Solutions — Important Questions

Polar solvents dissolve polar solutes and non-polar solvents dissolve non-polar solutes

Step 1: The phrase 'like dissolves like' refers to the polarity of substances. Step 2: Polar compounds (like common salt, NaCl) are soluble in polar solvents (like water). Step 3: Non-polar compounds (like fat, sulphur) are soluble in non-polar solvents (like ether, carbon disulphide). Step 4: This is why sulphur dissolves in CS₂ but not in water — sulphur is non-polar and water is polar. Step 5: Options C and D describe irrelevant physical properties (size, weight) that do not determine solubility.

They lose water of crystallization and turn colourless

Step 1: Copper sulphate pentahydrate (CuSO₄·5H₂O) is a hydrated salt — it has 5 molecules of water of crystallization. Step 2: When gently heated, the salt loses these 5 water molecules: CuSO₄·5H₂O → CuSO₄ + 5H₂O. Step 3: The blue colour of CuSO₄ crystals is due to the presence of water of crystallization. Step 4: After losing water, the resulting anhydrous CuSO₄ becomes colourless/white and amorphous. Step 5: This reaction is reversible — adding a few drops of water to anhydrous CuSO₄ makes it turn blue again, which is used as a test for moisture.

10 g per 100 g water

Step 1: Formula: Solubility = (Mass of solute / Mass of solvent) × 100. Step 2: Mass of solute = 1.5 g; Mass of solvent = 15 g. Step 3: Solubility = (1.5 / 15) × 100 = 10 g per 100 g of water. Step 4: This means 10 g of this solute can dissolve in 100 g of water to form a saturated solution at 298 K. Step 5: Option C (15 g) is a common error where students confuse the mass of solvent directly with solubility without scaling to 100 g.

Caustic soda (NaOH)

Step 1: Deliquescent substances absorb so much moisture from the air that they completely dissolve in the absorbed water. Step 2: Caustic soda (NaOH) is a classic deliquescent substance — it absorbs water from humid air and eventually forms a liquid solution. Step 3: Silica gel, Quick lime (CaO), and Phosphorus pentoxide (P₂O₅) are hygroscopic substances — they absorb moisture but do not dissolve. Step 4: The key difference is that deliquescent substances change their physical state (solid → liquid), while hygroscopic substances do not. Step 5: Deliquescence is maximum at low temperatures and