Acids, Bases and Salts

Acid-base indicator

A dye, or mixture of dyes, that shows by a colour change whether a substance is acidic or basic. Litmus (from lichen), turmeric and red cabbage are natural indicators; methyl orange and phenolphthalein are synthetic ones.

Olfactory indicator

A substance whose smell, rather than its colour, changes in an acidic or basic medium. Onion, vanilla essence and clove oil can be used, which lets the test be read by smell alone.

Alkali

A base that dissolves in water, producing hydroxide (OH-) ions. Not every base is an alkali, because many bases are insoluble. Alkalis are soapy to touch, bitter and corrosive.

Neutralisation reaction

The reaction of an acid with a base to give a salt and water, each cancelling the other's effect. In terms of ions it is simply H+(aq) + OH-(aq) --> H2O(l).

pH scale

A scale from 0 to 14 measuring hydrogen ion concentration in a solution. 7 is neutral, below 7 is acidic and above 7 is basic; the higher the hydronium ion concentration, the lower the pH.

Strong acid

An acid that produces a large number of H+ ions in solution. Hydrochloric acid is strong while acetic acid of the same concentration gives far fewer H+ ions and is weak, so strength is not the same as concentration.

Water of crystallisation

The set number of water molecules bound into each formula unit of a salt. Blue copper sulphate crystals, CuSO4·5H2O, turn white when heating drives this water off and turn blue again when it is added back.

Plaster of Paris

Calcium sulphate hemihydrate, CaSO4·½H2O, made by heating gypsum at 373 K. With water it sets back into gypsum as a hard mass, which is why it supports fractured bones and must be stored away from moisture.

Bleaching powder

A compound represented as Ca(ClO)2, made by passing chlorine over dry slaked lime. It bleaches cotton, linen and wood pulp, acts as an oxidising agent in industry, and makes drinking water free of germs.

Baking soda

Sodium hydrogencarbonate, NaHCO3: a mild, non-corrosive basic salt. Mixed with tartaric acid it forms baking powder, whose carbon dioxide makes cakes rise; it is also used in antacids and soda-acid fire extinguishers.

Washing soda

Sodium carbonate decahydrate, Na2CO3·10H2O, obtained by recrystallising sodium carbonate. It removes permanent hardness of water, and is used in the glass, soap and paper industries and to make borax.

Why does dry HCl gas not show acidic behaviour, while its solution in water does?

Acidic properties come from H+ ions, and HCl separates into ions only in the presence of water, forming hydronium ions (H3O+). Dry HCl gas therefore leaves dry blue litmus unchanged, but turns wet litmus red.

While diluting a concentrated acid, why must the acid be added to water and not water to the acid?

Dissolving an acid in water is highly exothermic. Adding water to concentrated acid releases so much heat locally that the mixture can splash out and burn, and the glass container may crack; adding acid slowly to water with stirring spreads the heat.

Why does an aqueous solution of an acid conduct electricity, while glucose and alcohol solutions do not?

Current in a solution is carried by ions. Acids release H+(aq) ions and anions in water, so the bulb in the test circuit glows; glucose and alcohol contain hydrogen but do not ionise, so they carry no current and are not acids.

How can you predict whether a salt solution will be neutral, acidic or basic?

From its parent acid and base. A strong acid with a strong base gives a neutral salt (pH 7); a strong acid with a weak base gives an acidic salt (below 7); a strong base with a weak acid gives a basic salt (above 7).

How does a change in pH cause tooth decay, and how is it prevented?

Bacteria break down sugar and food particles left in the mouth into acids. Once the pH falls below 5.5, tooth enamel (calcium hydroxyapatite) corrodes. Cleaning the mouth after eating, with a toothpaste that is generally basic, neutralises the excess acid.

How does an antacid relieve pain from indigestion?

The stomach produces hydrochloric acid for digestion, and during indigestion it produces too much, causing pain and irritation. An antacid is a mild base, such as milk of magnesia (magnesium hydroxide), that neutralises the excess acid.

What happens when dilute hydrochloric acid is added to sodium carbonate, and how is the gas identified?

Carbon dioxide is given off with effervescence: Na2CO3(s) + 2HCl(aq) --> 2NaCl(aq) + H2O(l) + CO2(g). Passed through lime water it forms a white precipitate: Ca(OH)2(aq) + CO2(g) --> CaCO3(s) + H2O(l).

What is the chlor-alkali process, and what is its equation?

Electricity is passed through brine, which decomposes: 2NaCl(aq) + 2H2O(l) --> 2NaOH(aq) + Cl2(g) + H2(g). Chlorine forms at the anode, hydrogen at the cathode and sodium hydroxide near the cathode; the name comes from chlorine and alkali.

How is bleaching powder prepared, and what is the equation?

Chlorine gas is passed over dry slaked lime: 2Ca(OH)2 + 2Cl2 --> Ca(ClO)2 + CaCl2 + 2H2O. The chlorine used usually comes from the chlor-alkali process.

What happens when baking soda is heated during cooking, and what is the equation?

It decomposes to sodium carbonate, water and carbon dioxide: 2NaHCO3 --heat--> Na2CO3 + H2O + CO2. Recrystallising the sodium carbonate formed this way gives washing soda.

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