Salt preparation

Preparation of salts. Learn how to decide which method to use and how o carry out that method according to the mark scheme.

WHAT METHOD

• Insoluble salt → precipitation

• Soluble Na⁺ / K⁺ / NH₄⁺ salt → titration

• Other soluble salts → acid + excess metal or insoluble base

Describe how to carry out an acid–alkali titration

M1 rinse burette with acid and fill it,

M2 use a pipette to measure a fixed volume of alkali into a conical flask,

M3 add a few drops of suitable indicator,

M4 slowly add acid from the burette while swirling,

M5 stop when indicator changes colour and record the final reading

Describe how to prepare a soluble salt from an acid and an insoluble base

M1 warm the dilute acid,

M2 add excess insoluble base until no more reacts,

M3 filter to remove unreacted solid,

M4 heat filtrate to evaporate water,

M5 allow solution to cool and crystallise

Describe how to prepare a soluble salt from an acid and a metal

M1 add excess metal to dilute acid,

M2 wait until fizzing stops,

M3 filter to remove excess metal,

M4 evaporate solution,

M5 leave to crystallise

Describe how to prepare an insoluble salt using precipitation

M1 mix two soluble salt solutions,

M2 an insoluble precipitate forms,

M3 filter to collect the precipitate,

M4 wash with distilled water,

M5 dry the solid

Explain why excess insoluble base is added when making a soluble salt

M1 ensures all the acid reacts,

M2 excess solid can be removed by filtration

Explain why titration must be used to make sodium, potassium or ammonium salts

M1 their salts are all soluble,

M2 excess reactant cannot be filtered off

Describe how to make sodium chloride crystals using titration

M1 titrate hydrochloric acid with sodium hydroxide using an indicator,

M2 repeat without indicator using same volumes,

M3 evaporate solution to form crystals

Describe how to obtain pure dry crystals from a solution

M1 heat solution to evaporate some water,

M2 leave to cool,

M3 crystals form,

M4 filter and dry crystals

Explain why an indicator is needed in a titration

M1 shows when neutralisation has occurred,

M2 indicated by a colour change

Explain why phenolphthalein is suitable for titration

M1 colourless in acid and pink in alkali, M2 sharp colour change at the end point

Describe how to improve accuracy in a titration

M1 repeat titration several times,

M2 use concordant results,

M3 read burette at eye level

Explain what is meant by the end point in a titration

M1 the point where the indicator changes colour,

M2 shows neutralisation is complete

Explain why the indicator is not added when making the salt solution

M1 indicator would contaminate the salt,

M2 affects purity

Describe how to test for sulfate ions

M1 add dilute hydrochloric acid,

M2 add barium chloride solution,

M3 white precipitate forms

Describe how to test for chloride ions

M1 add dilute nitric acid,

M2 add silver nitrate solution,

M3 white precipitate forms

Explain why distilled water is used when washing a precipitate

M1 removes soluble impurities,

M2 does not add new ions

Describe how to prepare copper sulfate crystals in the laboratory

M1 warm sulfuric acid,

M2 add excess copper oxide,

M3 filter off excess solid,

M4 evaporate filtrate,

M5 leave to crystallise

Explain why copper oxide is suitable for making copper sulfate

M1 it is insoluble,

M2 excess can be filtered off

Describe how to know when no more base should be added

M1 base remains unreacted,

M2 no further reaction occurs

Explain why crystals are dried after filtration

M1 to remove water,

M2 obtain a dry solid

Describe how to prepare lead(II) iodide

M1 mix lead nitrate and potassium iodide solutions,

M2 yellow precipitate forms,

M3 filter and wash precipitate

Explain why lead(II) iodide is insoluble

M1 insoluble salt formed from two soluble salts,

M2 precipitates out of solution

Describe what concordant titres mean

M1 results close together,

M2 within 0.1 cm³

Explain why swirling the flask during titration is important

M1 ensures complete mixing,

M2 prevents localised reaction

Explain why burette readings are taken at eye level

M1 reduces parallax error,

M2 improves accuracy