Acids, Bases & Salts

An acid is a substance that tastes sour and produces hydrogen ions when dissolved in water, and these few properties identify acids wherever we find them.

• Acids turn blue litmus red — the standard exam test for an acid.
• Acids release hydrogen ions (H⁺) in aqueous solution — this is the source of all their acidic behaviour.
• Acids have a pH less than 7 (pH < 7).
• Acids react with metals to release hydrogen gas (H₂), which burns with a "pop" sound.
• Acids react with carbonates and bicarbonates to give carbon dioxide gas (CO₂).
• Acids are corrosive and can damage skin, cloth and metal.

Acids are sorted in two useful ways — by how completely they ionise, and by where they come from.

• A weak acid ionises only partially in water and releases few H⁺ ions — e.g. CH₃COOH (acetic acid), carbonic acid H₂CO₃.

• Mineral (inorganic) acids are made from minerals and are usually strong — e.g. HCl, H₂SO₄, HNO₃.
• Organic acids occur in plants and animals and are usually weak — e.g. citric, lactic, tartaric, acetic and oxalic acids.

Many acids we name in the exam are organic acids found in everyday foods, alongside the strong mineral acids of the laboratory.

Sulphuric acid is so central to industry that the level of its production is taken as a measure of a country's industrial strength.

• It is a strong mineral acid and a powerful dehydrating agent.
• Used in car batteries (lead-acid batteries), fertilizers, detergents and metal refining.
• Concentrated H₂SO₄ chars sugar by removing water from it, showing its dehydrating nature.
• Adding water to acid is safe, but acid must never have water added the other way as it boils violently.

A base is the chemical opposite of an acid — it tastes bitter, feels soapy, and produces hydroxide ions in water.

• Bases turn red litmus blue — the standard exam test for a base.
• Bases release hydroxide ions (OH⁻) in aqueous solution.
• Bases have a pH greater than 7 (pH > 7).
• A base that dissolves in water is called an alkali — e.g. NaOH, KOH.
• Strong bases are corrosive (caustic) and can burn skin.

The important bases of the exam are the hydroxides, several of which are strong alkalis used in industry and at home.

The pH scale puts a number on how acidic or basic a solution is, running from strongly acidic at one end to strongly basic at the other.

• The pH scale runs from 0 to 14.
• pH = 7 is neutral (pure water).
• pH < 7 means acidic; the lower the value, the stronger the acid.
• pH > 7 means basic (alkaline); the higher the value, the stronger the base.
• The pH scale was introduced by S. P. L. Sorensen.

The pH of our surroundings matters for health, farming and survival, which is why it appears so often in the exam.

• Tooth decay begins when mouth pH falls below 5.5 — acid attacks the enamel.
• Antacids (mild bases) neutralise excess stomach acid to relieve acidity.
• Plants grow best in soil of suitable pH — too acidic or too alkaline harms crops.
• Acid rain has a pH below 5.6 and damages buildings, soil and aquatic life.
• A bee sting is acidic (cured by a base like baking soda); a wasp sting is basic (cured by a mild acid like vinegar).

An indicator is a substance that changes colour to reveal whether a solution is an acid or a base.

• Phenolphthalein stays colourless in acid and turns pink in base — a key exam fact.
• The universal indicator gives the whole range of pH colours, not just acid vs base.
• Olfactory indicators (e.g. onion, vanilla) change smell instead of colour.

When an acid and a base are mixed, they cancel each other's character and produce a salt and water.

• Example: HCl + NaOH → NaCl + H₂O (acid + base → common salt + water).
• The H⁺ of the acid and the OH⁻ of the base combine to form water.
• The reaction is exothermic — it releases heat.
• Everyday uses: antacids for acidity, lime to treat acidic soil, toothpaste to neutralise mouth acid.

A salt is the compound formed when the hydrogen of an acid is replaced by a metal, and salts can themselves be acidic, basic or neutral.

• A salt of a strong acid and strong base is neutral (pH 7) — e.g. NaCl.
• A salt of a strong acid and weak base is acidic (pH < 7) — e.g. NH₄Cl.
• A salt of a weak acid and strong base is basic (pH > 7) — e.g. Na₂CO₃.
• Water of crystallisation is the fixed water present in a crystalline salt — e.g. blue vitriol CuSO₄·5H₂O.

The everyday salts below are favourites of the exam, each with a common name, a formula and a familiar use.

A few of these salts carry extra facts that the exam loves to test.

• Used in soda-acid fire extinguishers because it releases CO₂.

• Used to remove permanent hardness of water and for cleaning.
• Has 10 molecules of water of crystallisation.

• Made by passing chlorine over slaked lime Ca(OH)₂.
• Used to disinfect drinking water and bleach cotton/linen.

• Made by heating gypsum to about 100°C (373 K).
• Sets into a hard solid on adding water, used for casts and statues.

• On heating it loses water and turns white; adding water turns it blue again.
• Used as a fungicide and in electroplating.