Hard Water Mystery

Why it matters

Soap lathers instantly in rainwater but turns into a sticky scum in tap water. The culprit is invisible dissolved salts.

Predict first

Why does soap fail to lather in hard water?

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

• The water is too cold
• Dissolved calcium and magnesium ions react with soap to form insoluble scum — correct
• The soap has expired
• Hard water lacks oxygen for the soap to react

What you do

Sort each dissolved salt by what it does to water. The rule: bicarbonates of calcium and magnesium cause temporary hardness; their chlorides and sulphates cause permanent hardness; sodium and potassium salts cause none at all.

Check yourself

Which ions are primarily responsible for water hardness?

Hardness is defined by the presence of dissolved calcium (Ca²⁺) and magnesium (Mg²⁺) ions.

What type of salts typically cause temporary hardness?

Temporary hardness is caused by dissolved calcium and magnesium bicarbonates, which can be removed by boiling.

Why is CaSO₄ considered a cause of permanent hardness?

Permanent hardness is caused by sulphates and chlorides, which do not precipitate out when the water is boiled.

• Sodium and Potassium ions
• Calcium and Magnesium ions — correct
• Iron and Copper ions
• Chlorides and Sulphates
• Bicarbonates — correct
• Nitrates
• It decomposes upon boiling to remove the calcium
• It remains dissolved even after boiling — correct
• It reacts with soap to form a gas