ChemistryICSE Class 8 · Chapter 8

Water ICSE Class 8: Solutions, Water of Crystallisation, Metals and Water, Hard and Soft Water

Updated 15 October 2026 · Fact-checked

ICSE Class 8 Water explains why water is the universal solvent, how salts dissolve, and the differences between solutions, suspensions and colloids. It covers unsaturated, saturated and supersaturated solutions, water of crystallisation, hygroscopic substances, how metals react with cold water, hot water and steam, and how hard water is softened.

What Water is about

Water covers about three-quarters of the Earth's surface and makes up most of your body. In chemistry it matters because it dissolves so many substances and reacts with many metals.

The CISCE Class VIII curriculum asks you to learn:

the dissolution of salts in water and why water is called the universal solvent;
solutions, suspensions and colloids;
unsaturated, saturated and supersaturated solutions;
water of crystallisation, hydrated and anhydrous substances, and hygroscopic substances;
the reactivity of metals with cold water, hot water and steam, with the products formed;
hard and soft water, the disadvantages of hard water, and softening by boiling or by washing soda.

A few words to fix first: the substance that dissolves is the solute, the liquid it dissolves in is the solvent, and the uniform mixture formed is the solution. Solute + solvent = solution.

Weightage: CISCE publishes no question paper, marking scheme or chapter weightage for Class VIII Chemistry: your school sets and marks the exam. Water sits in the Theme: Water theme of the CISCE Curriculum for Upper Primary Classes (VI-VIII).Source: CISCE Curriculum for Upper Primary Classes (VI-VIII), RDCD 2016

Key concepts

Dissolution and the universal solvent

Dissolution is the process in which a solute (such as a salt) spreads out evenly in a solvent to form a solution.

What happens when common salt dissolves in water:

The salt is made of tiny charged particles held together in a crystal.
Water particles attract these particles and pull them away from the crystal, one layer at a time.
The separated particles spread into the spaces between the water particles and move about with them.
The crystal slowly disappears, and the salt cannot be seen, but it is still there (the solution tastes salty, and evaporation gets the salt back).

Dissolving is faster when you stir, heat the water, or use powdered solute (more surface).

Universal solvent: water dissolves a larger number of substances than any other liquid, such as salts, sugar, many acids and alkalis, and gases such as oxygen and carbon dioxide. So it is called the universal solvent. It does not dissolve everything: oil, wax, sand and chalk do not dissolve in water.

Why it matters: dissolved oxygen lets fish breathe; dissolved minerals reach plants through roots; your blood carries dissolved food to cells.

Worked example

A spoon of salt is stirred into a glass of water and seems to disappear. Has the volume of water increased by the volume of the salt? Explain.

Try it first, then show the solutionSolution

Not by the full volume of the salt.
The salt particles move into the spaces between water particles, so the volume rises only slightly.

Answer: No: the salt particles fit into the spaces between water particles.

Solutions, suspensions and colloids

Property	True solution	Colloid	Suspension
Particle size	smallest (below about 1 nm)	in between (about 1 nm to 100 nm)	largest (above about 100 nm)
Looks	clear, transparent	translucent or cloudy, looks uniform	cloudy or opaque, not uniform
Type of mixture	homogeneous	appears homogeneous but is heterogeneous	heterogeneous
Settles on standing?	no	no	yes
Passes through filter paper?	yes	yes	no, it can be filtered
Scatters a beam of light?	no	yes (Tyndall effect)	yes, or blocks it
Examples	salt or sugar in water, copper sulphate solution	milk, ink, blood, starch in water, fog, smoke	chalk in water, muddy water, sand in water

(1 nm is one billionth of a metre.)

The Tyndall effect: a beam of light becomes visible as it passes through a colloid because the particles scatter the light, like sunbeams through a dusty room or headlights in fog.

Worked example

Classify: (i) milk (ii) muddy water (iii) sugar in water. Which one can be separated by filtration?

Try it first, then show the solutionSolution

Milk is a colloid; muddy water is a suspension; sugar in water is a true solution.
Only the suspension (muddy water) has particles large enough to be held back by filter paper.

Answer: Colloid, suspension, solution; muddy water can be filtered.

Unsaturated, saturated and supersaturated solutions

Unsaturated solution: can dissolve more solute at that temperature.
Saturated solution: can dissolve no more solute at that temperature; extra solute stays undissolved at the bottom.
Supersaturated solution: contains more solute than a saturated solution at that temperature. It is made by preparing a saturated solution at a high temperature and cooling it slowly and carefully without disturbing it. It is unstable: shaking it or dropping in a small crystal makes the extra solute crystallise out at once.

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.

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

For most solids, solubility increases with temperature; so a hot saturated solution becomes unsaturated when heated further and deposits crystals when cooled. Gases dissolve less in warm water.

Tests: add a little more solute. It dissolves → unsaturated. It does not dissolve → saturated. Crystals suddenly form → supersaturated.

Worked example

20 g of a salt makes a saturated solution with 50 g of water at 30 °C. Find its solubility at 30 °C.

Try it first, then show the solutionSolution

Solubility = (mass of solute ÷ mass of water) × 100.
= (20 ÷ 50) × 100 = 40 g per 100 g of water.

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

Water of crystallisation; hydrated, anhydrous and hygroscopic

Water of crystallisation is the fixed number of water molecules that are chemically combined in one formula unit of a crystalline salt. It gives the crystal its shape and often its colour.

Hydrated substance: contains water of crystallisation, e.g. blue copper sulphate crystals, $C u S O_{4} \cdot 5 H_{2} O$ .
Anhydrous substance: has no water of crystallisation, e.g. white anhydrous copper sulphate, $C u S O_{4}$ .

Common name	Formula
Blue vitriol (copper sulphate)	CuSO₄·5H₂O
Green vitriol (iron(II) sulphate)	FeSO₄·7H₂O
White vitriol (zinc sulphate)	ZnSO₄·7H₂O
Washing soda (sodium carbonate)	Na₂CO₃·10H₂O
Epsom salt (magnesium sulphate)	MgSO₄·7H₂O
Gypsum (calcium sulphate)	CaSO₄·2H₂O

On heating, blue copper sulphate loses its water and turns white:
$C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$
Adding a few drops of water turns the white powder blue again (and it gets warm). So anhydrous copper sulphate is used to test for water.

Hygroscopic substances absorb moisture from the air without dissolving in it. Examples: concentrated sulphuric acid, quicklime (calcium oxide), silica gel. They are used as drying agents, for example the silica gel sachets in shoe boxes and medicine bottles.

Worked example

50 g of blue copper sulphate crystals are heated until no further change. 250 g of the crystals contain 90 g of water. Find the mass of white powder left and the colour change.

Try it first, then show the solutionSolution

Water in 50 g = 50 × 90/250 = 18 g.
Anhydrous copper sulphate left = 50 − 18 = 32 g; blue turns white.

Answer: 32 g; blue to white

Metals with cold water, hot water and steam

How a metal reacts with water depends on its place in the reactivity series. Metals above hydrogen give hydrogen; with water they give the hydroxide, with steam the oxide.

Metal	Reacts with	Equation	Notes
Potassium	cold water, violently	$2 K + 2 H_{2} O \to 2 K O H + H_{2}$	hydrogen catches fire (lilac flame)
Sodium	cold water, vigorously	$2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$	darts about on the water, melts into a ball
Calcium	cold water, less vigorously	$C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$	sinks; solution turns milky
Magnesium	hot water (very slowly with cold)	$M g + 2 H_{2} O \to M g (O H)_{2} + H_{2}$
Magnesium	steam, burns brightly	$M g + H_{2} O \to M g O + H_{2}$	white oxide
Aluminium	steam	$2 A l + 3 H_{2} O \to A l_{2} O_{3} + 3 H_{2}$	oxide layer slows it
Zinc	steam	$Z n + H_{2} O \to Z n O + H_{2}$	oxide is yellow when hot, white when cold
Iron	red-hot iron with steam, reversible	$3 F e + 4 H_{2} O ⇌ F e_{3} O_{4} + 4 H_{2}$
Lead, copper, silver, gold	no reaction		below hydrogen or too unreactive

Sodium and potassium are stored under kerosene because they react with water and with moist air.

Worked example

Name one metal that reacts with cold water, one that needs steam, and one that does not react even with steam. Write the equation for the second.

