Water: composition, properties and hardness: WASSCE Chemistry

Water covers most of the earth and makes up about 60% of your body, yet it is chemically unusual: a small molecule that is liquid at room temperature, expands when it freezes and dissolves more substances than any other liquid. This lesson covers the composition and properties of water, how to test for it, and the hardness that makes soap hard to lather.

Learning objectives

By the end of this lesson you should be able to:

describe experiments showing the composition of water
state the physical properties of water and explain its unusual ones
describe the reactions of water with metals
describe chemical tests for water and a test for its purity
distinguish temporary and permanent hardness and describe how each is removed
state the advantages and disadvantages of hard water

Composition of water

Water is a compound of hydrogen and oxygen: H₂O.

Synthesis: hydrogen burns in oxygen to form water. Two volumes of hydrogen combine with one volume of oxygen.

2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}

\begin{gathered}2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \\[6pt] \rightarrow 2\text{H}_2\text{O(l)}\end{gathered}

Electrolysis of acidified water gives 2 volumes of hydrogen at the cathode for every 1 volume of oxygen at the anode.
By mass, water is 1 part hydrogen to 8 parts oxygen (2 g H : 16 g O).

Physical properties

Property	Value or behaviour
Appearance	Colourless, odourless, tasteless liquid
Boiling point	100 °C at 1 atm
Freezing point	0 °C
Maximum density	1 g/cm³ at 4 °C
Solvent power	Dissolves many ionic and polar substances: the "universal solvent"
Conductivity	Pure water is a very poor conductor

Unusual properties (anomalous behaviour) caused by hydrogen bonding:

a high boiling point for so small a molecule
ice is less dense than water, so it floats and lakes freeze from the top
high specific heat capacity, so water warms and cools slowly

Reactions of water with metals

Metal	Reaction	Equation
Potassium, sodium	Violent with cold water	2Na + 2H₂O → 2NaOH + H₂
Calcium	Steady with cold water	Ca + 2H₂O → Ca(OH)₂ + H₂
Magnesium	Very slow with cold water; burns in steam	Mg + H₂O(g) → MgO + H₂
Zinc, iron	Only with steam at red heat	3Fe + 4H₂O(g) ⇌ Fe₃O₄ + 4H₂
Copper, silver, gold	No reaction	—

Tests for water

Test	Result with water
Anhydrous copper(II) sulphate	White → blue
Anhydrous cobalt(II) chloride (paper)	Blue → pink

These tests show that water is present, not that it is pure. A liquid is pure water if it boils at exactly 100 °C and freezes at 0 °C at 1 atm.

Hardness of water

Definition

Hard water does not readily form a lather with soap. Instead it forms a grey scum. Hardness is caused by dissolved calcium and magnesium ions (Ca²⁺, Mg²⁺).

2\text{C}_{17}\text{H}_{35}\text{COO}^-\text{(aq)} + \text{Ca}^{2+}\text{(aq)} \rightarrow (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca(s)}

\begin{gathered}2\text{C}_{17}\text{H}_{35}\text{COO}^-\text{(aq)} + \text{Ca}^{2+}\text{(aq)} \\[6pt] \rightarrow (\text{C}_{17}\text{H}_{35}\text{COO})_2\text{Ca(s)}\end{gathered}

The product is scum (calcium stearate). Soap is wasted until all the Ca²⁺ is removed.

How hardness arises

Rainwater dissolves carbon(IV) oxide, forming weak carbonic acid. When it flows over limestone or chalk, it dissolves the rock:

\text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(aq)} \rightarrow \text{Ca(HCO}_3)_2\text{(aq)}

\begin{gathered}\text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(aq)} \\[6pt] \rightarrow \text{Ca(HCO}_3)_2\text{(aq)}\end{gathered}

Water flowing through rocks containing calcium sulphate (gypsum) or magnesium sulphate also becomes hard.

