(HC{O}_3{)}_2+N{a}_{2}C{O}_3\to CaC{O}_3+2NaHC{O}_3$

Potable water is water that is safe to drink. Steps in purification:

• Alum (potash alum): added as a coagulant; it makes fine suspended clay particles clump together and settle (sedimentation).
• Filtration: through beds of sand and gravel to remove remaining suspended matter.
• Chlorination: chlorine (or bleaching powder) kills germs: $C{l}_2+H_{2}O\to HCl+HOCl$; the hypochlorous acid is the germicide.
• Defluoridation: removing excess fluoride, which causes dental and skeletal fluorosis, e.g. by the Nalgonda technique (alum and lime) or by passing water through activated alumina.

Diseases from polluted water: diarrhoea, typhoid, cholera, dysentery, hepatitis A and polio (from germs in sewage-contaminated water); cancer and organ damage (from toxic chemicals such as heavy metals and pesticides).

Worked example

Water from a village well has permanent hardness due to magnesium sulphate. Can boiling remove it? Write the equation for a method that can.

Try it first, then show the solutionSolution

1. Boiling does not affect sulphates: no.
2. Add washing soda: $MgS{O}_4+N{a}_{2}C{O}_3\to MgC{O}_3+N{a}_{2}S{O}_4$; filter off MgCO₃.

Answer: No; washing soda can.

Chemistry in real life: Water

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Topics in this chapter

• Hard and Soft Water: Causes, Removal of Hardness and Purifying Water
• Solutions, Solubility and Solubility Curves: Crystallisation Calculations

Water formula sheet

Solubility

$S=\frac{m_{solute}}{m_{water}}\times 100$

g per 100 g water at a stated temperature.

Heating blue vitriol

$CuS{O}_4\cdot 5H_{2}O\to CuS{O}_4+5H_{2}O$

Blue → white.

Efflorescence of washing soda

$N{a}_{2}C{O}_3\cdot 10H_{2}O\to N{a}_{2}C{O}_3\cdot H_{2}O+9H_{2}O$

Crystals → powder.

Boiling removes temporary hardness

$Ca(HC{O}_3{)}_2\to CaC{O}_3+H_{2}O+C{O}_2$

Also Mg(HCO₃)₂.

Washing soda removes permanent hardness

$CaC{l}_2+N{a}_{2}C{O}_3\to CaC{O}_3+2NaCl$

Also MgSO₄.

Chlorination

$C{l}_2+H_{2}O\to HCl+HOCl$

HOCl kills germs.

Water: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Water

The question patterns to expect, and the method that works for each.

Tricks to remember Water

Short memory aids. Say each one aloud once, then use it on a question the same day.

• E-D-H by mass

  Efflorescent Exits water (mass down); Deliquescent Drinks till it Dissolves; Hygroscopic Holds water but stays solid.

  Use it when: Three definitions.

• Vitriols by colour

  Blue vitriol = copper (5 waters); Green vitriol = iron (7 waters).

  Use it when: Hydrated examples.

• Temporary = bicarbonate = boil

  B for Bicarbonate and Boil; permanent needs the soda.

  Use it when: Hardness types.

• Curve shapes: 'Potassium Peaks, Salt Stays, Calcium Sulphate Sinks'

  KNO₃ steep rise, NaCl nearly flat, CaSO₄ falls at higher temperature.

  Use it when: Solubility curves.

Water: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 2 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Water

The slips examiners see most often, and what to write instead.

• ✗Mistake: Solubility per 100 g of solution

  Students divide by the mass of solution instead of the mass of water.

  Wrong working

  20 g in 60 g solution → solubility = 20/60 × 100 = 33.3 g.

  Right working

  Water = 40 g; solubility = 20/40 × 100 = 50 g per 100 g water.

  ✓Do this: Subtract the solute first; divide by water.

  Usually costs: 1-2 marks.

• ✗Mistake: Hygroscopic called deliquescent

  Both absorb moisture; the difference is whether the solid dissolves.

  Wrong working

  Quicklime is deliquescent.

  Right working

  Quicklime is hygroscopic: it absorbs moisture but stays solid.

  ✓Do this: Deliquescent = Dissolves.

  Usually costs: 1 mark.

• ✗Mistake: Boiling to remove permanent hardness

  Students forget which salts cause which hardness.

  Wrong working

  Boiling removes CaSO₄ hardness.

  Right working

  Only bicarbonates decompose on boiling; sulphates and chlorides need washing soda.

  ✓Do this: Bicarbonate = boil.

  Usually costs: 1 mark.

• ✗Mistake: Drying agent and dehydrating agent treated as the same

  Conc. H₂SO₄ is both, so students assume quicklime is too.

  Wrong working

  Quicklime chars sugar.

  Right working

  Only conc. H₂SO₄ removes the elements of water from sugar.

  ✓Do this: Drying removes moisture; dehydrating removes H and O as water.

  Usually costs: 1 mark.

• ✗Mistake: Wrong role of alum

  Alum is confused with a germicide.

  Wrong working

  Alum kills germs in water.

  Right working

  Alum coagulates suspended particles so they settle; chlorine kills germs.

  ✓Do this: Alum settles, chlorine sterilises.

  Usually costs: 1 mark.

Practice questions on Water

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · MCQExam level

   Which is a colloid?
   (a) sugar solution
   (b) chalk in water
   (c) milk
   (d) copper sulphate solution

   Hint

   Tyndall effect, does not settle.