Try it first, then show the solutionSolution

Cold water: sodium. Steam: zinc. No reaction: copper.
$Z n + H_{2} O \to Z n O + H_{2}$

Answer: Sodium; zinc ( $Z n + H_{2} O \to Z n O + H_{2}$ ); copper

Hard and soft water

Soft water forms lather easily with soap. Hard water does not form lather easily; it forms a white scum instead.

Hardness is caused by dissolved calcium and magnesium salts:

Temporary hardness: due to hydrogencarbonates (bicarbonates) of calcium and magnesium. Removed by boiling.
Permanent hardness: due to chlorides and sulphates of calcium and magnesium. Not removed by boiling; removed by washing soda.

Disadvantages of hard water:

Wastes soap: soap first reacts with calcium and magnesium salts to form scum, and only then lathers.
Scale (fur) forms in kettles, geysers and boilers: it wastes fuel because it conducts heat poorly, and it can block pipes or even make boilers burst.
Clothes washed in it look dull and feel rough; it is unsuitable for dyeing.
Food such as pulses takes longer to cook.

Softening by boiling (temporary hardness):
$C a (H C O_{3})_{2} boil C a C O_{3} + H_{2} O + C O_{2}$
The insoluble calcium carbonate settles and is filtered off.

Softening with washing soda (both kinds of hardness):
$C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$
$M g S O_{4} + N a_{2} C O_{3} \to M g C O_{3} + N a_{2} S O_{4}$
Calcium and magnesium are removed as insoluble carbonates; the sodium salts left do not cause hardness.

Worked example

A sample of water lathers well after boiling and filtering. What kind of hardness did it have? Write the equation for the change during boiling.

Try it first, then show the solutionSolution

Hardness removed by boiling is temporary hardness, caused by calcium (or magnesium) hydrogencarbonate.
$C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$

Answer: Temporary hardness; $C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$

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.

Scale in a kettle

Suppose your family's electric kettle has a white crust inside after a few months, and it now takes longer to boil water.

What is the crust, where did it come from, and why does the kettle take longer?

Try it first, then show the solutionSolution

The tap water has temporary hardness (calcium hydrogencarbonate). Boiling changes it to insoluble calcium carbonate, which settles as scale.
Scale conducts heat poorly, so more heat and time are needed to boil the water.

Answer: Calcium carbonate scale from temporary hardness; it insulates the heating element.

Memory hook: Hard water + heat = scale, and scale wastes fuel.

Supersaturated solution

Imagine you make very thick sugar syrup in hot water, let it cool undisturbed, and then drop in one sugar crystal: crystals suddenly grow all through the jar.

What kind of solution was the cooled syrup, and why did the crystal start the change?

Try it first, then show the solutionSolution

When cooled carefully, the syrup held more sugar than a saturated solution can at room temperature: a supersaturated solution.
It is unstable; the added crystal gives the extra sugar a surface to build on, so it crystallises out.

Answer: Supersaturated; the seed crystal triggers crystallisation.

Memory hook: Super = over-full and unstable.

Hygroscopic drying agent

Suppose a new pair of shoes comes with a small paper sachet marked 'silica gel, do not eat'.

What does the sachet do, and what is this property called?

Try it first, then show the solutionSolution

Silica gel absorbs moisture from the air in the box.
It absorbs water without dissolving, so it is hygroscopic; it keeps the shoes dry and free of mould.

Answer: Absorbs moisture; it is hygroscopic.

Memory hook: Hygro = moisture-grabbing.

Solubility and cooling

Suppose salt Y has a solubility of 110 g at 60 °C and 32 g at 20 °C, and a saturated solution in 200 g of water at 60 °C is cooled to 20 °C.

What mass of crystals separates?

Try it first, then show the solutionSolution

Per 100 g of water: 110 − 32 = 78 g comes out.
For 200 g of water: 2 × 78 = 156 g.

Answer: 156 g of crystals

Memory hook: Difference in solubility × (water ÷ 100).

Topics in this chapter

Water formula sheet

Solubility: $solubility = \frac{mass of solute}{mass of water} \times 100$; Grams per 100 g of water at a stated temperature, for a saturated solution.
Heating blue vitriol: $C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$; Blue to white; add water to turn it blue again.
Sodium with cold water: $2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$; Cold water gives the hydroxide and hydrogen.
Magnesium with steam: $M g + H_{2} O \to M g O + H_{2}$; Steam gives the oxide and hydrogen.
Removing temporary hardness: $C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$; By boiling.
Removing permanent hardness: $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$; By washing soda.

