TNPSC Daily Notes: Solutions, Colloids & Surface Chemistry — Concentration, Hard Water, Tyndall Effect, Emulsions & Adsorption | கரைசல்கள், கூழ்மங்கள் மற்றும் மேற்பரப்பு வேதியியல் — 11 November 2026

1. Solutions and the language of concentration

A solution is a homogeneous mixture of two or more substances in which the component present in the larger amount is the solvent and the one present in the smaller amount is the solute. Water dissolves so many substances that it is called the universal solvent. Solutions are not restricted to solids in liquids: soda water is a gas in a liquid, brass is a solid solution (an alloy) of copper and zinc, and air is a solution of gases. A solution that can dissolve no more solute at a given temperature is saturated; one that holds more than its saturation value, like the syrup in a crystal-growing experiment, is supersaturated and deposits crystals on the slightest disturbance.

Examiners prefer concentration terms because they can be tested with one line of arithmetic. Mass percentage is the mass of solute in 100 g of solution. Molarity (M) is the number of moles of solute dissolved in one litre of solution, so it falls slightly when the solution is warmed and expands. Molality (m) is the number of moles of solute per kilogram of solvent, and because mass does not change with temperature, molality is temperature-independent — the single most frequently asked distinction in this chapter. Mole fraction is the ratio of the moles of one component to the total moles, and parts per million (ppm) is used for very dilute solutions such as fluoride in drinking water. One mole of any substance contains the Avogadro number, 6.022 × 10²³, particles, and one mole of any gas occupies 22.4 litres at STP.

2. Solubility, Henry’s law and everyday proof

Solubility is the maximum mass of solute that dissolves in 100 g of solvent at a stated temperature. For most solids the solubility rises with temperature — sugar dissolves faster and more fully in hot tea. For gases the trend is reversed: solubility falls as temperature rises and rises as pressure rises, a relationship stated by Henry’s law. This single rule explains a surprising number of exam questions: a soft-drink bottle fizzes when opened because the pressure above the liquid drops and dissolved carbon dioxide escapes; boiled water tastes flat because its dissolved air has been driven off; fish suffocate in water warmed by industrial discharge because warm water holds less dissolved oxygen; and deep-sea divers who surface too quickly suffer decompression sickness (the bends) as dissolved nitrogen bubbles out of the blood.

3. Hard water and soft water

Water that does not give a good lather with soap is hard water, and the cause is dissolved salts of calcium and magnesium. Hardness is reported as the equivalent amount of calcium carbonate, in milligrams per litre or ppm; by the standard classification water is soft up to 60 mg/L, moderately hard from 61 to 120, hard from 121 to 180 and very hard above 180 mg/L. Temporary hardness comes from the bicarbonates of calcium and magnesium and can be destroyed simply by boiling, which precipitates insoluble calcium carbonate and magnesium hydroxide, or by Clark’s method, in which a calculated quantity of slaked lime, Ca(OH)₂, is added. Permanent hardness comes from the chlorides and sulphates of the same metals and survives boiling; it is removed by adding washing soda, Na₂CO₃, by the Calgon process using sodium hexametaphosphate, or by the Permutit (zeolite) ion-exchange process, in which hydrated sodium aluminium silicate trades its sodium ions for calcium and magnesium ions and is later regenerated with brine. Hard water wastes soap, leaves scale inside boilers and pipes that wastes fuel and can cause bursting, and spoils dyeing and tanning operations.

4. Colloids: between a solution and a suspension

When the dispersed particles measure roughly 1 to 1000 nanometres the mixture is a colloid — larger than the sub-nanometre particles of a true solution but too small to settle like a suspension. A colloid has a dispersed phase and a dispersion medium, and the familiar names follow from their combination: a sol is solid in liquid (ink, starch), a gel is liquid trapped in solid (jelly, cheese), an emulsion is liquid in liquid (milk, butter), an aerosol is solid or liquid in gas (smoke, fog, mist) and a foam is gas in liquid or solid (shaving cream, sponge). Colloids that are strongly attracted to the medium and therefore stable and reversible are lyophilic (starch, gum); those with little attraction, easily coagulated and irreversible, are lyophobic (gold sol, arsenic sulphide sol).

Four properties of colloids are examination favourites. The Tyndall effect is the scattering of light that makes a beam visible as it crosses a colloid — the reason a cinema projector beam, headlights in fog and sunbeams through a dusty window can be seen. Brownian movement is the ceaseless zig-zag motion of the particles under molecular bombardment, and it is what stops a colloid from settling. Electrophoresis is the migration of charged colloidal particles towards an electrode, used industrially in the Cottrell smoke precipitator to strip carbon from factory flue gases. Coagulation is the clumping of particles when an electrolyte is added, and the Hardy–Schulze rule states that the higher the charge on the oppositely charged ion the greater its coagulating power — so for a negatively charged sol, Al³⁺ beats Ca²⁺ which beats Na⁺. That is precisely why alum clarifies muddy water and why a delta forms where a river’s clay sol meets the electrolytes of the sea. Colloids are purified by dialysis, in which small ions escape through a semipermeable membrane while the colloid is retained — the working principle of the artificial kidney.

5. Surface chemistry: adsorption and catalysis

Adsorption is the accumulation of a substance only on the surface of a solid or liquid, whereas absorption means uniform penetration through the whole bulk; chalk dipped in ink adsorbs the dye on its surface while the water is absorbed throughout. Weak van der Waals attachment is physisorption (reversible, favoured by low temperature) and chemical-bond attachment is chemisorption (irreversible, needs higher temperature). Activated charcoal, with its enormous surface area, adsorbs poisonous gases in gas masks and colouring matter in sugar refining; silica gel packets adsorb moisture; animal charcoal decolourises. A catalyst alters the rate of a reaction without being consumed: iron in the Haber process for ammonia, vanadium pentoxide in the contact process for sulphuric acid, nickel in the hydrogenation of oils into vanaspati, and enzymes as the biological catalysts of digestion. A promoter increases catalytic activity (molybdenum with iron) and a poison destroys it. Because catalysis works on surfaces, finely divided and colloidal catalysts are the most efficient — the thread that ties the whole chapter together.