Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.17 to 1.18

Fresh Water, Hardness and pH.

Almost all the water on Earth is salty sea water. Even the fresh part is not all equally good: some of it will not lather with soap, and some is slightly acidic. This note shows where fresh water sits, what hard water is and how it is softened, and how pH is used to judge water.

1. How much of the Earth's water is fresh

Of all the water on Earth, 97.3% is salty sea water. The remaining 2.7% is fresh. Inside that fresh share, glaciers and snow make up about 2%, groundwater 0.6%, and lakes and rivers together about 0.01%.

Now the split inside the fresh water itself: glaciers and snow 74%, groundwater 22.2%, lakes 0.3%, rain 0.035%, rivers 0.03%. Most fresh water is locked up as ice, and rivers hold the smallest share. Put the sources in order, from the biggest share to the smallest.

2. Sources, uses and the yardsticks

Usable water comes from two places. Surface water: rivers, glaciers, lakes, springs and dams. Groundwater: alluvial layers, bedrock, artesian basins and aquifers.

About 70% of the human body is water. That is why at least 1.6 litres of pure water must be drunk every day.

Water is not only for drinking. Industry needs it for boilers and cooling. Agriculture needs it for irrigation, as with IRRI rice. Aquatic plants keep the dissolved oxygen (DO) right by photosynthesis. And the whole ecosystem depends on water.

Six yardsticks tell how pure surface water is: pH, hardness, DO, BOD, COD and TDS. Anything outside this list, such as SDS (a detergent), is not a yardstick. This note takes the first two; the other four come in the next note.

3. Hard water and its two kinds

Fresh water that holds dissolved Ca²⁺, Mg²⁺ and Fe²⁺ is called hard water. It forms insoluble floating scum with soap, so it will not lather.

Hardness comes in two kinds, and the difference is the anion of the salt. Temporary hardness is due to bicarbonate (HCO₃⁻) salts and goes on boiling. Permanent hardness is due to the chlorides and sulphates of Ca²⁺, Mg²⁺ and Fe²⁺ and does not go on boiling. Say which group each salt belongs to.

Boiling removes temporary hardness like this: Ca(HCO₃)₂ → CaCO₃ + CO₂ + H₂O (heat).

Why does the calcium leave? First 2HCO₃⁻ ⇌ CO₂ + H₂O + CO₃²⁻. Then Ca²⁺ + CO₃²⁻ → CaCO₃(s). The insoluble CaCO₃ settles out and takes the Ca²⁺ with it.

The damage of hard water shows in boilers: scale builds up there, and the scale lowers heat transfer.

Boiling cannot take out permanent hardness, so we use permutit or ion exchange. The resin holds on to Ca²⁺ and gives out Na⁺ in return: 2R-SO₃⁻Na⁺ + Ca²⁺ → (RSO₃)₂Ca + 2Na⁺. When the resin is used up, a KCl or NaCl solution regenerates it. Ion exchange removes permanent hardness too.

4. pH and the WHO limits

pH measures how acidic water is. The formula is pH = −log[H⁺], where [H⁺] is the molar concentration of hydrogen ions. The pH of pure water is 7.

Surface water holds dissolved H₂CO₃, so its pH is 6 to 6.5.

By the WHO limit, the pH of drinking water must lie between 6.5 and 8.5. For aquatic animals the pH should be 7.0 to 7.5.

The sum is short. In 0.001 M HCl, [H⁺] = 10⁻³ M, so pH = −log 10⁻³ = 3. Drop the minus sign from the exponent and you have the pH. Try it below: guess the pH first and type it in.

5. Check yourself

Three questions in the exam's style. Start with the first.

Now the second.

In the last one, keep an eye on the anion and the sign of the exponent.

The whole note on a few cards.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.17 to 1.18.