Preparing a Soluble Salt

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 2.39 — Preparing a pure dry soluble salt from an insoluble reactant
• 2.42 — Practical: prepare pure dry hydrated copper(II) sulfate crystals from copper(II) oxide (required practical)

Specification section 2(h) Chemical tests — also taught in Titrations, Preparing an Insoluble Salt, Tests for Gases, and for Water, Flame Tests.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

To make a soluble salt you need the acid to be used up completely — any acid left over would contaminate the crystals. The difficulty is knowing when it has been used up, since both the acid and the salt are colourless solutions and nothing visible marks the moment.

When the other reactant is INSOLUBLE, there is a beautifully simple answer: add TOO MUCH of it. The acid is then certainly all used up, and the excess solid, being insoluble, can simply be filtered off. You do not need to know when to stop, because stopping accurately was never required.

Think of it like using too many teabags and then lifting them out

Making a very strong pot of tea, you could try to work out exactly how many bags are needed and drop in precisely that number. Or you could throw in plenty, wait, and fish out the ones that are left — which is easier and gives a better result, because the tea is certainly as strong as it can be. That only works because a teabag can be lifted out. If the bags dissolved, you would have to count them exactly. That is the whole logic of this page: an insoluble reactant can be used in excess and removed, and a soluble one cannot, which is why the titration method exists at all.

Statement 2.39 — the method

1. WARM the dilute acid gently in a beaker. Warming speeds the reaction up; it is not essential but it makes the preparation practical in a lesson.
2. ADD the insoluble solid — a metal, a metal oxide, a metal hydroxide or a carbonate — a little at a time, stirring.
3. CONTINUE until no more reacts and some solid remains undissolved. That leftover solid is how you KNOW the acid is used up.
4. FILTER to remove the excess solid, leaving a clear solution of the salt in the filtrate.
5. EVAPORATE the filtrate gently until crystals just begin to form at the edges — the point of crystallisation.
6. Take it off the heat there and let the basin cool undisturbed, so that large well-formed crystals grow rather than a fine powder.
7. Separate the crystals from the remaining liquid and pat them dry.

Step three is the one that carries the whole method and it is worth stating as an observation rather than an instruction: WHEN SOLID STOPS DISAPPEARING, THE ACID HAS RUN OUT. That is the signal, and it is why the excess is deliberate rather than careless.

The reaction is copper(II) oxide + sulfuric acid → copper(II) sulfate + water, or CuO + H₂SO₄ → CuSO₄ + H₂O. Black solid disappearing into a blue solution makes the progress of this one unusually easy to follow.

The required practical uses copper(II) oxide with sulfuric acid, and it is chosen partly because the colours make everything visible. The copper oxide is BLACK and does not dissolve in water; the copper sulfate solution is BLUE. So the acid disappearing and the solution turning blue happen in front of you, and the black solid sitting at the bottom at the end is unmistakable evidence of excess.

The final crystals are HYDRATED copper(II) sulfate, CuSO₄·5H₂O — blue crystals containing water of crystallisation. That is why the solution is not evaporated to dryness: driving off all the water would remove the water of crystallisation too, leaving a white anhydrous powder rather than blue crystals.

Choosing the right method from the reactants

Which method would you use to prepare (a) zinc sulfate from zinc oxide, (b) sodium chloride from sodium hydroxide, (c) magnesium chloride from magnesium metal?

1. (a) Zinc oxide is INSOLUBLE, so it can be added in excess and filtered off. Use the excess-and-filter method.
2. (b) Sodium hydroxide is an ALKALI and therefore soluble — an excess could not be filtered off, because it would dissolve. A TITRATION is required.
3. (c) Magnesium metal is insoluble and is above hydrogen, so it reacts with the acid. Excess metal can be filtered off, so use excess-and-filter.
4. The deciding question in every case is whether the reactant DISSOLVES.
5. Notice that (c) also fizzes, since acid with a metal gives hydrogen — the fizzing stopping is a second signal that the acid has been used up.

Answer: (a) and (c) by excess-and-filter; (b) by titration — decided entirely by whether the reactant dissolves.

That comparison is the point of the page. There is no judgement involved in choosing a method: look up whether the reactant dissolves, and the method follows. A soluble reactant forces a titration; an insoluble one permits the far easier excess-and-filter route.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Statement 2.39 — preparing a soluble salt from an insoluble reactant

1. Warm the dilute acid gently in a beaker.
2. Add the insoluble solid a little at a time, stirring, until no more reacts and some remains undissolved.
3. The leftover solid shows the acid has all been used up.
4. Filter to remove the excess solid; the filtrate is a solution of the salt.
5. Evaporate the filtrate gently until crystals just begin to form at the edges.
6. Leave to crystallise slowly, then filter off the crystals and dry between filter papers.

Required practical 2.42 — hydrated copper(II) sulfate crystals

1. Measure about 25 cm³ of dilute sulfuric acid into a beaker and warm it gently with a Bunsen burner, without boiling.
2. Add a spatula of black copper(II) oxide and stir; it dissolves and the solution turns blue.
3. Continue adding copper(II) oxide a little at a time until some black solid remains undissolved even after stirring.
4. This excess shows that all of the acid has reacted.
5. Filter the mixture to remove the unreacted copper(II) oxide, collecting the blue filtrate.
6. Pour the filtrate into an evaporating basin and heat gently until crystals begin to appear at the edges.
7. Remove from the heat and leave the solution to cool and crystallise slowly.
8. Filter off the blue crystals and dry them between two pieces of filter paper.

