Sec 3 Combined Science (Chemistry) Acids & Bases notes

An acid is a substance that produces hydrogen ions, H⁺(aq), when dissolved in water. Hydrochloric acid gives H⁺ and Cl⁻; sulfuric acid gives H⁺ and SO₄²⁻; nitric acid gives H⁺ and NO₃⁻; ethanoic acid gives H⁺ and CH₃COO⁻. Without water there are no mobile H⁺ ions — which is why dry hydrogen chloride gas does not turn indicator red, but its solution does.

An alkali is a soluble base that produces hydroxide ions, OH⁻(aq), in water. Sodium hydroxide gives Na⁺ and OH⁻; potassium hydroxide gives K⁺ and OH⁻; calcium hydroxide gives Ca²⁺ and OH⁻; aqueous ammonia reacts with water to give NH₄⁺ and OH⁻. Every alkali shares the same working ion: OH⁻ — exactly as every acid shares H⁺.

Both families affect Universal Indicator — a mixture of dyes that shows a smooth colour change across the whole pH range, unlike litmus which only flips red/blue. Acids turn it red, orange or yellow; alkalis turn it blue, indigo or violet.

The pH scale runs from 0 to 14 and ranks solutions by their relative acidity or alkalinity. A neutral solution — pure water is the standard example — has pH 7, where the H⁺ and OH⁻ concentrations are equal. Below 7 the solution is acidic (more H⁺ than OH⁻); above 7 it is alkaline (more OH⁻ than H⁺).

"Relative acidity" is a comparison, not a measurement: a solution of pH 2 is more acidic than one of pH 5 because it has the greater concentration of H⁺ ions. You are never asked to calculate a pH in Combined Science — you only need to order solutions, read a colour, and state which ion is in excess.

Worked equations for each pattern: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g) and Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) show the metal pattern. CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) shows a base dissolving in acid. CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g) shows the carbonate pattern — the reaction that eats limestone buildings in acid rain.

The salt is the ionic compound left when the acid's H⁺ has been replaced by a metal or ammonium ion: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates, ethanoic acid makes ethanoates.

Neutralisation is what happens when an acid meets an alkali: the hydrogen ion and the hydroxide ion combine to make a water molecule, written as the ionic equation H⁺(aq) + OH⁻(aq) → H₂O(l). The other ions (for example Na⁺ and Cl⁻ from NaOH and HCl) are spectators that form the salt in solution. When exactly equal amounts react, the solution is neutral — neither ion is in excess, and Universal Indicator shows green.

Gas identification closes the loop: hydrogen burns with a squeaky pop with a lighted splint; carbon dioxide turns limewater milky (it reacts with dissolved calcium hydroxide to make insoluble calcium carbonate). Those two tests decide almost every "identify the gas" question in this topic.

Most crop plants grow best in slightly acidic to neutral soil (roughly pH 6–7). Soil becomes too acidic through acid rain, heavy fertiliser use and the decay of organic matter. When the pH drops well below 6, nutrients dissolve and wash away and the crop suffers.

The treatment is liming: spreading calcium hydroxide, Ca(OH)₂ (slaked lime) on the field. Its hydroxide ions neutralise the excess H⁺ in the soil water — H⁺ + OH⁻ → H₂O — lifting the pH back towards neutral. The choice of calcium hydroxide matters: it is a soluble base, cheap, and mildly alkaline, so it works without making the soil caustic.

Control is the point: too much lime overshoots and leaves the soil too alkaline, which locks up nutrients in a different way and damages the crop. Farmers test the soil pH first and add only the calculated amount.

A base is a metal oxide or hydroxide that neutralises acids. If it dissolves in water it is called an alkali (NaOH, KOH, Ca(OH)₂, aqueous NH₃). Insoluble bases such as copper(II) oxide, CuO, and iron(III) oxide, Fe₂O₃, are bases but not alkalis — they still react with acids, just without producing an alkaline solution on their own.

Two characteristic reactions: a base + acid → salt + water (CuO + H₂SO₄ → CuSO₄ + H₂O), and — the one that surprises students — a base + ammonium salt → salt + water + ammonia. Heating is needed: Ca(OH)₂(s) + 2NH₄Cl(s) → CaCl₂ + 2H₂O(l) + 2NH₃(g). Farmers exploit this: spreading lime (a base) at the same time as an ammonium fertiliser wastes the fertiliser, because the ammonia escapes as a gas before the crop can use it.

The ammonia test: it is the only common alkaline gas — it turns damp red litmus blue. In the reaction above the hydroxide ions displace ammonia from the ammonium ion, NH₄⁺ + OH⁻ → NH₃ + H₂O — the mirror image of neutralisation.

An oxide is a compound of an element with oxygen. The classification follows the metal/non-metal split of the parent element, with a short exceptions list you memorise:

Amphoteric means "both ways": zinc oxide reacts with hydrochloric acid (ZnO + 2HCl → ZnCl₂ + H₂O) and with sodium hydroxide (ZnO + 2NaOH → Na₂ZnO₂ + H₂O). Aluminium oxide behaves the same. The neutral oxides carbon monoxide and water sit outside every reaction pattern in this chapter — they are the distractors, not the players.