Water: diagrams to practise

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

Magnesium with steam

Draw the arrangement to show that magnesium reacts with steam and gives hydrogen.

Draw

A boiling tube held sideways with wet sand (or glass wool soaked in water) at the closed end.
Magnesium ribbon in the middle of the tube.
Burner flames under both the magnesium and the wet sand.
A delivery tube leading to a gas jar inverted over water.

Label

Wet sand (source of steam)
Magnesium ribbon
Heat
Delivery tube
Hydrogen collected over water

Watch out: Heat the metal first, then the wet sand so steam passes over hot metal. Write $M g + H_{2} O \to M g O + H_{2}$ beneath.

Tyndall effect

Show how a beam of light can tell a colloid from a true solution.

Draw

Two beakers side by side: one with salt solution, one with dilute milk or starch solution.
A torch beam shining through both from the side.
The path of light visible in the colloid, invisible in the solution.

Label

Torch
True solution: beam not seen
Colloid: beam seen (Tyndall effect)

Watch out: Draw the beam as a bright band only in the colloid.

How the school exam asks Water

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

Question type	How it is asked	Method
Solution, colloid or suspension	Classify mixtures; give differences in particle size, settling, filtration and light.	Use the comparison table: size first, then the tests.
Saturation and solubility sums	Find solubility; decide whether a solution is saturated; find the mass of crystals on cooling.	Always scale to 100 g of water.
Water of crystallisation	Define; give formulae of hydrated salts; describe heating blue copper sulphate.	Hydrated = with water; anhydrous = without; blue ↔ white.
Metals and water	Complete equations for Na, Ca, Mg, Zn, Fe with water or steam; arrange metals by reactivity.	Cold water → hydroxide + H₂; steam → oxide + H₂.
Hard water	Causes, disadvantages, and softening equations.	Bicarbonates → boil; chlorides and sulphates → washing soda.

Tricks to remember Water

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

Size order S-C-S
Solution Small, Colloid Centre, Suspension Settles.
Use it when: Comparing the three mixtures.
Cold gives hydroxide, steam gives oxide
Steam is hotter and 'drier', so the metal ends up as the oxide; water at low temperature leaves the hydroxide.
Use it when: Writing metal and water equations.
Boil the Bi
Bicarbonate hardness is removed by Boiling; for the rest, use washing Soda.
Use it when: Temporary versus permanent hardness.
Copper sulphate traffic light
Blue means water is there; white means it has gone.
Use it when: Hydrated and anhydrous copper sulphate; test for water.
Per hundred
Solubility always talks about 100 g of water.
Use it when: Solubility calculations.

Water: exam-level solved questions

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

Example 1: Colloid · 1 mark · MCQExam level

Which of these shows the Tyndall effect?
(a) salt solution
(b) sugar solution
(c) milk diluted with water
(d) copper sulphate solution

Try it first, then show the solutionSolution

Only colloids scatter a beam of light.
Diluted milk is a colloid; the others are true solutions.

Answer: (c)

Example 2: Hydrated and anhydrous · 2 marks · Short answerExam level

Define water of crystallisation. Give the formula of washing soda and of green vitriol.

Try it first, then show the solutionSolution

Water of crystallisation is the fixed number of water molecules chemically combined in one formula unit of a crystalline salt.
Washing soda: Na₂CO₃·10H₂O. Green vitriol: FeSO₄·7H₂O.

Answer: As above

Example 3: Is it saturated? · 3 marks · Short answerHarder exam level

The solubility of salt Z at 40 °C is 40 g per 100 g of water. A student dissolves 15 g of Z in 50 g of water at 40 °C. Is the solution saturated? If not, how much more Z will dissolve?

Try it first, then show the solutionSolution

50 g of water can hold 40 × 50/100 = 20 g of Z at 40 °C.
Only 15 g is dissolved, so the solution is unsaturated.
More that can dissolve = 20 − 15 = 5 g.

Answer: Unsaturated; 5 g more

Example 4: Metals and water · 4 marks · Long answerHarder exam level

Write balanced equations for: (i) calcium with cold water (ii) magnesium with steam (iii) iron with steam. Name the gas formed in each.