Temporary and permanent hardness

	Temporary hardness	Permanent hardness
Caused by	Calcium and magnesium hydrogencarbonates, Ca(HCO₃)₂ and Mg(HCO₃)₂	Calcium and magnesium sulphates and chlorides
Removed by boiling?	Yes	No

Removing hardness

Temporary hardness:

Boiling decomposes the hydrogencarbonate. The CaCO₃ forms the "fur" or scale inside kettles.

\text{Ca(HCO}_3)_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}

\begin{gathered}\text{Ca(HCO}_3)_2\text{(aq)} \\[6pt] \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\end{gathered}

Clark's method: add a calculated amount of lime water (calcium hydroxide).

\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \rightarrow 2\text{CaCO}_3 + 2\text{H}_2\text{O}

\begin{gathered}\text{Ca(HCO}_3)_2 + \text{Ca(OH)}_2 \\[6pt] \rightarrow 2\text{CaCO}_3 + 2\text{H}_2\text{O}\end{gathered}

Both types:

Washing soda (sodium carbonate) precipitates the calcium ions.

\text{Ca}^{2+}\text{(aq)} + \text{CO}_3^{2-}\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)}

\begin{gathered}\text{Ca}^{2+}\text{(aq)} + \text{CO}_3^{2-}\text{(aq)} \\[6pt] \rightarrow \text{CaCO}_3\text{(s)}\end{gathered}

Ion exchange: water passes through a resin that swaps Ca²⁺ and Mg²⁺ for Na⁺ ions. The resin is regenerated with concentrated salt solution.
Distillation: removes all dissolved solids, but is expensive.

Advantages and disadvantages of hard water

Advantages	Disadvantages
Calcium is good for bones and teeth	Wastes soap (scum forms)
Pleasant taste	Scale (fur) in kettles, boilers and pipes wastes fuel and can block pipes
Calcium coating reduces dissolving of lead from old pipes	Scum stains baths and clothes
Good for brewing some beers	Unsuitable for some industrial processes and dyeing

In Sierra Leone

Most water in Sierra Leone is soft, because the underlying rocks are mainly granite and laterite rather than limestone. Soap lathers easily, so hardness is rarely a problem here. Travellers to limestone areas notice the difference at once: more soap is needed, and kettles quickly fur up.

Common mistake

Boiling removes only temporary hardness. Permanent hardness, caused by sulphates and chlorides, is unaffected by boiling and needs washing soda, ion exchange or distillation.

Key points

Water is H₂O: 2 volumes H₂ + 1 volume O₂; 1 : 8 by mass.
Hydrogen bonding gives water a high boiling point, and makes ice float (maximum density at 4 °C).
Tests: anhydrous CuSO₄ white → blue; cobalt chloride blue → pink. Purity: boils at 100 °C, freezes at 0 °C.
Hard water contains Ca²⁺ and Mg²⁺ and forms scum with soap.
Temporary hardness (hydrogencarbonates): removed by boiling or Clark's method. Permanent hardness (sulphates and chlorides): removed by washing soda, ion exchange or distillation.

WASSCE examination focus

WAEC tests this

Expect to describe tests for water, to explain the causes of temporary and permanent hardness, to give methods of removal with equations, and to list advantages and disadvantages of hard water. Questions on reactions of metals with water or steam are also common.

Quick check

Give a chemical test for water and the result.
What causes temporary hardness?
Write an equation for the removal of temporary hardness by boiling.
Why does washing soda remove both temporary and permanent hardness?

Answers:

Anhydrous copper(II) sulphate turns from white to blue (or cobalt(II) chloride from blue to pink).
Dissolved calcium and magnesium hydrogencarbonates.
Ca(HCO₃)₂ → CaCO₃ + H₂O + CO₂.
Its carbonate ions precipitate all the Ca²⁺ and Mg²⁺ ions as insoluble carbonates, whatever their anion.

Water: composition, properties and hardness