CARA'S TIPSAir, water, solubility & colloids
JAMBWAECNECO
What the syllabus asks
JAMB: Air; water (hardness, treatment, solubility, water of crystallisation, efflorescent, deliquescent and hygroscopic substances); solutions, solubility curves; suspensions and colloids
WAEC: Air: composition, separation by fractional distillation of liquid air, air as a mixture; water: sources, treatment for town supply, hardness (temporary and permanent) and its removal, water as a solvent, water of crystallisation, efflorescence, deliquescence and hygroscopy; solubility, solubility curves, saturated and supersaturated solutions; suspensions and colloids, Tyndall effect
NECO: Air: composition, separation by fractional distillation of liquid air, air as a mixture; water: sources, treatment for town supply, hardness (temporary and permanent) and its removal, water as a solvent, water of crystallisation, efflorescence, deliquescence and hygroscopy; solubility, solubility curves, saturated and supersaturated solutions; suspensions and colloids, Tyndall effect
What you will learn
- Show that air is a mixture and state its composition.
- Describe experiments to find the percentage of oxygen in air and the separation of air by fractional distillation.
- Describe the treatment of water for town supply.
- Explain temporary and permanent hardness, their causes, advantages, disadvantages and removal.
- Define solubility, interpret solubility curves and calculate solubility and crystal yields.
- Distinguish efflorescent, deliquescent and hygroscopic substances and calculate percentage water of crystallisation.
- Distinguish true solutions, colloids and suspensions and describe types and uses of colloids.
Key points
- Air is a mixture: N₂ 78%, O₂ 21%, Ar 0.9%, CO₂ 0.04%, water vapour variable.
- Oxygen content: pass air over hot copper; volume falls by about 21%.
- Liquid air is fractionally distilled: N₂ (−196 °C) boils off before O₂ (−183 °C).
- Town water: screening → alum coagulation → sedimentation → filtration → chlorination.
- Hard water contains Ca²⁺/Mg²⁺ and forms scum with soap.
- Temporary hardness (hydrogencarbonates) removed by boiling or lime; permanent (sulphates) by washing soda, ion exchange or distillation.
- Solubility = g per 100 g solvent (or mol/dm³) at saturation; crystals on cooling = difference in solubilities.
- Water of crystallisation: CuSO₄·5H₂O is 36.1% water; anhydrous CuSO₄ tests for water.
- Efflorescent lose water; deliquescent absorb and dissolve; hygroscopic absorb without dissolving.
- Colloids (1 nm to 1 μm) do not settle, pass filter paper and show the Tyndall effect.
Start here: Air is a mixture
Air is all around us, yet chemistry students often forget that it is a mixture of gases, not a single substance or a compound. Several facts prove this:
- Its composition varies slightly from place to place: air over a busy road in Onitsha has more carbon dioxide and carbon monoxide than air over a farm in Jos; air in the Harmattan carries more dust and less water vapour.
- Its components can be separated by physical means (fractional distillation of liquid air).
- When liquid air is formed, there is no heat change characteristic of a chemical reaction, and the components keep their own properties (oxygen still supports burning).
- Air has no fixed formula, and its composition does not match any whole-number ratio.
- When air dissolves in water, the dissolved air is richer in oxygen than ordinary air, because oxygen is more soluble than nitrogen. A compound would dissolve with a fixed composition.
| Gas | % by volume (dry air) | Importance |
|---|---|---|
| Nitrogen | 78 | Dilutes oxygen; source of nitrogen for fertilisers (Haber process) |
| Oxygen | 21 | Respiration and combustion |
| Argon and other noble gases | about 0.9 | Filling bulbs, welding shields, neon signs |
| Carbon dioxide | about 0.04 | Photosynthesis |
| Water vapour | variable | Weather, humidity |
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Practice questions
- nitrogen
- oxygen
- carbon dioxide
- argon
Show answer
A: nitrogen
About 78%.
- 78%
- 1%
- 0.04%
- 21%
Show answer
D: 21%
About one-fifth.
- Argon
- Neon
- Carbon dioxide
- Nitrogen
Show answer
C: Carbon dioxide
CO₂ + H₂O → glucose + O₂.
- magnesium sulphate
- calcium hydrogencarbonate
- calcium sulphate
- sodium chloride
Show answer
B: calcium hydrogencarbonate
It is removed by boiling.
- adding washing soda
- boiling
- filtration
- chlorination
Show answer
A: adding washing soda
Na₂CO₃ precipitates Ca²⁺ and Mg²⁺.
Key words
- Mixture
- Two or more substances physically combined in any proportion and separable by physical means.
- Fractional distillation
- Separation of liquids by their different boiling points.
- Coagulation
- Clumping of fine suspended particles using a chemical such as alum.
- Chlorination
- Adding chlorine to water to kill germs.
- Hard water
- Water that does not form lather readily with soap, due to dissolved calcium or magnesium salts.
- Scum
- Insoluble solid formed when soap reacts with calcium or magnesium ions.
- Solubility
- Mass of solute that saturates 100 g of solvent at a given temperature (or mol/dm³).
- Saturated solution
- A solution that can dissolve no more solute at that temperature, in contact with undissolved solute.
- Supersaturated solution
- An unstable solution holding more solute than the saturated amount at that temperature.
- Water of crystallisation
- A fixed number of water molecules chemically combined in a crystal.
- Efflorescence
- Loss of water of crystallisation to the atmosphere.
- Deliquescence
- Absorption of so much atmospheric moisture that a solid dissolves to form a solution.
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