Try it first, then show the solutionSolution

(i) $C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$
(ii) $M g + H_{2} O \to M g O + H_{2}$
(iii) $3 F e + 4 H_{2} O ⇌ F e_{3} O_{4} + 4 H_{2}$
The gas in each case is hydrogen.

Answer: As above; hydrogen

Example 5: Hard water · 4 marks · Long answerHarder exam level

Distinguish between temporary and permanent hardness and give one method of removing each, with an equation.

Try it first, then show the solutionSolution

Temporary hardness is caused by hydrogencarbonates of calcium and magnesium; permanent hardness by their chlorides and sulphates.
Temporary: boiling: $C a (H C O_{3})_{2} \to C a C O_{3} + H_{2} O + C O_{2}$ .
Permanent: washing soda: $C a S O_{4} + N a_{2} C O_{3} \to C a C O_{3} + N a_{2} S O_{4}$ .
The insoluble carbonate is filtered off.

Answer: As above

Example 6: Percentage of water of crystallisation · 3 marks · Short answerHOTS / competency

250 g of blue vitriol contain 90 g of water of crystallisation. (i) Find the percentage of water in it. (ii) What mass of anhydrous salt is left on heating 25 g?

Try it first, then show the solutionSolution

(i) Percentage = 90/250 × 100 = 36%.
(ii) Water lost from 25 g = 36% of 25 = 9 g.
Anhydrous salt = 25 − 9 = 16 g.

Answer: (i) 36% (ii) 16 g

Example 7: Identify the metals · 4 marks · Long answerHOTS / competency

Metal A reacts violently with cold water and the gas catches fire. Metal B does not react with cold water but burns brightly in steam, leaving a white solid. Metal C does not react even with steam. Suggest A, B and C, arrange them by reactivity, and write the equation for B.

Try it first, then show the solutionSolution

A: potassium (or sodium), reacting violently with cold water.
B: magnesium, which burns in steam to give white magnesium oxide.
C: copper (below hydrogen).
Order A > B > C; $M g + H_{2} O \to M g O + H_{2}$ .

Answer: A potassium, B magnesium, C copper; A > B > C

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 8-mark question split into 2 + 3 + 3 marks.

A school water project

Suppose your class collects three water samples: P from a deep well, Q from rain, R from a river after a storm. P gives very little lather with soap; after boiling and filtering it lathers well. Q lathers at once. R is brown and cloudy, and on standing a layer of mud settles.

(i) · 2 marks
Which sample is soft? Which has temporary hardness? Give reasons.
Answer
Answer: Q soft; P temporarily hard
Q lathers easily, so it is soft (rain water has almost no dissolved salts). P lathers only after boiling, so its hardness was temporary (hydrogencarbonates).
(ii) · 3 marks
Is R a solution, colloid or suspension? Give two reasons, and how can it be cleared?
Answer
Answer: Suspension; settles and can be filtered
R is a suspension: it is cloudy and not uniform, and its particles settle on standing. It can be cleared by letting the mud settle and filtering (or decanting).
(iii) · 3 marks
If P still did not lather after boiling, what would you add, and why? Write one equation.
Answer
Answer: Washing soda; permanent hardness
Hardness that boiling cannot remove is permanent (chlorides or sulphates). Washing soda precipitates calcium as carbonate: $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$ .

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.

Define a saturated solution. How can you show that a given solution is saturated? (2 marks)

Step	Marks
A solution that cannot dissolve any more solute at that temperature	1
Add a little more solute and stir at the same temperature; it does not dissolve	1

What happens when blue copper sulphate crystals are heated, and when water is added to the residue? Write the equation. (3 marks)

Step	Marks
The blue crystals turn into a white powder as water of crystallisation is lost	1
$C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$	1
On adding water, the white powder turns blue again and becomes warm	1

Give four disadvantages of using hard water. (4 marks)

Step	Marks
Wastes soap, forming scum	1
Forms scale in kettles and boilers, wasting fuel	1
Boilers or pipes may block or burst	1
Clothes look dull; unsuitable for dyeing; pulses cook slowly	1

