Soluble salts — AQA GCSE Combined Science
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Soluble salts explained
A soluble salt can be prepared by reacting an acid with an insoluble solid: a metal, metal oxide, hydroxide or carbonate.
Read the full explanation
The solid is added in small portions to the warm acid and stirred until no more dissolves, so the acid is fully used up and a little solid remains. That excess solid is then removed by filtration, leaving a solution containing the salt. The method works because the solid is insoluble while the salt is soluble, so unreacted solid can be separated physically. For example, copper(II) oxide is added to warm dilute sulfuric acid until excess black powder settles; filtration gives copper(II) sulfate solution. The solution can then be crystallised to obtain the solid salt.
Salt solutions can be crystallised to produce solid salts.
Crystallisation recovers a dissolved salt from its solution by removing the solvent. A salt solution contains water plus dissolved ions, for example sodium chloride solution contains Na⁺ and Cl⁻ ions. Gently warming the solution evaporates some water, increasing the concentration until the solution becomes saturated. Once saturated, further evaporation removes more solvent, or cooling lowers the solubility limit of the salt. Both processes cause the dissolved salt to exceed its solubility and precipitate out of solution as solid crystals. The crystals are separated from the remaining liquid, then dried. This links to soluble salt preparation: after reacting an acid with a base, the solution is filtered to remove excess solid, then crystallised to give a pure, dry solid salt.
Students should be able to describe how to make pure, dry samples of named soluble salts from information provided.
To make a pure, dry sample of a named soluble salt, choose the right acid and solid reactant so the required salt forms. For example, copper(II) sulfate can be made from copper(II) oxide and dilute sulfuric acid. Warm the acid, add the solid in small amounts until it is in excess, then filter to remove unreacted solid. The filtrate contains the salt solution. Crystallise it by gentle evaporation and cooling, then separate and dry the crystals. The excess solid ensures all the acid is used up, so the product is not contaminated by acid. Naming the salt correctly depends on the acid used: hydrochloric acid gives chlorides, sulfuric acid gives sulfates, and nitric acid gives nitrates.
Required practical activity 8: 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.
This practical prepares a soluble salt from an insoluble oxide or carbonate. You warm dilute acid using a Bunsen burner, add the insoluble solid in excess, and stir until no more dissolves. Filter to remove unreacted solid, keeping the filtrate. Then crystallise the filtrate by gentle evaporation using a water bath or electric heater, not a direct Bunsen flame, to avoid spitting and decomposition. Cool, filter off crystals, and dry them between filter papers or in a warm place. For example, copper(II) oxide with dilute sulfuric acid gives blue copper(II) sulfate crystals. The method separates the soluble salt from insoluble impurities and controls evaporation to obtain a pure, dry sample.
Your focus
- Describe a method to prepare a soluble salt from an acid and an insoluble solid.
- Explain why excess solid is added and how filtration separates it from the salt solution.
- Identify suitable reactants for producing a named soluble salt.
Show all 12 objectives
- Describe how a salt solution can be crystallised to produce a solid salt.
- Explain why evaporation and cooling cause crystals to form from a saturated solution by exceeding the solubility limit.
- Identify the steps needed to separate and dry the crystals safely.
- Describe a method to prepare a pure, dry sample of a named soluble salt.
- Explain why excess solid reactant is used and why the mixture is filtered.
- Name the salt formed from a given acid and solid reactant.
- Describe the steps to prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate.
- Explain why excess insoluble solid is added and why the mixture is filtered.
- Explain how gentle evaporation and cooling produce crystals and how drying removes water.
Soluble salts exam tips
Marking Points
- Choose an insoluble reactant that matches the required salt, for example a metal, metal oxide, hydroxide or carbonate of the appropriate metal.
- Add the solid in small portions to the acid, warming if needed, and stir until no more dissolves.
- Recognise that excess solid is added deliberately so that all the acid reacts and the solution is neutral.
- Filter the mixture to remove unreacted excess solid, collecting the salt solution as the filtrate.
- Identify that the salt remains in solution because it is soluble, while the excess reactant is removed as residue.
- Link the method to a later step such as crystallisation or evaporation to obtain the solid salt.
- Crystallisation separates a dissolved salt from its solution by removing water, usually through gentle heating and then cooling.
- Evaporating some water makes the solution saturated; further evaporation removes more solvent, causing crystals to form.
- Cooling a saturated solution lowers the solubility limit of the salt, causing the solid to precipitate out as crystals.
