Temporary hardness of water is primarily due to the presence of:
So, calcium and magnesium are the main hardness-causing ions. For temporary hardness, their bicarbonates are the culprits. When water is boiled, the bicarbonates decompose into carbonates, which precipitate out. That's why boiling removes temporary hardness. The chemical reaction would be something like Ca(HCO3)2 → CaCO3 + CO2 + H2O when heated.
Looking at the options, if the correct answer is calcium bicarbonate, then the other options might be other salts. Let me think: if the options were sodium sulfate, magnesium chloride, calcium chloride, and calcium bicarbonate, then the correct one would be calcium bicarbonate. The other options are either permanent hardness contributors or not related.
Wait, magnesium bicarbonate could also contribute, but the primary is calcium. So if the options include calcium bicarbonate, that's the answer. The incorrect options would be sulfates, chlorides, or maybe sodium salts. Sodium doesn't contribute to hardness because it's not a cation that forms insoluble salts.
So the core concept here is the difference between temporary and permanent hardness. The key is that temporary hardness is from bicarbonates of calcium and magnesium, which decompose upon boiling. The correct answer should be calcium bicarbonate. The wrong options are other salts that don't decompose with heat. The clinical pearl here is that temporary hardness can be removed by boiling, unlike permanent hardness. Students should remember that bicarbonates are the key here.
**Core Concept** Temporary hardness of water is caused by dissolved bicarbonates of calcium and magnesium. Boiling decomposes these bicarbonates into insoluble carbonates, which precipitate and remove hardness. This contrasts with permanent hardness, caused by sulfates or chlorides of these cations.
**Why the Correct Answer is Right** Calcium bicarbonate (Ca(HCO₃)₂) is the primary contributor to temporary hardness. When heated, it decomposes into calcium carbonate (CaCO₃), CO₂, and H₂O:
**Ca(HCO₃)₂ → CaCO₃↓ + CO₂↑ + H₂O**. This reaction removes hardness by precipitation. Magnesium bicarbonate behaves similarly but is less abundant in natural water.
**Why Each Wrong Option is Incorrect**
**Option A:** Sodium sulfate (Na₂SO₄) does not contribute to hardness, as sodium is not a hardness-causing cation.
**Option B:** Magnesium chloride (MgCl₂) causes permanent hardness, as chlorides do not decompose on boiling.
**Option D:** Calcium chloride (CaCl₂) also causes permanent hardness, as chlorides remain soluble when heated.
**Clinical Pearl / High-Yield Fact** Remember the acronym **BIC** for temporary hardness: **B**icarbonates of **I**onized **C**alcium and magnesium. Boiling removes temporary hardness but not permanent hardness. A common exam trap is confusing bicarbonates (temporary) with sulfates/chlorides (permanent).
**Correct Answer: