Soluble salts — AQA GCSE Chemistry
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Soluble salts explained
Soluble salts covers 5 specification points, including Soluble salts can be made from acids by… and The solid is added to the acid until no more….
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Open a point below for its explanation and revision guidance.
Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates.
A soluble salt can be prepared by reacting an acid with an insoluble solid. The solid may be a metal, a metal oxide, a metal hydroxide or a metal carbonate. The acid dissolves the solid and a salt forms in solution. For example, copper oxide reacts with sulfuric acid to make copper sulfate, and magnesium reacts with hydrochloric acid to make magnesium chloride. The method works because the solid is insoluble, so it can be added in excess and then removed by filtration. The reaction stops when no more solid dissolves. The resulting solution contains the soluble salt, which can be crystallised. This is a key preparation method for soluble salts in the laboratory.
The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt.
In this preparation, the insoluble solid is added gradually to the acid. The solid reacts and dissolves as the salt forms. You keep adding the solid until no more will react, which shows that all the acid has been used up. At this point there is excess solid sitting at the bottom of the mixture. The mixture is then filtered. The filter paper traps the unreacted excess solid, while the solution of the soluble salt passes through as the filtrate. This gives a clean salt solution without leftover acid. The solution can then be evaporated or left to crystallise to obtain the solid salt. For example, adding copper oxide to warm sulfuric acid until no more dissolves, then filtering, gives copper sulfate solution.
Salt solutions can be crystallised to produce solid salts.
Crystallisation recovers a dissolved salt as a solid. A salt solution contains water plus dissolved ions, for example sodium chloride from hydrochloric acid neutralised by sodium hydroxide. Gently warming evaporates some water, raising the concentration until the solution becomes saturated. Cooling then lowers solubility for many salts, so crystals form; the solid is separated by filtration, washed with a little cold distilled water to remove soluble impurities, and dried between filter papers or in a warm oven. Strong heating must be avoided because it can drive off water of crystallisation or decompose the salt. The method works because solubility usually falls as temperature falls, so the dissolved salt leaves the solution as an ordered solid lattice.
Students should be able to describe how to make pure, dry samples of named soluble salts from information provided.
To make a named soluble salt, choose a dilute acid and an insoluble reactant (base, carbonate, or metal) that supply the required ions. For copper(II) sulfate, warm dilute sulfuric acid and add insoluble copper(II) oxide until it is in excess, ensuring all the acid reacts. Filter off the unreacted solid oxide to leave a solution containing only the salt and water. Evaporate some water to reach saturation, cool to crystallise, filter, wash with a little cold distilled water and dry gently. The excess solid ensures no unreacted acid remains, and filtration removes the excess solid, giving a pure salt. The same pattern works for other named salts, such as magnesium sulfate from magnesium oxide and sulfuric acid.
Required practical 1: preparation of a pure, dry sample of a soluble salt from an insoluble oxide or carbonate using a Bunsen burner to heat dilute acid and a water bath or electric heater to evaporate the solution.
In this required practical, warm dilute acid using a Bunsen burner, then add the insoluble oxide or carbonate a little at a time until it is in excess and no more dissolves. Filter the mixture to remove the excess solid, keeping the filtrate, which contains the soluble salt. Evaporate the filtrate using a water bath or electric heater rather than a direct flame, because gentle heating avoids spitting and decomposition. Stop when crystals begin to appear, then cool, filter, wash with a little cold distilled water and dry gently. The practical develops safe heating, filtration and crystallisation skills while producing a pure, dry named salt such as copper(II) sulfate or magnesium sulfate.
Your focus
- Describe how a soluble salt can be prepared from an acid and an insoluble solid.
- Select suitable reactants to produce a named soluble salt.
- Explain why the solid is added in excess and removed by filtration.
Show all 15 objectives
- Describe how to add an insoluble solid to an acid until no more reacts.
- Explain why the excess solid is filtered off and how this produces a salt solution.
- Identify the filtrate as the solution of the soluble salt.
- Describe how a salt solution is concentrated and cooled to form crystals.
- Explain why crystals are washed with cold distilled water and dried gently.
- Apply the crystallisation sequence to a named soluble salt.
- Select suitable reactants to prepare a named soluble salt.
- Describe the sequence that produces a pure, dry sample of that salt.
- Explain how excess solid and filtration improve the purity of the product.
- Carry out the preparation of a named soluble salt from an insoluble oxide or carbonate.
- Explain why a water bath or electric heater is used instead of a direct flame.
- Produce and dry a pure sample, justifying each separation step.
Soluble salts exam tips
Marking Points
- Choose an insoluble solid reactant that contains the required positive ion, such as a metal, metal oxide, metal hydroxide or metal carbonate.
- Choose the correct acid to supply the required anion, for example hydrochloric acid for a chloride salt or sulfuric acid for a sulfate salt.
- Add the solid to the acid and observe the reaction, which may produce bubbles if a carbonate is used or heat if a metal reacts.
- Continue adding the solid until no more dissolves or reacts, showing that the acid has been neutralised.
- Filter the mixture to remove the excess insoluble solid and collect the solution containing the soluble salt.
- Obtain the solid salt from the solution by evaporation or crystallisation.
- Add the insoluble solid a little at a time to the acid, stirring to help it react.
- Continue adding until no more solid reacts or dissolves, indicating that the acid has been neutralised.
- Observe that excess solid remains in the mixture after the reaction is complete.
