Hard Water Trouble

Why it matters

Soap lathers instantly in rainwater but turns into a sticky, insoluble scum in tap water.

Predict first

Why does soap fail to lather in hard water?

Hard water contains dissolved calcium and magnesium salts. These ions react with soap to form an insoluble precipitate called scum, preventing lather.

• The water is too cold to dissolve soap
• Calcium and magnesium ions react with soap to form insoluble scum — correct
• Hard water contains too much oxygen
• Soap molecules are too large to fit in hard water

What you do

Build the dissolved salts that make water 'hard' — calcium sulphate (CaSO4) and magnesium chloride (MgCl2) — by crossing valencies. Their Ca²⁺ and Mg²⁺ ions are what curdle soap into scum.

Check yourself

Which ions are primarily responsible for water hardness?

Hard water is water that does not lather readily with soap. This hardness is caused mainly by dissolved calcium and magnesium salts — typically bicarbonates, chlorides, or sulphates.

What happens when you boil hard water containing temporary hardness (bicarbonates)?

Boiling decomposes calcium/magnesium bicarbonates into insoluble carbonates (scale), which can be filtered out, leaving softer water.

Why is soap scum a problem in washing machines?

The insoluble calcium/magnesium stearate (scum) deposits on clothes, making them look grey and stiff, and can clog machine pipes.

• Sodium (Na+) and Potassium (K+)
• Calcium (Ca2+) and Magnesium (Mg2+) — correct
• Iron (Fe2+) and Copper (Cu2+)
• Chloride (Cl-) and Sulphate (SO4 2-)
• It becomes harder
• Insoluble carbonates precipitate out, softening the water — correct
• Nothing changes
• It turns into steam and disappears
• It makes clothes smell bad
• It forms an insoluble precipitate that sticks to fabrics and machine parts — correct
• It increases the water temperature
• It dissolves the rubber seals