Solutions, Gases and Chemistry in Everyday Life

A solution is a homogeneous mixture of two or more substances, which means it looks the same throughout and the particles cannot be seen even with a microscope. The substance that dissolves is the solute and the substance that does the dissolving is the solvent. The solvent is usually the component present in larger amount, and it decides the physical state of the solution. Salt in water, air, soda water and brass are all solutions; water is called the universal solvent because it dissolves more substances than any other common liquid.

How much solute is dissolved is the concentration of the solution. Chemists measure it in several ways. The most common mistake is forgetting that the mass of the solution is the solute plus the solvent. Suppose Radhika dissolves 40 g of salt in 200 g of water. The solution has a mass of 40 + 200 = 240 g, so the mass per cent of salt is 40 ÷ 240 × 100 = 16.67 per cent, not 20 per cent.

Henry's law: at a fixed temperature, the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the liquid. This is why soft-drink bottles are sealed under high pressure (more CO₂ dissolves, and the fizz rushes out when you open the cap and the pressure drops). Scuba divers breathe air at high pressure, so more nitrogen dissolves in the blood; if they rise too fast the dissolved gas bubbles out and causes 'the bends'. Raoult's law: the partial vapour pressure of each volatile component in a solution is proportional to its mole fraction.

In a gas the particles are far apart, move randomly at high speed and have very weak forces between them. So a gas has no fixed shape and no fixed volume, it fills whatever container it is in, it is highly compressible, and it exerts pressure equally in all directions. Four quantities describe a gas: pressure (P), volume (V), temperature (T) and amount in moles (n). Every gas law is simply a statement of how these four are tied together.

Always convert temperature to kelvin (K = °C + 273) before using any gas law. Absolute zero, 0 K or about −273 °C, is the lowest possible temperature. At STP (0 °C and 1 atm) one mole of any ideal gas occupies 22.4 litres and holds 6.022 × 10²³ molecules, the Avogadro number. Radhika's quick check: 5.6 L of oxygen at STP is 5.6 ÷ 22.4 = 0.25 mole, so it holds 0.25 × 6.022 × 10²³ = 1.5055 × 10²³ molecules.

Dry air is mostly nitrogen (about 78 per cent) and oxygen (about 21 per cent), together about 99 per cent of the volume. Argon is about 0.93 per cent and carbon dioxide only a trace, around 0.04 per cent. Competitive exams do not ask the percentages as much as the uses: which gas fills a bulb, which cools a surgeon's cut, which sits in your kitchen cylinder, and which one is a silent poison.

Every household substance has a common name, a chemical name and a formula, and exams love to ask you to join the three. The safest way to learn them is in families: the sodium family (baking soda, washing soda, caustic soda), the calcium family (lime, slaked lime, limestone, gypsum, plaster of Paris, bleaching powder) and the 'vitriol' family of hydrated sulphates (blue, green, white). Radhika's trick is to say the name aloud with the formula, as in 'baking soda, NaHCO₃', until the pair sticks.

Drugs are chemicals of low molecular mass that interact with biological targets and produce a medical effect. Chemistry also keeps our food safe and sweet: preservatives slow spoilage, antioxidants stop fats going rancid, and artificial sweeteners give sweetness with almost no calories. The exam trick is to attach one example to every class: aspirin for analgesics, penicillin for antibiotics, sodium benzoate for preservatives, saccharin for sweeteners.

A food preservative stops growth of micro-organisms or slows chemical spoilage. Common salt, sugar and edible oil preserve pickles, jams and squashes; sodium benzoate and potassium metabisulphite are chemical preservatives; antioxidants such as BHA and BHT stop fats turning rancid. Pasteurisation, named after Louis Pasteur, is gentle heating followed by quick cooling that destroys harmful micro-organisms in milk without spoiling its taste. Black pepper is added for flavour, not as a preservative.

Chemistry also builds our fields, houses and festivals. Fertilisers supply the three primary nutrients nitrogen, phosphorus and potassium (NPK); cement and glass are made by heating limestone and sand with other minerals; ceramics are clay products hardened by fire; and fireworks and explosives are mixtures that release gas and heat in a flash. Learn each as 'raw materials, process, product, use'. Radhika's tip is to link the raw materials: limestone appears in cement, glass and whitewash alike.