SEMI FINAL STAGE 2013
MfantsiMan Girls
Adisadel College
St Peter's SHS
QUESTION
Name any three cations whose presence in water may cause permanent hardness.
ANSWER: Calcium²⁺, Magnesium²⁺, and Iron²⁺
SOLUTION:
These form insoluble salts with soap and are not removed by boiling (unlike temporary hardness).
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PRACTICE QUESTIONS
1. QUESTION: Name two anions associated with permanent hardness of water.
ANSWER: Sulfate ($SO_4^{2-}$) and chloride ($Cl^-$)
SOLUTION:
Calcium and magnesium sulfates and chlorides are not removed by boiling.
2. QUESTION: Name the anion responsible for temporary hardness of water.
ANSWER: Hydrogencarbonate ($HCO_3^-$)
SOLUTION:
Calcium and magnesium hydrogencarbonates decompose on boiling.
3. QUESTION: Name one method of removing permanent hardness.
ANSWER: Any one of: adding washing soda, ion exchange, or distillation
SOLUTION:
These remove the $Ca^{2+}$ and $Mg^{2+}$ ions, which boiling cannot.
4. QUESTION: Name the cation, other than $Ca^{2+}$, mainly responsible for the hardness of water.
ANSWER: Magnesium ion ($Mg^{2+}$)
SOLUTION:
Calcium and magnesium ions both form scum with soap.
5. QUESTION: Name one method of removing temporary hardness.
ANSWER: Boiling (or adding slaked lime)
SOLUTION:
Both convert the hydrogencarbonates into insoluble carbonates.
6. QUESTION: Name the insoluble substance deposited when temporarily hard water is boiled.
ANSWER: Calcium carbonate (scale)
SOLUTION:
$Ca(HCO_3)_2\rightarrow CaCO_3+CO_2+H_2O$.