- Filter the mixture using filter paper and a funnel to separate the excess solid from the salt solution.
- Collect the filtrate, which is the solution of the soluble salt.
- Explain that filtration removes the excess insoluble solid but keeps the soluble salt in solution.
- A solution is warmed to evaporate some solvent and produce a saturated or nearly saturated solution.
- The concentrated solution is left to cool so that crystals of the salt form.
- Crystals are separated from the remaining solution by filtration.
- Crystals are washed with a small volume of cold distilled water to remove soluble impurities.
- Crystals are dried gently, for example between filter papers or in a warm oven, not by strong direct heating.
- The method relies on the solubility of the salt decreasing as the temperature falls.
- Select reactants that provide the required cation and anion, using an insoluble oxide, hydroxide, carbonate or metal with a dilute acid.
- Warm the dilute acid and add the insoluble solid until it is in excess, so the acid is fully neutralised.
- Filter the mixture to remove unreacted excess solid and obtain a clear salt solution.
- Evaporate some water, cool to crystallise, then filter, wash with cold distilled water and dry gently.
- Name the salt correctly and link its formula to the ions supplied by the chosen reactants.
- Explain that excess solid guarantees the acid is used up, improving purity of the product.
- Warm the dilute acid using a Bunsen burner before adding the insoluble oxide or carbonate.
- Add the solid in small portions until it is in excess and no more dissolves.
- Filter the mixture and retain the filtrate as the salt solution.
- Evaporate the filtrate using a water bath or electric heater, not a direct Bunsen flame.
- Stop evaporation when crystals begin to appear, then cool, filter, wash and dry the crystals.
- Use the named salt and its reactants correctly, for example copper(II) oxide with dilute sulfuric acid.
Examiner Tips
- 💡State clearly that the solid must be insoluble so it can be removed by filtration.
- 💡Describe adding the solid in excess until no more reacts, then filtering to remove the excess.
- 💡Link the choice of solid to the positive ion needed in the salt and the choice of acid to the negative ion needed.
- 💡Mention crystallisation or evaporation as the final step to obtain the solid salt.
- 💡Use the phrase 'until no more reacts' or 'until in excess' to show you know when to stop adding solid.
- 💡State that the excess solid is filtered off and the filtrate is the salt solution.
- 💡Explain that filtration works because the excess solid is insoluble while the salt is soluble.
- 💡Mention that the salt solution can be crystallised to obtain the solid salt.
- 💡Name the salt and the solution it comes from so the answer is specific rather than generic.
- 💡Sequence the steps clearly: evaporate some water, cool, filter, wash, dry.
- 💡Explain why each step is done, especially why gentle heating and cold washing are used.
- 💡State the named salt and give the two reactants with their formulae.
- 💡Use the words excess, filter, crystallise, wash and dry in a logical order.
- 💡Explain the purpose of each key step rather than listing steps only.
- 💡Learn the order: warm acid, add solid to excess, filter, evaporate gently, cool, filter, wash, dry.
- 💡Name the equipment used for gentle evaporation and explain why a direct flame is unsuitable.
- 💡Link each step to its purpose, such as excess solid ensuring complete neutralisation.
Common Mistakes
- Choosing a soluble solid such as sodium hydroxide when the method requires an insoluble solid; the correction is to use an insoluble base such as copper oxide or an insoluble carbonate.
- Adding too little solid so that acid remains; the correction is to add solid until no more reacts, then filter off the excess.
- Filtering before the reaction is complete, which leaves unreacted acid in the solution; the correction is to wait until no further reaction occurs before filtering.
- Using a metal that reacts dangerously or not at all, such as sodium, instead of a suitable metal like magnesium; the correction is to select a metal that reacts safely and is appropriate for the salt required.
- Stopping the addition of solid too early, which leaves unreacted acid in the solution; the correction is to add until no more reacts.
- Filtering the mixture before the reaction is complete, which wastes reactants and leaves acid; the correction is to wait until no further reaction occurs.
- Using filter paper that is too coarse or torn, which lets solid through; the correction is to use a clean, correctly folded filter paper.
- Evaporating the filtrate to dryness too quickly, which can decompose the salt or cause spitting; the correction is to evaporate slowly or crystallise.
- Evaporating the solution to complete dryness over a strong flame: this can decompose the salt or remove water of crystallisation, so the solution should be evaporated only until crystals begin to appear and then cooled.
- Washing crystals with hot water: this redissolves much of the product, so cold distilled water should be used sparingly.
- Describing filtration as the step that forms crystals: filtration only separates crystals already formed, so crystallisation must be explained as the cooling and saturation step.
- Adding too little solid so acid remains: the salt would be contaminated with acid, so the solid must be added until no more dissolves.
- Choosing two soluble reactants (like NaOH and HCl) and adding an excess of one: the excess soluble reactant cannot be filtered off and will contaminate the target salt. An insoluble reactant must be used, or a titration performed.
- Forgetting to filter before crystallising: unreacted solid would contaminate the crystals, so filtration must remove it first.
- Heating the evaporating basin directly with a roaring Bunsen flame: this can spit or decompose the salt, so a water bath or electric heater should be used.
- Discarding the filtrate and keeping the residue: the soluble salt is in the filtrate, so the filtrate must be kept.
- Adding all the solid at once and assuming it has reacted: excess solid must remain undissolved to show the acid is used up, so it should be added gradually.