Chemistry — PPSC previous year questions

A reaction that releases heat and raises the temperature of the mixture is exothermic (e.g. combustion, neutralisation, quicklime reacting with water). An endothermic reaction absorbs heat and cools the surroundings, as when ammonium chloride dissolves in water. Whether a catalyst is needed has nothing to do with the temperature change.

Domestic LPG (liquefied petroleum gas) is a mixture of butane and propane, hydrocarbons that liquefy under modest pressure and are stored in cylinders. Ethyl mercaptan is added to give the odourless gas its characteristic smell for leak detection. Methane is the main constituent of CNG/PNG, not LPG.

Coal seams release methane, which miners call firedamp. A methane–air mixture of roughly 5–15% methane explodes on the slightest spark or flame. This causes most colliery explosions, and it is why Davy's safety lamp was invented. Fine coal dust stirred up by the first blast often sets off a secondary explosion. Hydrogen, acetylene and carbon dioxide are not normally present in mine atmospheres.

Diamond is pure carbon. Each atom is joined by covalent bonds to four others in a rigid tetrahedral lattice. That makes it the hardest natural substance. Graphite is the other common crystalline allotrope of carbon. Sand is silica (SiO₂). Diamond has no carbonates and no phosphates.

Galvanisation is coating iron or steel with a thin layer of zinc. Zinc is more reactive than iron, so it corrodes preferentially and protects the iron even where the coating is scratched (sacrificial protection). Tin-plating is used for food cans, and chromium for stainless steel and decorative plating.

Iodised salt is ordinary sodium chloride to which a few parts per million of potassium iodide (or potassium iodate) have been added. No reaction takes place between the two, so it is a mixture, not a compound. Molecular iodine is not used because it is volatile and would be lost, and the tiny additive is nowhere near enough to make the salt potassium iodide alone.

Natural gas is mainly methane (CH₄), typically 70–90% by volume, with smaller amounts of ethane, propane and butane and traces of CO₂, nitrogen and hydrogen sulphide. CNG used in vehicles is simply compressed natural gas, so it too is essentially methane.

Atmospheric composition is quoted by volume: nitrogen about 78%, oxygen about 21%, argon 0.93% and carbon dioxide about 0.04%. By mass nitrogen is roughly 75.5% and oxygen 23%, because oxygen molecules are heavier.

Diamond is a pure crystalline form of the element carbon, so it is a substance, not a mixture. Air is a mixture of gases, sea water a solution of salts in water, and soil a mixture of minerals, organic matter, water and air.

Methane (CH4) is called marsh gas because it is produced by anaerobic bacterial decay of vegetation at the bottom of marshes and paddy fields and bubbles up to the surface. It is also the main constituent of natural gas and biogas, and the same gas is called fire-damp in coal mines.