Variables

Independent (changed) — The mass of copper(II) oxide added to the acid
Dependent (measured) — Whether all the acid has reacted, judged by solid remaining undissolved

Sources of error

The vocabulary of this preparation

Filtrate

The liquid that passes through the filter paper. Here it is the solution of the salt.

Residue

The solid left on the filter paper. Here it is the excess insoluble reactant.

Hydrated salt

A salt whose crystals contain water of crystallisation, such as CuSO₄·5H₂O — blue rather than white.

Point of crystallisation

The stage at which crystals first appear at the edges of the evaporating basin. Stop heating here.

Why each step is done

• WARM the acid — speeds the reaction up so the preparation is practical
• ADD IN EXCESS — guarantees all the acid is used up, so none contaminates the crystals
• SOLID REMAINING is the signal that the acid has run out
• FILTER — removes the excess, which is only possible because the reactant is INSOLUBLE
• CRYSTALLISE SLOWLY — evaporating to dryness would give a powder and drive off the water of crystallisation

Model answer [3 marks]

Explain why an excess of copper(II) oxide is added to the sulfuric acid, and how you know when enough has been added. [3]

An excess is added so that all of the sulfuric acid is used up in the reaction, because any acid remaining would contaminate the crystals and the salt would not be pure. You know enough has been added when some black copper(II) oxide remains undissolved even after stirring, which shows there is no acid left for it to react with. The excess solid can then be removed by filtration, which is possible because copper(II) oxide is insoluble in water.

Model answer [3 marks]

Explain why the copper(II) sulfate solution is not evaporated to dryness. [3]

The crystals produced are hydrated copper(II) sulfate, which contain water of crystallisation as part of their structure. Evaporating the solution to dryness would drive off that water of crystallisation as well as the solvent, leaving a white anhydrous powder rather than the blue crystals required. Heating is therefore stopped once crystals begin to appear at the edges, and the solution is left to crystallise slowly.

Not this: Adding an excess of the solid is careless, because some reactant is wasted.

This: The excess is DELIBERATE and is the whole method. It guarantees the acid is completely used up, and the leftover solid is both the signal that this has happened and something that can simply be filtered away.

Mark-losing trap. The excess is ADDED ON PURPOSE. Solid remaining is the signal the acid has run out.

Mark-losing trap. This method works ONLY because the reactant is INSOLUBLE and can be filtered off.

Mark-losing trap. Do NOT evaporate to dryness — the water of crystallisation would be driven off.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — How do you know that enough copper(II) oxide has been added to the sulfuric acid?
2. Grade 7 · Explain [2 marks] — Explain why the excess copper(II) oxide can be removed by filtration.
3. Grade 8 · Explain [3 marks] — A student wants to prepare sodium chloride from sodium hydroxide and hydrochloric acid. Explain why the excess-and-filter method cannot be used.
4. Grade 9 · Describe [6 marks] — Select every statement that belongs in a full-mark description of how to prepare pure dry crystals of hydrated copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
5. 9+ · Analyse [6 marks] — A student prepares copper(II) sulfate but stops adding copper(II) oxide as soon as the solution turns blue. Their final crystals are blue but the yield is low and a pH test on a dissolved sample gives pH 3. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who argued that purity is something you have to demonstrate, and the one who had to invent a preparation before he could study anything. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Robert Boyle — the one who argued that purity is something you have to demonstrate

Everything on this page exists to produce a PURE salt — the excess, the filtering, the slow crystallisation — and the idea that purity is a property requiring proof rather than an assumption is Boyle's. He argued that a substance should be judged by tests rather than by its reputation or its source, and that a chemist should say what was done to establish it. Crystallisation was one of his own criteria: a well-formed crystal was evidence of a single substance, since impurities disrupt the regular shape.

• “How would you show a substance is pure?”
• “Why does a good crystal suggest a pure substance?”
• “What did chemists take on trust that they should not have?”
• “Why publish exactly how a preparation was carried out?”
• “How pure could anything be made in your laboratory?”

Humphry Davy — the one who had to invent a preparation before he could study anything

A school preparation follows a known recipe, and Davy's work was the opposite: to study potassium he first had to devise a way of making it at all, with no precedent and no certainty that it existed. He is the right person to ask about the part of a method that never appears in the write-up — deciding what to try, recognising when something has worked, and knowing whether the substance in front of you is the thing you were after or a contaminant. Those judgements are what a printed procedure quietly removes.

• “How do you make something nobody has made before?”
• “How did you know the substance was what you thought it was?”
• “What did you try that did not work?”
• “How do you tell a product from a contaminant?”
• “What does a written procedure leave out?”

Carry on through the course

• Previous lesson: The Reactions of Acids
• Next lesson: Preparing an Insoluble Salt
• Group 1 — the Alkali Metals — Metals soft enough to cut with a knife, which set fire to water — and get worse as you go down.
• Group 7 — the Halogens — A green gas, a brown liquid and a grey solid — and a trend that runs the opposite way to Group 1.
• The Gases in the Air — Four gases, one of which is doing almost all the chemistry — and how to measure it without ever touching it.
• Combustion and Carbonates — Three things burning in oxygen, one carbonate breaking apart, and the 0.04% gas that changes the climate.
• The Reactivity Series — Putting eleven metals in order by watching what each one will and will not do.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification