Water

Test for water (presence).

Test 1 — Anhydrous copper(II) sulfate.

• Anhydrous (no water) CuSO₄ is WHITE.
• Add the unknown liquid.
• If water is present: $\text{CuSO}_{4} + 5\text{H}_{2}\text{O} \to \text{CuSO}_{4}\cdot 5\text{H}_{2}\text{O}$ — turns BLUE.

Test 2 — Cobalt(II) chloride paper.

• Anhydrous cobalt chloride is BLUE.
• Add water → PINK ($\text{CoCl}_{2}\cdot 6\text{H}_{2}\text{O}$).

These tests confirm that water is PRESENT but not whether it's PURE.

Worked qualitative. A student adds liquid X to anhydrous copper sulfate. The white powder turns blue. Conclusion: liquid X contains water. (Could be pure water, salt water, juice — anything with water.)

Cambridge tip. Be precise about colour changes. White-to-blue is the standard answer for CuSO₄; blue-to-pink for cobalt chloride paper.

Purity test. Pure water has a very specific m.p. and b.p.:

• Melts/freezes at exactly $0\degree\text{C}$.
• Boils at exactly $100\degree\text{C}$ (at $1\,\text{atm}$).

Impure water (e.g. salt water, sugar water) has:

• LOWER m.p. (freezes below $0\degree\text{C}$).
• HIGHER b.p. (boils above $100\degree\text{C}$).
• A RANGE of temperatures, not a sharp point.

Why? Dissolved particles disrupt the water lattice → harder to freeze, harder to escape → b.p. rises and m.p. falls.

Worked qualitative. A student measures the b.p. of two water samples. Sample A boils at $100\degree\text{C}$ exactly; sample B boils at $102\degree\text{C}$. Sample A is pure; sample B contains a dissolved solute.

Practical applications. Salt on icy roads in winter LOWERS the m.p. of water → ice melts even when air is below $0\degree\text{C}$. Anti-freeze in car coolant LOWERS the m.p. so the coolant doesn't freeze in winter.

Treating water for human consumption (potable water).

Step 1: Sedimentation. Water from rivers/reservoirs is allowed to stand; heavy particles settle. Sometimes alum is added as a flocculant — it traps small particles into clumps that sink faster.

Step 2: Filtration. Water passes through SAND BEDS to remove suspended solids and microbes.

Step 3: Chlorination. Chlorine added (or chloramines, chlorine dioxide) — kills bacteria and viruses.

Step 4 (some regions): Fluoridation. Fluoride added at low concentrations ($\sim 1\,\text{ppm}$) — reduces tooth decay. Controversial — some communities oppose it as enforced medication.

Distilled water. Boil water, condense the steam back to liquid. Removes all dissolved solids and most volatile organics. NOT used for drinking (lacks minerals; tastes flat) but useful in labs and industry.

Desalination. Removing salt from sea water:

1. Distillation: heat sea water; pure water vapour rises, condenses; salt remains.
2. Reverse osmosis: high-pressure pump pushes water through a semi-permeable membrane; salt blocked.

Worked qualitative. Why is sea water not used for drinking? Salt content too high — kidneys can't excrete it; salt accumulates → death from dehydration paradoxically.