Water
ICSE Class 9 · Chemistry · Chapter 3
What this chapter covers
- Water as a universal solvent
- Mixtures, solutions, colloids and suspensions
- Solutions of solids in water
- Water of crystallisation
- Efflorescence, deliquescence and hygroscopy
- Drying and dehydrating agents
- Soft water and hard water
- Removal of temporary and permanent hardness
- Making water potable
Key points
- Water dissolves more substances than any other common liquid because its molecules are polar; it is called the universal solvent.
- Solutions (particles under 1 nm), colloids (1–100 nm) and suspensions (over 100 nm) differ in clarity, settling, filtration and the Tyndall effect.
- A saturated solution can hold no more solute at that temperature; an unsaturated one can; a supersaturated one holds more than saturation and is unstable.
- Solubility is the maximum mass in grams dissolving in 100 g of water at a given temperature; for most solids it rises with temperature.
- Potassium nitrate's solubility curve rises steeply, sodium chloride's is nearly flat, and calcium sulphate's is very low and falls above about 40 °C.
- Water of crystallisation is the fixed number of water molecules combined in a salt's crystals: CuSO4 · 5H2O, FeSO4 · 7H2O, Na2CO3 · 10H2O, Na2SO4 · 10H2O; NaNO3, KCl, PbCl2, K2SO4 are anhydrous.
- Efflorescent substances lose water to air (washing soda); deliquescent ones absorb water and dissolve (ferric chloride); hygroscopic ones absorb water without changing state (quicklime, conc. sulphuric acid).
- A drying agent removes free moisture (quicklime); a dehydrating agent removes combined water (conc. sulphuric acid).
- Hard water does not lather with soap because of calcium and magnesium salts; temporary hardness (hydrogen carbonates) is removed by boiling, and both kinds by washing soda.
- Potable water is made by filtration, loading with alum, chlorination and, where needed, defluoridation; polluted water spreads diarrhoea, typhoid, cholera, dysentery, hepatitis A and polio, and toxic chemicals cause cancer and organ damage.
Questions answered
Why is water called a universal solvent? Name one substance that does not dissolve in it.
Water is called a universal solvent because it dissolves more substances than any other common liquid, owing to its polar molecules. Oil does not dissolve in water.
Give two differences between a colloid and a suspension.
Colloid particles are 1–100 nm and do not settle; suspension particles are larger than 100 nm and settle on standing. Colloid particles pass through filter paper; suspension particles do not.
How would you show that a given solution of potassium nitrate is unsaturated?
Add a small crystal of potassium nitrate at the same temperature; if it dissolves, the solution is unsaturated.
How is a supersaturated solution prepared, and why is it called unstable?
A hot saturated solution is cooled slowly without disturbance. It is unstable because a jerk or a seed crystal makes the extra solute crystallise at once.
Describe the shapes of the solubility curves of potassium nitrate, sodium chloride and calcium sulphate.
Potassium nitrate rises steeply; sodium chloride is almost flat, rising slightly; calcium sulphate stays very low, rising slightly to about 40 °C and then falling.
Define water of crystallisation. Give the formulae of green vitriol and Glauber's salt.
Water of crystallisation is the fixed number of water molecules chemically combined with one formula unit of a salt in its crystals. FeSO4 · 7H2O and Na2SO4 · 10H2O.
Name two anhydrous salts.
Potassium chloride and potassium sulphate (also sodium nitrate, lead chloride).
Distinguish between efflorescence and deliquescence, with one example of each.
In efflorescence a hydrated crystal loses water to the air and becomes powder, as washing soda does. In deliquescence a substance absorbs moisture and dissolves in it, as ferric chloride does.
Past board exam questions
ICSE 2019 sample paper · 1 mark
Which of the following has water of crystallization?
- Potassium chloride
- Sodium chloride
- Sodium nitrate
- Washing soda crystals
Show answer
Washing soda crystals
ICSE 2019 sample paper · 5 marks
Match Column A with Column B. | Column A | Column B | |---|---| | (i) Causes hardness in water | A. Calcium oxide | | (ii) Causes ozone depletion | B. Concentrated sulphuric acid | | (iii) Dehydrating agent | C. Magnesium bicarbonate | | (iv) Causes acid rain | D. Chlorofluorocarbon | | (v) Drying agent | E. Nitrogen dioxide |
Show answer
(i) – C. Magnesium bicarbonate
(ii) – D. Chlorofluorocarbon
(iii) – B. Concentrated sulphuric acid
(iv) – E. Nitrogen dioxide
(v) – A. Calcium oxide
ICSE 2019 sample paper · 3 marks
State if the solubility increases, decreases or remains the same with rise in temperature for each of the following compounds: (i) Calcium sulphate (ii) Potassium nitrate (iii) Sodium chloride
Show answer
(i) Calcium sulphate: decreases
(ii) Potassium nitrate: increases
(iii) Sodium chloride: remains (almost) the same
ICSE 2019 sample paper · 3 marks
(i) What is the difference between temporary hard water and permanent hard water? (ii) Write a balanced chemical equation for the removal of: 1. Permanent hardness 2. Temporary hardness
Show answer
(i) Temporary hardness is caused by the bicarbonates of calcium and magnesium and can be removed by boiling; permanent hardness is caused by the chlorides and sulphates of calcium and magnesium and cannot be removed by boiling (it is removed by adding washing soda).
(ii) 1. Permanent hardness (washing soda): CaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaCl (or CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄)
2. Temporary hardness (boiling): Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂↑