Cambridge O Level Chemistry · Syllabus 5070 · Acids, Bases and Salts
Acid
What is Acid?
A substance that acts as a proton donor. In aqueous solution an acid produces hydrogen ions, H+(aq), and it 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.
This definition is part of the Acids, Bases and Salts chapter in Cambridge O Level Chemistry.
Acid in context
Acids, bases and salts are three connected ideas: an acid is a proton donor that produces hydrogen ions, H+(aq), in solution; a base is a proton acceptor — a metal oxide or metal hydroxide — that neutralises an acid to form a salt and water; and a salt is the compound formed when the hydrogen ion of an acid is replaced by a metal ion or the ammonium ion. An alkali is simply a base that is soluble in water, so every alkali is a base but not every base is an alkali, since insoluble bases such as copper(II) oxide neutralise acids without ever forming a solution.
Common mistakes with Acid
- “A base is the same thing as an alkali.” Repair A base is a metal oxide or metal hydroxide. 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.
- “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.
- “Neutralisation always ends at pH 7.” Repair Neutralisation is defined by what reacts — an acid with a base — 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 requires. Thymolphthalein is colourless in acid and blue in alkali. The three required 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 statement 7.1.12 is asking for.
- “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.
- “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.
- “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.
Examiner tips on Acid
- The coefficient that gets dropped Sulfuric acid releases two hydrogen ions per formula unit, so the coefficient 2 in front of H+(aq) is not decoration — without it the charges do not balance, because SO42− carries 2−. Check charge as well as atoms on every ionic equation you write.
Questions students ask about Acid
What is the difference between a base and an alkali?
A base is a metal oxide or metal hydroxide that neutralises an acid; an alkali is a base that is also soluble in water. Copper(II) oxide is a base — it reacts with acid to give a salt and water — but it is insoluble, so no solution of it exists to turn litmus blue, and it is not an alkali.
What is the difference between a strong acid and a concentrated acid?
Strong describes how completely the acid dissociates into ions in solution: a strong acid dissociates completely, a weak acid only partially. Concentrated describes how much acid is dissolved per unit volume. The two are independent, so a dilute strong acid and a concentrated weak acid can both exist — hydrochloric, nitric and sulfuric are the strong acids on this syllabus, and ethanoic acid is weak.
Why can't you make a soluble salt from an acid and an alkali by filtering?
Both the acid and the alkali are dissolved, and so is the salt formed, so there is no undissolved solid for a filter to remove. The exact volume of alkali that reacts with the acid must instead be found by titration, using an indicator, and then the same volumes are mixed again without indicator so the crystals are not contaminated.
Why does acid plus carbonate not give hydrogen gas?
Fizzing shows a gas is forming but does not by itself say which gas. The carbonate ion, CO3(2-), supplies the carbon and oxygen that become carbon dioxide, so acid plus a carbonate gives a salt, water and carbon dioxide. Only an acid reacting with a reactive metal releases hydrogen gas, because the metal has no oxygen or hydroxide to form water with.
What are the colours of the three required indicators in acid and in alkali?
Litmus is red in acid and blue in alkali. Thymolphthalein is colourless in acid and blue in alkali. Methyl orange is red in acid and yellow in alkali. Each indicator must be learned as a complete pair, since half a pair — such as remembering only that methyl orange is red — is not enough to answer a question correctly.