Hard Water | Mandatory Practical Activity 17 | Softening Hard Water by Ion Exchange | Water Treatment for Domestic Use
Dissolved Salts in Water
Water is so good at dissolving many different substances that it is known as the ‘universal solvent’.
Many salts, because they are ionic compounds, dissolve easily in water.
Water in nature is rarely pure; it usually contains a variety of dissolved substances including soluble salts.
It is difficult to form lather when using soap with hard water.
A lather is a layer sudsy froth at the surface of the water.
Hard water contains the soluble ions of calcium and/or magnesium (Ca2+, Mg2+ )
The sources of these ions are certain common soluble salts.
Calcium Hydrogen Carbonate Ca(HCO3) 2
Magnesium Hydrogen Carbonate Mg(HCO3)2
Calcium Sulphate CaSO4
Magnesium Sulphate MgSO4
The calcium and magnesium ions react with soap making a curdy scum.
A lather will form only when there are no calcium or magnesium ions in the water.
Therefore extra soap is needed to form a lather in hard water.
The extra soap is used to remove the calcium and/or magnesium ions.
The extra soap is used to soften the water.
Less soap is needed to make a lather with soft water.
Hard Water Advantages
1. Supplies calcium which is needed for strong bones.
2. Good for brewing beer and ale.
3. Good for tanning i.e. converting animal skin to leather.
4. Some prefer the taste of hard water.
Hard Water Disadvantages
1. Limescale wastes fuel – it insulates the kettles, steam irons and boilers.
2. The scum causes problems when washing clothes.
3. Limescale blocks hot water pipes and radiators.
4. Wastes soap – extra soap is needed to form lather.
Limescale is the layer of calcium carbonate (limestone rock, CaCO3 ) that forms on the inside walls when water hard with calcium hydrogen carbonate is heated.
Ca(HCO3)2 + Heat » CaCO3 + H2O + CO2
Mandatory Practical Activity 17
(a) Conduct a Qualitative Experiment to Detect the Presence of Dissolved Salts in Water Samples.
1. Place an evaporating dish on top of a beaker of water.
2. Pour some of the water sample into the evaporating dish.
3. Heat the water sample by heating the beaker of water with a Bunsen.
4. The steam from the boiling water heats the evaporating dish.
5. Keep heating until the evaporating dish is dry.
6. Note the fine layer of grey residue on the inside of the evaporating dish – this is the soluble solids that were in solution in the water.
7. If a drop of dilute hydrochloric acid is placed in the dish and produces a fizz then a carbonate salt is present.
(b) Test Water for Hardness (Soap Test)
Use identical small flasks and stoppers.
Have the same volume of water in each flask.
A pipette will deliver an exact volume of the water sample to the flask.
Set up a control: use soft water e.g. deionised water or distilled water.
Add one drop of soap solution from a small dropper to each flask.
Stopper and shake each flask with equal effort for 30 seconds.
Check for a permanent lather i.e. a lather that lasts for one minute.
Keep adding soap solution, drop by drop, until a permanent lather is formed.
A water sample is hard if it needs more soap flakes than the deionised soft water.
The more soap flakes needed, the harder the water.
Softening Hard Water by Ion Exchange
Water is hard because it has soluble calcium and/or magnesium ions.
To soften water the ions of calcium and magnesium must be removed.
Swop the calcium and magnesium ions in the water for sodium ions.
Pass the hard water through a column of porous sodium aluminium silicates or over sodium ion exchange resins.
Soft water emerges.
The sodium ions in the column pass into the water.
The calcium and magnesium ions pass from the water into the column.
Sodium ions in the water do not cause hardness.
Some Other Methods of Softening Water
Distillation – the distilled water is soft.
Add washing soda i.e. sodium carbonate – dishwashers and washing machines.
Remove all ions from the water: deionised water is soft.
Carry Out a Simple Distillation, and obtain a Sample of Water from Seawater.
Water Treatment for Domestic Use (‘HATS’)
Healthy: free of poisons and disease-causing organisms (bacteria, viruses).
Attractive: must look good – clear, colourless and no suspended material.
Taste: nice taste.
Smell: no smell.
Five Major Treatment Processes
The water is passed through wire meshes to remove large suspended debris such as twigs, leaves, plastic bottles, paper cartons etc..
2. Settling or Sedimentation
The water flows into a reservoir or large tanks.
Small dense suspended material like fine sand and clay sinks to bottom.
Clear water flows out at the top at the far end.
The water is assed through filter beds of fine sand, coarse sand and gravel.
Coarse sand and gravel used to keep the layer of fine sand in place.
The smallest suspended material is trapped in the layer of fine sand.
The tiny spaces between the grains of fine sand are too small to let even the finest suspended material to pass through.
Chlorine gas or a soluble chlorine compound is dissolved in the water.
Bacteria are killed – water is now sterile and safe to drink.
In some treatment plant ozone gas is used to sterilise the water.
A fluoride salt is dissolved in the water to reduce tooth decay.