Where students lose marks in Water

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

Mistake: Writing the hydroxide for metal with steam
Steam gives the metal oxide, not the hydroxide.
Wrong working
Magnesium with steam gives $M g (O H)_{2}$
Right working
$M g + H_{2} O \to M g O + H_{2}$
Do this: Cold or hot water: hydroxide. Steam: oxide.
Usually costs: 1 mark.
Mistake: Forgetting the 100 g base in solubility
Solubility is per 100 g of water, not per the mass given.
Wrong working
20 g in 50 g water, so solubility 20
Right working
20 × 100/50 = 40 g per 100 g of water
Do this: Always scale to 100 g of water and state the temperature.
Usually costs: 1-2 marks.
Mistake: Saying boiling removes all hardness
Boiling removes only temporary hardness (hydrogencarbonates).
Do this: Permanent hardness needs washing soda.
Usually costs: 1 mark.
Mistake: Calling milk a true solution
Its particles scatter light and are larger than those in a solution.
Do this: Milk, ink, blood, starch solution: colloids.
Usually costs: 1 mark.
Mistake: Mixing up hygroscopic and anhydrous
Anhydrous means without water of crystallisation; hygroscopic means it absorbs moisture from air without dissolving.
Do this: Learn one example of each: anhydrous CuSO₄; silica gel or quicklime.
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.

Q1 · 1 mark · MCQExam level
A solution that can dissolve more solute at the same temperature is:
(a) saturated
(b) unsaturated
(c) supersaturated
(d) a suspension
Hint
Room for more.
Answer
(b)
Q2 · 1 mark · Very short answerExam level
Name the substance used to test for water, and state the colour change.
Hint
Anhydrous...
Answer
Anhydrous copper sulphate; white to blue.
Q3 · 2 marks · Short answerExam level
Give two differences between a true solution and a suspension.
Hint
Settling, filtration, appearance.
Answer
Any two: a solution is transparent and homogeneous, a suspension cloudy and heterogeneous; a solution does not settle, a suspension does; a solution passes through filter paper, a suspension can be filtered.
Q4 · 2 marks · Short answerExam level
Give the formula of: (i) Epsom salt (ii) gypsum.
Hint
Sulphates with water.
Answer
(i) MgSO₄·7H₂O (ii) CaSO₄·2H₂O.
Q5 · 1 mark · Assertion-reasonHarder exam level
Assertion (A): A supersaturated solution deposits crystals when a small crystal of the solute is dropped in.
Reason (R): A supersaturated solution is unstable.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
Hint
Why does the crystal trigger it?
Answer
(c)
Q6 · 2 marks · Short answerHarder exam level
18 g of a salt saturates 60 g of water at 25 °C. (i) Find the solubility at 25 °C. (ii) If 20 g of the water now evaporates at 25 °C, what mass of crystals separates?
Hint
Scale to 100 g; then find what 40 g of water can hold.
Answer
(i) 18 × 100/60 = 30 g per 100 g of water. (ii) 40 g of water can hold 30 × 40/100 = 12 g, so 18 − 12 = 6 g of crystals separate.
Q7 · 2 marks · Short answerHarder exam level
The solubility of common salt at a certain temperature is 36 g. How much salt is needed to saturate 250 g of water at that temperature, and what is the mass of the saturated solution formed?
Hint
36 per 100; solution = solute + solvent.
Answer
Salt = 36 × 250/100 = 90 g; saturated solution = 250 + 90 = 340 g.
Q8 · 3 marks · Short answerHarder exam level
Write the equations for sodium with cold water and zinc with steam. Why is the product of the first alkaline but the second not?
Hint
Hydroxide versus oxide.
Answer
$2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$ ; $Z n + H_{2} O \to Z n O + H_{2}$ . Sodium hydroxide is a soluble base (an alkali), so the solution is alkaline; zinc oxide is insoluble (and amphoteric), so it gives no alkaline solution.
Q9 · 2 marks · Short answerHarder exam level
Write equations for the removal of hardness caused by (i) calcium chloride (ii) magnesium sulphate, using washing soda.
Hint
Carbonate precipitates.
Answer
(i) $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$ (ii) $M g S O_{4} + N a_{2} C O_{3} \to M g C O_{3} + N a_{2} S O_{4}$ .
Q10 · 3 marks · Short answerHarder exam level
Explain why water is called the universal solvent, and give two examples from daily life where its dissolving power is useful.
Hint
Many solutes.
Answer
It dissolves more substances than any other liquid. Examples: oxygen dissolved in water lets fish breathe; minerals dissolved in soil water reach plants; blood carries dissolved food; we wash away dirt and salts.
Q11 · 2 marks · Short answerHarder exam level
Why does the hydrogen catch fire when potassium is dropped into water, but not when calcium is? Use the reactivity series.
Hint
Heat given out.
Answer
Potassium is far more reactive than calcium; its reaction is so fast and gives out so much heat that the hydrogen formed catches fire. Calcium reacts less vigorously, so not enough heat is given out to ignite the hydrogen.
Q12 · 3 marks · Short answerHOTS / competency
74 g of a saturated solution of salt X at 50 °C is cooled to 10 °C. The solubility of X is 85 g at 50 °C and 25 g at 10 °C. Find the mass of water in the solution and the mass of crystals that form. Is the solution left at 10 °C saturated?
Hint
At 50 °C, 85 g of X and 100 g of water make 185 g of saturated solution.
Answer
Water = 74 × 100/185 = 40 g (and X = 34 g). At 10 °C, 40 g of water holds 25 × 40/100 = 10 g, so 34 − 10 = 24 g of crystals form. The solution left is saturated, because crystals are in contact with it.
Q13 · 3 marks · Short answerHOTS / competency
A student says: 'I boiled my hard water sample for ten minutes, but the soap still did not lather, so my kettle must be faulty.' Explain the real reason and suggest a fix with an equation.
Hint
Which hardness survives boiling?
Answer
The sample has permanent hardness (chlorides or sulphates of calcium or magnesium), which boiling does not remove. Add washing soda: $C a S O_{4} + N a_{2} C O_{3} \to C a C O_{3} + N a_{2} S O_{4}$ , then filter.

