3.3 - Water Hardness

Hard water and the two kinds of hardness

• As rain soaks through rocks such as limestone, chalk and gypsum, it dissolves compounds containing calcium or magnesium.
• Hard water contains a high concentration of dissolved calcium ions (Ca²⁺) and magnesium ions (Mg²⁺).
• Soft water contains little or none of these ions.

Telling hard water from soft water

• The test is what happens when soap is added:
  • Soft water forms a lather readily.
  • Hard water reacts with soap to form an insoluble solid called scum, which wastes soap and prevents a lather until all the Ca²⁺ and Mg²⁺ ions have reacted.
• Hard water needs far more soap than soft water before it will lather.

Temporary and permanent hardness

• Temporary hardness is caused by dissolved calcium hydrogencarbonate and magnesium hydrogencarbonate. Boiling removes it.
• Permanent hardness is caused by dissolved sulfates (mainly calcium sulfate and magnesium sulfate). Boiling does not remove it.
• A water sample can contain temporary hardness, permanent hardness, or both. Boiling a sample with both makes it softer, but does not make it completely soft.

Softening hard water

Softening hard water means removing dissolved calcium and magnesium ions so soap lathers easily without forming scum.

Boiling

• Boiling removes temporary hardness only.
• It is simple and cheap for small volumes, but it is too expensive and impractical for softening a whole household water supply.

Adding sodium carbonate

• Sodium carbonate, Na₂CO₃ (washing soda), is soluble and is simply added to the water.
• It removes both temporary and permanent hardness.
• Drawback: the precipitated carbonate forms limescale, which can build up and block pipes.

Ion exchange columns

• Water is passed through an ion exchange column, where a resin swaps the calcium and magnesium ions in the water for sodium ions, which don't cause hardness.
• This removes both kinds of hardness.
• It can treat a continuous flow of water, making it suitable for household supplies.
• The column eventually runs out of sodium ions and is regenerated by flushing with concentrated sodium chloride solution.
• Drawback: ion exchange columns are expensive to buy.

The chemistry of softening

Softening works by removing dissolved Ca²⁺ and Mg²⁺ ions from solution.

Boiling

Heating decomposes dissolved hydrogencarbonate into insoluble carbonates, water, and carbon dioxide:

Ca(HCO₃)₂(aq) → CaCO₃(s) + H₂O(l) + CO₂(g)

Mg(HCO₃)₂(aq) → MgCO₃(s) + H₂O(l) + CO₂(g)

• The insoluble calcium and magnesium carbonates precipitate out as limescale.
• Dissolved sulfates do not decompose when heated, so boiling leaves permanent hardness unchanged.

Adding sodium carbonate

Sodium carbonate is soluble, so adding it puts carbonate ions into the water:

sodium carbonate + calcium sulfate → calcium carbonate + sodium sulfate

Na₂CO₃(aq) + CaSO₄(aq) → CaCO₃(s) + Na₂SO₄(aq)

Dissolved carbonate ions (CO₃^2-) react with Ca²⁺ and Mg²⁺ ions to form insoluble precipitates:

Ca²⁺(aq) + CO₃^2-(aq) → CaCO₃(s)

Mg²⁺(aq) + CO₃^2-(aq) → MgCO₃(s)

Because the carbonate ions precipitate any dissolved Ca²⁺ and Mg²⁺, sodium carbonate removes both temporary and permanent hardness.

Ion exchange columns

• The column contains an insoluble resin coated with sodium ions, Na⁺.
• As hard water passes through, the resin swaps its sodium ions for calcium and magnesium ions:
  • Na₂Resin(s) + Ca²⁺(aq) → CaResin(s) + 2Na⁺(aq)
• Two Na⁺ ions are released for every Ca²⁺ or Mg²⁺ ion removed.
• Dissolved sodium ions do not cause hardness, so the water leaves soft.
• Once the resin is saturated with Ca²⁺ and Mg²⁺, flushing with concentrated sodium chloride solution displaces them and recharges the column with Na⁺.

What hardness does, good and bad

Advantages of hard water

• Dissolved calcium ions help build and maintain strong teeth and bones.
• Drinking hard water is linked to a reduced risk of heart disease.

Disadvantages of hard water

• Heating hard water precipitates limescale (mostly calcium carbonate) inside kettles, boilers, and hot water pipes:
  • Scale on a heating element is a poor thermal conductor, so appliances take longer and use more energy to heat water.
  • Scale inside pipes reduces water flow and can eventually block them.
• Hard water wastes soap and produces unsightly scum.

Worked example - Hardness of a water sample

Two water samples were each tested by adding soap solution until a lasting lather formed, once before boiling and once after boiling, alongside distilled water as a control.

Work out the total, temporary and permanent hardness of each sample, and say what type of hardness each sample contains.