Cambridge IGCSE Chemistry · Syllabus 0620 · Acids, Bases and Salts
Salt
What is Salt?
A compound formed when the hydrogen ion of an acid is replaced by a metal ion or by the ammonium ion. The metal or ammonium part comes from the base, metal or carbonate used, and the negative part comes from the acid, so hydrochloric acid gives chlorides, nitric acid gives nitrates and sulfuric acid gives sulfates.
This definition is part of the Acids, Bases and Salts chapter in Cambridge IGCSE Chemistry.
Common mistakes with Salt
- “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.
- “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.
- “Filter the solution to remove the dissolved salt from the water.” Repair Filtration only separates an undissolved solid from a liquid. Dissolved ions pass straight through the paper. To recover a dissolved salt you must concentrate the solution by gentle evaporation and then let it cool and crystallise.
- “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.
- “Evaporate the mixture to make the precipitate appear.” Repair A precipitate forms the instant the two solutions meet, because the ions of an insoluble salt cannot stay in solution together. Evaporating would only dump every dissolved spectator salt on top of your product. Mix, filter, wash the residue with distilled water, dry.
- “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.
- “Ammonium chloride + sodium hydroxide → sodium chloride + ammonium hydroxide.” Repair There is no such compound as ammonium hydroxide in an answer at this level. On warming, an alkali reacts with an ammonium salt to give a salt, water and ammonia gas. Write NH3(g) and H2O(l) as two separate products.
Examiner tips on Salt
- The one habit worth building for this topic Whenever you write a preparation step, immediately write the word because after it. “Filter the mixture because the excess copper(II) oxide is insoluble and the copper(II) sulfate is dissolved.” The command word for most of statement 7.3.1 is describe, and a described method is one where each step is tied to the property that makes it work; a step with no reason is a recipe, and a recipe cannot be adapted to a salt you have not met.