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.Why is water called the universal solvent?Show answer
It dissolves more substances than any other liquid.
2.Particle size of a colloid?Show answer
About 1 nm to 100 nm.
3.Which mixture settles on standing?Show answer
A suspension.
4.Formula of blue vitriol?Show answer
CuSO₄·5H₂O.
5.Name a hygroscopic substance.Show answer
Silica gel, quicklime or concentrated sulphuric acid.
6.Metal that reacts with steam but not cold water?Show answer
Zinc (or magnesium, aluminium, iron).
7.Cause of temporary hardness?Show answer
Hydrogencarbonates of calcium and magnesium.
8.Chemical used to remove permanent hardness?Show answer
Washing soda (sodium carbonate).

Best order to study it

Learn solute, solvent, solution and why water is the universal solvent.
Learn the solution, colloid and suspension table.
Practise saturation and solubility sums.
Learn water of crystallisation with the table of hydrated salts.
Learn the metals and water table with equations.
Finish with hard water: causes, disadvantages, softening equations.

Quick revision checklist

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

Can you explain what happens to the particles when salt dissolves?
Can you give three differences between a solution and a suspension?
Can you calculate solubility and crystals on cooling?
Can you write formulae of five hydrated salts?
Can you write the equations for Na, Ca, Mg, Zn and Fe with water or steam?
Can you write equations for removing both kinds of hardness?

Water: frequently asked questions

Is a colloid homogeneous or heterogeneous?

It looks homogeneous to the eye, but it is really heterogeneous, because its particles are big enough to scatter light (the Tyndall effect).

Why are sodium and potassium kept under kerosene?

They react quickly with water and with moisture and oxygen in the air. Kerosene does not react with them and keeps air and moisture away.

Is a hygroscopic substance the same as a deliquescent one?

No. A hygroscopic substance absorbs moisture without dissolving. A deliquescent substance, such as sodium hydroxide, absorbs so much moisture that it dissolves in it. The CISCE Class VIII curriculum names only hygroscopic substances.

Why does washing soda remove both kinds of hardness?

It reacts with all soluble calcium and magnesium salts (hydrogencarbonates, chlorides and sulphates), turning them into insoluble carbonates that can be filtered off.

Does hot water dissolve more gas?

No. Gases are less soluble in warm water, which is why bubbles appear in water as it is heated, long before it boils.

Based on: CISCE Curriculum for Upper Primary Classes (VI-VIII), Science: Chemistry (Class VIII); CISCE RDCD: Curriculum Pre-School to Class VIII