Cambridge IGCSE Chemistry · Syllabus 0620 · Acids, Bases and Salts
Acid
What is Acid?
A substance whose aqueous solution contains hydrogen ions, H+(aq). An acid reacts with reactive metals to give a salt and hydrogen, with bases to give a salt and water, and with carbonates to give a salt, water and carbon dioxide, and it turns litmus red.
This definition is part of the Acids, Bases and Salts chapter in Cambridge IGCSE Chemistry.
Common mistakes with Acid
- “A base is the same thing as an alkali.” Repair A base is an oxide or a hydroxide of a metal. An alkali is a base that dissolves in water. Copper(II) oxide is a base — it neutralises acids — but it is insoluble, so it is not an alkali and no solution of it exists to turn litmus blue.
- “Neutralisation always ends at pH 7.” Repair Neutralisation is defined by what reacts — an acid with an alkali, producing water — not by a number. A mixture only sits at pH 7 when exactly enough of each has been added and the salt formed does not itself make the solution acidic or alkaline. Add one drop too much acid and the reaction was still a neutralisation.
- “Acid plus carbonate gives hydrogen — you can see it fizzing.” Repair Fizzing tells you a gas is made, not which gas. Acid + carbonate gives carbon dioxide, along with a salt and water. Only acid + a reactive metal gives hydrogen.
- “Methyl orange is purple in alkali.” Repair Methyl orange is red in acid and yellow in alkali. Purple belongs to litmus in a neutral solution. Learn each indicator as a complete pair; half a pair is worth nothing in a multiple-choice question.
- “Thymolphthalein is the pink one.” Repair That is phenolphthalein, a different indicator and not one of the three this syllabus names. Thymolphthalein is colourless in acid and blue in alkali. The three named indicators are litmus, thymolphthalein and methyl orange.
- “Universal indicator just tells you acid or alkali.” Repair Litmus does that. Universal indicator gives a graded colour, so it lets you compare how acidic or alkaline two solutions are, and match the colour to an approximate pH. That is what Core statement 7.1.7 is asking for.
- “Mix the acid and the alkali, then filter off the salt.” Repair There is nothing solid to filter. Both reactants are dissolved and so is the salt. That is precisely why an acid–alkali preparation must be done by titration — you have to measure the right volume, because you cannot remove an excess afterwards.
- “A strong acid is a concentrated acid, or a corrosive one.” Repair Strong describes the extent of dissociation in aqueous solution: complete for a strong acid, partial for a weak one. Concentrated describes how much acid there is per unit volume. They are independent, so a dilute strong acid and a concentrated weak acid both exist.
- “Amphoteric means neutral — it sits in the middle.” Repair The opposite. An amphoteric oxide reacts with both acids and bases to give a salt and water. A neutral oxide reacts with neither. Aluminium oxide and zinc oxide are the two you need; they are defined by evidence of reaction, never by position on a table.
- “Magnesium + hydrochloric acid → magnesium chloride + water.” Repair A metal has no oxygen and no hydroxide to give, so no water can be made. The hydrogen released by the acid has nowhere to go except into H2(g). Products: salt + hydrogen.
- “Copper(II) oxide + sulfuric acid → copper(II) sulfate + hydrogen.” Repair The oxide ion accepts the hydrogen ions, so they end up as water, not as hydrogen gas. Copper is also below hydrogen in the reactivity series and would not displace it anyway. Products: salt + water, and no effervescence at all.
- “It fizzed, so calcium carbonate + acid must give hydrogen.” Repair The carbonate ion CO32− supplies the carbon and oxygen for carbon dioxide. Test it: the gas turns limewater milky and does not pop. Products: salt + water + carbon dioxide.