Acid = pH below 7, three salt reactions
An acid has an aqueous solution of pH less than 7 and shows three characteristic reactions, each forming a salt: acid + metal salt + hydrogen (bubbles of gas that pop with a lit splint); acid + base salt + water; acid + carbonate salt + water + carbon dioxide (effervescence; the gas turns limewater milky). Acids turn litmus red and methyl orange red.
Base = metal oxide or metal hydroxide
A base is a metal oxide or a metal hydroxide; copper(II) oxide and iron(III) oxide are insoluble ones. An alkali is a base that dissolves in water -- sodium hydroxide, potassium hydroxide, aqueous ammonia. Every alkali is a base; not every base is an alkali. Any base reacts with an acid to give a salt and water only: . Alkalis turn litmus blue and methyl orange yellow.
Universal indicator gives a pH number
Universal indicator (paper or solution) shows a range of colours; read against a pH colour chart each colour gives a pH number on a scale running from below 0 to above 14. pH below 7 is acidic (red/orange/yellow -- the lower the number, the more strongly acidic), pH 7 is neutral (green), pH above 7 is alkaline (blue/violet/purple). Neutralisation is the reaction of an acid with an alkali to give a salt and water only; no ionic equation is required at 0654.
Drawn from real examiner reports.
pH change needs numbers, not words
Asked how the pH changes as alkali is added to an acid, candidates write "it is neutralised" or "it becomes less acidic". Both describe an outcome, not the change. The mark scheme wants the scale used: the pH number rises from a low value towards 7, and keeps rising past 7 if excess alkali is added.
Flagged Jun 2022 P41 Q5ai
Colour alone is not a pH answer
Universal indicator questions ask you to compare relative acidity or alkalinity. Giving the colour alone ("A went orange, B went green") misses the second step the question needs: match each colour to a pH number on the pH colour chart, then compare the numbers -- "A is pH 3-4, B is pH 7, so A is the more acidic".
Flagged Nov 2023 P11 Q19 · Jun 2023 P31 Q11aii
Sulfate ≠ sulfide ≠ sulfite
Sulfuric acid always gives a sulfate salt: the polyatomic ion passes through the reaction intact and is never broken apart. Potassium hydroxide with sulfuric acid gives potassium sulfate, -- not potassium sulfide or sulfite, which are different sulfur-containing ions. Nitrate/nitrite is the same trap.
Flagged Jun 2022 P41 Q5aiii
Metal oxides basic, non-metal acidic
Oxide classification is often given the wrong way round. Metal oxides are basic -- they react with acids, and where soluble their solutions have a pH above 7. Non-metal oxides such as carbon dioxide and sulfur dioxide are acidic, giving a pH below 7. Predict from whether the element forming the oxide is a metal or a non-metal.
Flagged Jun 2023 P11 Q22 · Nov 2023 P11 Q20
Every alkali is a base, not vice versa
"Base" and "alkali" are not interchangeable. A base is any metal oxide or metal hydroxide, soluble or not; only a soluble base is an alkali. Writing "an alkali is a base" is true but incomplete and drops the mark -- the word soluble has to appear. Copper(II) oxide is a base but not an alkali; sodium hydroxide is both.
Dropping the water from salt + water
Water is a product of every acid + base, acid + alkali and acid + carbonate reaction, and it is routinely left out -- sodium carbonate with sulfuric acid written as giving only the salt and carbon dioxide. Neutralisation produces a salt and water only, so the water is half the answer, not an optional extra.
Insoluble does not mean unreactive
"Not an alkali" does not mean "will not react". Copper(II) oxide is insoluble in water, so it is not an alkali -- but it is a metal oxide, therefore a base, and it dissolves in warm dilute acid to give a salt and water. Solubility decides only whether a base is ALSO called an alkali, never whether it reacts with an acid.
Classify the reactant, then predict
Identify what the acid is reacting with before writing anything: a metal gives salt + hydrogen, a base or alkali gives salt + water, a carbonate gives salt + water + carbon dioxide. The whole product pattern follows from that one classification.
Build the salt name from the acid
The salt takes its second word from the acid, unchanged: hydrochloric gives a -chloride, sulfuric a -sulfate, nitric a -nitrate. The first word comes from the metal in the base, metal or carbonate used. Never invent a different anion.
Pick the indicator the question needs
Litmus and methyl orange only answer "acid or alkali?". Universal indicator with a pH colour chart answers "how acidic?". If the question says compare, or asks for a pH value, reach for universal indicator -- and quote both the colour AND the number it maps to.
Cambridge 0654 spec reference: Section C7 "Acids, bases and salts", sub-topic C7.1 (Core only -- no Extended/Supplement statement for this leaf). Covers the characteristic reactions of acids (with metals, bases, carbonates) and bases (with acids), the effect of acids/alkalis on litmus and methyl orange, the base/alkali distinction, comparing acidity/alkalinity with universal indicator and the pH scale, and neutralisation.
| Term | Mark-scheme-precise meaning |
|---|---|
| Acid | A substance whose aqueous solution has pH less than 7; reacts with metals, bases and carbonates to form salts; turns litmus red and methyl orange red |
| Base | A metal oxide or metal hydroxide; reacts with an acid to form a salt and water only |
| Alkali | A soluble base -- dissolves in water; turns litmus blue and methyl orange yellow |
| Salt | A compound formed when the hydrogen ion of an acid is replaced by a metal (or ammonium) ion |
| pH scale | A numerical scale, running from below 0 to above 14, that indicates how acidic or alkaline a solution is |
| Neutralisation | The reaction between an acid and an alkali to produce a salt and water only (no ionic equation required at 0654) |
Characteristic reactions (Core, all form a salt):
| Acid reacts with | General equation | Extra product/observation |
|---|---|---|
| a metal | acid + metal salt + hydrogen | bubbles of gas; pops with a lit splint |
| a base | acid + base salt + water | insoluble base dissolves/disappears as it reacts |
| a carbonate | acid + carbonate salt + water + carbon dioxide | effervescence; gas turns limewater milky |
| an alkali (= neutralisation) | acid + alkali salt + water | pH of the mixture rises toward 7 |
Naming the salt: the salt's name always combines the metal/base/carbonate name with the acid's characteristic ending -- hydrochloric acid "-chloride"; sulfuric acid "-sulfate"; nitric acid "-nitrate". The polyatomic ion from the acid (e.g. ) stays intact through the reaction.
Define neutralisation.
(a) Zinc granules are added to dilute sulfuric acid; bubbles of gas are given off. Write the balanced symbol equation for this reaction. (2 marks)
(b) Sodium carbonate powder is added to dilute sulfuric acid; effervescence occurs and the gas turns limewater milky. Write the balanced symbol equation for this reaction. (2 marks)
