Basic Chemistry: Notes for UPSC, SSC & Railway Exams

Acids, bases and the pH scale

Acids taste sour, turn blue litmus red and release hydrogen ions in water. Bases taste bitter, feel soapy, turn red litmus blue and release hydroxide ions. Soluble bases are called alkalis. An acid and a base react to form a salt and water — neutralisation.

The pH scale runs from 0 to 14. pH 7 is neutral; lower values are acidic and higher values basic. The scale is logarithmic, so pH 3 is ten times more acidic than pH 4.

• Indicators: litmus (red in acid, blue in base); phenolphthalein (colourless in acid, pink in base); methyl orange (red in acid, yellow in base); turmeric (yellow, turns reddish-brown in base).
• Tooth enamel starts to corrode when the pH in the mouth falls below about 5.5; toothpastes are basic.
• Acidity (excess stomach acid) is treated with antacids such as milk of magnesia or baking soda.
• Acidic soil is treated with quicklime or slaked lime; basic soil with organic matter.

Everyday compounds and their chemical names

Metals and non-metals

• Mercury is the only metal that is liquid at room temperature; bromine is the only liquid non-metal.
• Gallium and caesium melt at temperatures close to body temperature (gallium can melt in the palm).
• Sodium and potassium react violently with water and air, so they are stored under kerosene.
• Silver is the best conductor of electricity, followed by copper; gold is the most malleable and ductile metal.
• Tungsten has the highest melting point among metals, so it was used for bulb filaments.
• Exceptions among non-metals: graphite conducts electricity, iodine is lustrous, and diamond (carbon) is the hardest natural substance.
• Abundance in the Earth's crust: oxygen first, silicon second, aluminium third (the most abundant metal), iron fourth.

Corrosion and its prevention

Rust is hydrated iron(III) oxide. Iron rusts only when both oxygen (air) and water are present, which is why rusting is faster near the sea. Silver turns black (silver sulphide) and copper develops a green coat (basic copper carbonate) by similar slow reactions.

• Painting, oiling or greasing keeps air and moisture away.
• Galvanisation: coating iron with zinc. Zinc is more reactive, so it corrodes first and protects the iron even where scratched.
• Tinning: coating with tin, used for food cans.
• Alloying: stainless steel (iron with chromium, nickel and a little carbon) does not rust easily.

Alloys

Pure gold is 24 carat. 22-carat jewellery has 22 parts gold and 2 parts of copper or silver to make it harder.

Gases and fuels around us

• LPG (liquefied petroleum gas): mainly butane with propane. It is odourless, so a strong-smelling sulphur compound (ethyl mercaptan) is added to detect leaks.
• CNG (compressed natural gas) and biogas: mainly methane. CNG burns cleaner than petrol or diesel.
• Dry ice: solid carbon dioxide; it sublimes directly into gas. Used for cooling and stage fog.
• Laughing gas: nitrous oxide (N2O), still used as a mild anaesthetic, for example in dentistry.
• Carbon monoxide is poisonous because it binds to haemoglobin far more strongly than oxygen.
• Noble gases: neon in advertising signs, argon in bulbs, helium in balloons (light and non-flammable).
• Hard water contains calcium and magnesium salts. Temporary hardness (bicarbonates) is removed by boiling; permanent hardness (chlorides and sulphates) needs washing soda or ion exchange.