Chemical Reactions, Acids, Bases and Salts

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Physical and chemical change

A physical change alters only form or state and makes no new substance (melting ice, dissolving sugar); a chemical change makes new substances (burning, rusting, curdling of milk, digestion, cooking). Its signs are a change of colour or state, evolution of a gas, a change in temperature or the formation of a precipitate. A chemical equation is balanced because mass is neither created nor destroyed (Lavoisier's law of conservation of mass): 2H2 + O2 gives 2H2O.

Types of reactions

In combination two or more substances form one, as when quicklime reacts vigorously with water to form slaked lime (CaO + H2O gives Ca(OH)2), used for whitewash. In decomposition one substance breaks up, by heat (limestone, CaCO3, gives CaO and CO2), by electricity (electrolysis of water gives hydrogen and oxygen in the ratio 2:1 by volume) or by light (silver chloride turns grey in sunlight, the basis of black-and-white photography). In displacement a more reactive metal pushes a less reactive one out of its salt solution: an iron nail turns blue copper sulphate solution green and gets a brown coat of copper. In double displacement ions are exchanged, often forming a precipitate, as barium chloride and sodium sulphate give white barium sulphate; neutralisation is of this type. Reactions that release heat are exothermic (burning, respiration); those that absorb heat are endothermic (most decompositions, photosynthesis).

Oxidation, reduction, corrosion and rancidity

Oxidation is the gain of oxygen or loss of hydrogen (loss of electrons); reduction is the loss of oxygen or gain of hydrogen (gain of electrons). They happen together in redox reactions: in CuO + H2 giving Cu + H2O, copper oxide is reduced and hydrogen is oxidised. Corrosion eats metals away: iron rusts to reddish-brown hydrated iron(III) oxide, which needs both air and moisture, silver turns black (silver sulphide) and copper gets a green coat (basic copper carbonate). Painting, oiling, galvanising with zinc, chrome plating and alloying prevent rusting. Rancidity is the oxidation of fats and oils in food, which spoils its smell and taste; antioxidants, airtight containers, refrigeration and filling chips packets with nitrogen slow it.

Acids and bases

Acids taste sour, turn blue litmus red and give H+ ions in water; bases taste bitter, feel soapy, turn red litmus blue and give OH- ions in water. Bases that dissolve in water are alkalis (sodium, potassium and calcium hydroxides), so every alkali is a base but not every base is an alkali. Natural acids: vinegar (acetic), lemon and orange (citric), tamarind (tartaric), tomato (oxalic), curd (lactic), ant and nettle stings (methanoic or formic) and our stomach (hydrochloric). Aqua regia, 3 parts concentrated hydrochloric acid to 1 part concentrated nitric acid, dissolves gold and platinum. Acids give hydrogen with metals, which burns with a pop sound, and carbon dioxide with carbonates, which turns lime water milky. Acid is always added slowly to water, never water to acid, because mixing releases great heat.

Indicators

Litmus, obtained from lichens, is red in acid and blue in base. Turmeric turns reddish-brown in base, so a turmeric stain turns red when washed with soap; red cabbage and some flower petals are other natural indicators. Phenolphthalein is colourless in acid and pink in base, and methyl orange is red in acid and yellow in base. Olfactory indicators change smell: onion, vanilla and clove oil cannot be smelt in a basic solution. A universal indicator shows a range of colours that give the pH.

The pH scale

pH, introduced by S. P. L. Sørensen in 1909, measures hydrogen ion concentration on a scale of 0 to 14: 7 is neutral, below 7 acidic and above 7 basic, each step being a tenfold change. Approximate values: gastric juice 1.2, lemon juice 2.2, pure water 7, blood 7.4, milk of magnesia 10 and sodium hydroxide solution 14. NCERT says our body works within pH 7.0 to 7.8; blood itself is kept close to 7.4. Tooth decay starts when the pH in the mouth falls below 5.5, which is why toothpastes are basic, and rain with a pH below 5.6 is acid rain. Antacids such as milk of magnesia and baking soda relieve acidity, baking soda soothes an acidic bee sting, and lime corrects acidic soil.

Common salts

A salt forms when an acid neutralises a base. Electrolysis of brine (common salt solution), the chlor-alkali process, gives sodium hydroxide, chlorine and hydrogen. Baking soda (NaHCO3) releases carbon dioxide on heating and makes cakes soft and spongy; baking powder is baking soda with a mild edible acid such as tartaric acid; baking soda is also an antacid and is used in soda-acid fire extinguishers. Washing soda (Na2CO3.10H2O) removes the permanent hardness of water and is used in making glass, soap and paper. Bleaching powder (CaOCl2), made by passing chlorine over dry slaked lime, bleaches cotton and wood pulp and disinfects drinking water. Plaster of Paris (CaSO4.1/2H2O), made by heating gypsum (CaSO4.2H2O) at 373 K, sets hard with water and is used for plasters on fractured bones, for statues and for smooth ceilings. Blue copper sulphate crystals (CuSO4.5H2O) turn white on heating as they lose their water of crystallisation.

Key facts

Combination: CaO + H2O gives Ca(OH)2 (whitewash, exothermic); thermal decomposition: CaCO3 gives CaO + CO2; electrolysis of water gives hydrogen and oxygen in the ratio 2:1 by volume.
Silver chloride turns grey in sunlight (decomposition by light, used in black-and-white photography); an iron nail turns copper sulphate solution from blue to green (displacement).
Oxidation: gain of oxygen, loss of hydrogen or loss of electrons; reduction: the reverse; CuO + H2 gives Cu + H2O is a redox reaction.
Rust is hydrated iron(III) oxide; silver tarnishes black (silver sulphide); copper turns green (basic copper carbonate); chips packets are filled with nitrogen to prevent rancidity.
Natural acids: vinegar - acetic; lemon, orange - citric; tamarind - tartaric; tomato - oxalic; curd - lactic; ant and nettle sting - methanoic (formic); stomach - hydrochloric.
Litmus (from lichen): red in acid, blue in base; phenolphthalein: colourless in acid, pink in base; methyl orange: red in acid, yellow in base; turmeric: reddish-brown in base.
pH: gastric juice about 1.2, lemon juice about 2.2, pure water 7, blood about 7.4, milk of magnesia about 10; tooth decay below 5.5; acid rain below 5.6.
Aqua regia = 3 parts concentrated HCl + 1 part concentrated HNO3; it dissolves gold and platinum.
Baking soda NaHCO3 (cakes, antacid, fire extinguisher); washing soda Na2CO3.10H2O (hard water, glass); bleaching powder CaOCl2 (disinfects water); plaster of Paris CaSO4.1/2H2O (fractures, statues).

Quick revision

Combination, decomposition, displacement, double displacement; electrolysis of water gives H2 and O2 in 2:1; respiration exothermic, photosynthesis endothermic; oxidation = gain of oxygen or loss of electrons; rust is hydrated iron oxide; nitrogen in chips packets stops rancidity; alkalis are soluble bases; tomato oxalic, curd lactic, ant sting formic; phenolphthalein pink in base; pH 7 neutral, blood 7.4, acid rain below 5.6; aqua regia 3 HCl : 1 HNO3; baking soda NaHCO3; bleaching powder CaOCl2; POP from gypsum at 373 K.

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