   Answer

   (c)

2. Q2 · 1 mark · MCQExam level

   Temporary hardness of water is due to:
   (a) calcium chloride
   (b) magnesium sulphate
   (c) calcium bicarbonate
   (d) sodium chloride

   Hint

   Boil.

   Answer

   (c)

3. Q3 · 3 marks · Very short answerExam level

   Name: (a) a hydrated salt that is blue, (b) an anhydrous potassium salt, (c) a substance used as both drying and dehydrating agent.

   Hint

   Vitriol; KCl or K₂SO₄; acid.

   Answer

   (a) CuSO₄·5H₂O (b) KCl or K₂SO₄ (c) Concentrated sulphuric acid.

4. Q4 · 3 marks · Short answerHarder exam level

   Concentrated sulphuric acid is added to sugar. Write the equation, give two observations and name the property shown.

   Hint

   Charring.

   Answer

   $C_{12}{H}_{22}{O}_{11}\to 12C+11H_{2}O$. The white sugar turns black and swells into a spongy mass; much heat and steam are given out. Dehydrating property.

5. Q5 · 3 marks · Short answerHarder exam level

   A solution of a salt at 70 °C is cooled slowly without disturbance to 20 °C and no crystals appear, though the solubility at 20 °C is much lower. (a) What kind of solution is it now? (b) What happens if a crystal of the salt is dropped in? (c) Is the change exothermic?

   Hint

   Unstable.

   Answer

   (a) Supersaturated. (b) The excess salt crystallises rapidly until the solution is just saturated. (c) Yes: crystallisation releases heat, so the flask warms.

6. Q6 · 2 marks · Short answerExam level

   Give two advantages and two disadvantages of hard water.

   Hint

   Minerals; soap, scale.

   Answer

   Advantages: provides calcium for bones and teeth; does not dissolve lead from pipes (also better taste). Disadvantages: wastes soap (scum); forms scale in kettles and boilers.

7. Q7 · 2 marks · Short answerHarder exam level

   Explain why soap works poorly in sea water but a detergent works well. (Use your knowledge of hardness.)

   Hint

   Ca²⁺ and Mg²⁺.

   Answer

   Sea water has many calcium and magnesium ions, which react with soap to form insoluble scum, using it up. Detergents do not form insoluble calcium and magnesium salts, so they still lather.

8. Q8 · 2 marks · Short answerHarder exam level

   Why are fish found dead in a pond near a factory that discharges hot water, even though the water is not poisonous?

   Hint

   Gas solubility.

   Answer

   Oxygen, like other gases, is less soluble in warm water. The hot discharge lowers the dissolved oxygen, and the fish suffocate.

9. Q9 · 3 marks · Short answerHarder exam level

   A sample of water contains both calcium bicarbonate and calcium sulphate. A student boils it, filters it and finds it still does not lather. Explain, and suggest a treatment with an equation.

   Hint

   Two types of hardness.

   Answer

   Boiling removed the bicarbonate (temporary hardness), but calcium sulphate (permanent hardness) remains. Add washing soda: $CaS{O}_4+N{a}_{2}C{O}_3\to CaC{O}_3+N{a}_{2}S{O}_4$, then filter.

10. Q10 · 4 marks · Long answerHOTS / competency

    A student heats 12.47 g of hydrated copper(II) sulphate until the mass stops changing and finds 7.97 g of white powder left. Use this to show that the formula is CuSO₄·5H₂O. (Cu = 63.5, S = 32, O = 16, H = 1)

    Hint

    Compare the ratio of water to CuSO₄ by mass.

    Answer

    Water lost = 4.50 g. Mass ratio water : CuSO₄ = 4.50 : 7.97. For x waters: 18x : 159.5. So 18x/159.5 = 4.50/7.97, giving x = 4.50 × 159.5/(7.97 × 18) = 5.0. Formula: CuSO₄·5H₂O.

11. Q11 · 3 marks · Short answerHOTS / competency

    Two samples of water, P and Q, each lather poorly. P lathers well after boiling; Q lathers only after washing soda is added. A third sample R lathers well but tastes flat. (a) Identify the hardness in P and Q. (b) What is R likely to be, and why is it not ideal for drinking every day?

    Hint

    Soft water lacks minerals.

    Answer

    (a) P: temporary hardness (bicarbonates). Q: permanent hardness (chlorides or sulphates). (b) R is soft water (e.g. distilled or rainwater); it lacks dissolved calcium and other minerals the body needs.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Formula of green vitriol?Show answer

   FeSO₄·7H₂O.

2. 2.An efflorescent substance?Show answer

   Washing soda, Na₂CO₃·10H₂O.

3. 3.A deliquescent substance?Show answer

   Ferric chloride, FeCl₃.

4. 4.Two hygroscopic substances?Show answer

   Quicklime and concentrated sulphuric acid.

5. 5.Cause of permanent hardness?Show answer

   Chlorides and sulphates of calcium and magnesium.

6. 6.Role of alum?Show answer

   Coagulates suspended particles so they settle.

7. 7.Which salt's solubility falls at higher temperature?Show answer

   Calcium sulphate.

8. 8.Name two diseases spread by polluted water.Show answer

   Cholera and typhoid (also diarrhoea, dysentery, hepatitis A, polio).

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you define a supersaturated solution and say how to make one crystallise?
• ☐Can you calculate solubility and crystals formed on cooling?
• ☐Can you name four hydrated and four anhydrous substances with formulae?
• ☐Can you tell efflorescent, deliquescent and hygroscopic apart by mass change and appearance?
• ☐Can you write removal equations for both types of hardness?
• ☐Can you explain the role of alum, chlorine and defluoridation?

Water: frequently asked questions