- The solid salt is separated from the remaining solution, for example by filtration or by decanting off the liquid.
- The crystals are dried, for example in a warm place or using filter paper, to obtain a dry solid sample.
- Select the correct acid and solid reactant to produce the named salt, for example sulfuric acid and copper(II) oxide for copper(II) sulfate.
- Add the solid reactant in excess to the warm acid so that all the acid reacts and no acid remains in the product.
- Filter the mixture to remove the excess unreacted solid, keeping the filtrate as the salt solution.
- Crystallise the filtrate by gentle evaporation and cooling, then separate and dry the crystals to obtain a pure, dry sample.
- Name the salt from its ions, for example copper(II) sulfate contains Cu²⁺ and SO₄²⁻ ions.
- Warm the dilute acid using a Bunsen burner to increase the rate of reaction safely before adding the solid.
- Add the insoluble oxide or carbonate in excess, a little at a time, until no more dissolves, so all the acid is neutralised.
- Filter the mixture to remove unreacted solid, collecting the filtrate containing the soluble salt.
- Evaporate the filtrate slowly using a water bath or electric heater to the point of crystallisation, then cool to form crystals.
- Separate crystals by filtration and dry them between filter papers or in a warm place to obtain a pure, dry sample.
Examiner Tips
- 💡Name the specific reactants and the salt produced, rather than describing the method only in general terms.
- 💡Explain the purpose of each step: excess solid ensures complete reaction; filtration separates unreacted solid from the salt solution.
- 💡Use the terms residue and filtrate precisely when describing separation by filtration.
- 💡If asked why the mixture is warmed or stirred, link this to increasing the rate of reaction and helping the solid dissolve.
- 💡Name the equipment and the key step: evaporating basin, gentle heating, then cooling to form crystals.
- 💡Explain why gentle heating is used rather than boiling to dryness, linking to safety and to avoiding decomposition.
- 💡State clearly that the crystals are separated and dried, so the final product is described as pure and dry.
- 💡Link the named salt to its acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
- 💡Use the words excess, filter, filtrate, crystallise and dry in the correct order.
- 💡Explain why excess solid is used: it ensures all the acid reacts, so the salt is not contaminated.
- 💡State clearly why excess solid is used: to ensure all the acid reacts, so the filtrate contains only the salt and water.
- 💡Describe evaporation as gentle, using a water bath or electric heater, and explain that this prevents spitting or decomposition.
- 💡When asked about purity, refer to removing unreacted solid by filtration and drying the crystals to remove water.
Common Mistakes
- Adding all the solid at once and assuming the reaction is complete. Correction: add portions gradually and stop when solid remains undissolved, showing the acid is used up.
- Filtering before excess solid has been added, leaving unreacted acid in the solution. Correction: continue adding solid until no more reacts, then filter.
- Choosing a soluble reactant such as sodium hydroxide solution, which cannot be removed by filtration. Correction: select an insoluble metal, oxide, hydroxide or carbonate for this method.
- Confusing residue and filtrate. Correction: the excess solid is the residue on the filter paper; the salt solution is the filtrate.
- Heating the solution strongly until all the water boils away: this can spit and may decompose some salts; the correction is to evaporate gently to the point of crystallisation and then let crystals form.
- Thinking the solid salt is made by a chemical reaction during crystallisation: crystallisation is a physical separation, not a new reaction; the salt was already present as dissolved ions.
- Stating that the salt becomes insoluble during crystallisation. Correction: the salt remains soluble, but its solubility limit is exceeded as solvent is removed or the solution is cooled, causing it to precipitate.
- Adding too little solid reactant so acid remains: the correction is to add solid until no more dissolves, ensuring it is in excess.
- Filtering before the reaction is complete or filtering the wrong part: the filtrate is the salt solution, while the residue is the excess solid.
- Heating the filtrate strongly to dryness: the correction is gentle evaporation followed by cooling, so pure crystals form rather than a decomposed or spattered product.
- Heating the solution directly with a Bunsen flame during evaporation: this can cause spitting, loss of product, or decomposition. Correction: use a water bath or electric heater for gentle evaporation.
- Adding insufficient oxide or carbonate: some acid remains, so the salt is contaminated. Correction: add the solid in excess until no more dissolves.
- Evaporating to dryness before cooling: this may decompose the salt or trap impurities. Correction: evaporate to the point of crystallisation, then cool slowly.