Salt Preparation Worked Examples | Hougang Secondary 4 Chemistry

How do you work through actual salt-preparation questions in Secondary 4 Chemistry? Begin with the product and test its solubility, then apply the correct route to a concrete example. Copper(II) sulfate can be made from a suitable insoluble copper compound and sulfuric acid; sodium sulfate from appropriately measured acid and soluble alkali; and insoluble barium sulfate from suitable solutions whose ions form a precipitate. Each worked case is a test of reaction, solubility and recovery—not just a list of apparatus.

For Hougang families seeking Chemistry tuition, soluble and insoluble salts worked examples, practical revision or G3 K324 subject support, this page focuses on four worked cases and the reasons behind each step. A student who knows the names of methods but cannot explain why a solution is filtered or a precipitate washed needs diagnosis before another practical worksheet. Compare this page with the salt-preparation decision-tree guide for the general method, and match any exam practice to the student’s actual syllabus and year.

The real question is not “Which method do I remember?”

Picture a student in Hougang looking at a school question: “Describe how to prepare a pure dry sample of copper(II) sulfate crystals.” The student remembers filtration and crystallisation but cannot decide which comes first.

Here is the reason: they are choosing apparatus without tracking the materials. Is there an excess solid to remove? Is the desired product currently dissolved? Would the product pass through filter paper? Which substance must stay and which must leave?

Once the student maps those questions, the order of operations stops being mysterious. Chemistry becomes a series of justified decisions.

The simple principle is that preparation makes the desired salt and separation and purification isolate it from unwanted materials. These are connected stages but they answer different questions.

Start with four useful solubility rules

At school level, a short set of reliable generalisations helps students choose the route. The detailed rules include exceptions that the syllabus expects learners to recognise.

• All common nitrates are soluble. If the target is a nitrate, it normally belongs to the soluble-salt category.
• Group 1 metal salts and ammonium salts are generally soluble. This helps explain why sodium or potassium salts are not usually recovered by first precipitating them from water.
• Most chlorides are soluble, with important exceptions such as silver chloride and lead(II) chloride, whose solubility is limited under typical school conditions.
• Many sulfates are soluble, with barium sulfate and lead(II) sulfate insoluble and calcium sulfate sparingly soluble.
• Most carbonates are insoluble except, for example, Group 1 and ammonium carbonates.

For hydroxides, too, solubility differs by metal. The correct method should be chosen using the stated solubility information, not a vague “all salts dissolve” assumption.

The 2027 K324 Chemistry syllabus includes the general solubility rules as part of salt preparation. Knowing the exceptions helps a student avoid an elegant method that simply cannot work for the specified product.

Worked case 1 — copper(II) sulfate using an insoluble base

Target: pure dry crystals of copper(II) sulfate, CuSO₄.

Choose dilute sulfuric acid and an insoluble copper(II) oxide. The word equation is copper(II) oxide + sulfuric acid → copper(II) sulfate + water. The balanced formula equation is:

CuO + H₂SO₄ → CuSO₄ + H₂O.

The basic school-laboratory logic is:

• Add excess copper(II) oxide to a suitable quantity of dilute sulfuric acid in supervised laboratory conditions.
• Allow the reaction to proceed with controlled warming if instructed by the teacher.
• Filter to remove unreacted solid copper(II) oxide; the copper(II) sulfate remains in the filtrate.
• Concentrate the filtrate carefully as directed, then allow it to cool so crystals can form.
• Recover the crystals and dry them using the appropriate school procedure.

Why add an excess of the insoluble oxide? To help ensure the acid has reacted completely. Any oxide left over is solid, so it can be filtered away. This choice creates a cleaner separation than adding an uncontrolled amount of soluble alkali.

Why not filter first and expect the copper(II) sulfate crystals on the paper? Because at that stage, the product is dissolved in the liquid. It passes into the filtrate, whereas the unreacted solid is retained.

That single distinction explains most sequence errors in this question.

Worked case 2 — zinc sulfate from a metal and an acid

Target: zinc sulfate, ZnSO₄.

A suitable school-level reaction is zinc + dilute sulfuric acid → zinc sulfate + hydrogen:

Zn + H₂SO₄ → ZnSO₄ + H₂.

Zinc is a reactive metal that can react with suitable dilute acid. If excess zinc remains, it can be separated from the resulting salt solution. The salt can then be crystallised using the approved laboratory technique.

A crucial safety point is that hydrogen is flammable. Experiments involving metals and acids belong in a supervised school laboratory, not a kitchen improvisation.

The reasoning question is this: why does the product solution still need attention after the visible reaction stops? Because the salt may remain dissolved and must be separated and purified. Producing a salt and collecting pure crystals are different operations.

Students sometimes suggest using copper metal and dilute sulfuric acid to prepare copper(II) sulfate under ordinary school conditions. That is not the appropriate route with dilute acid: copper does not react with dilute sulfuric acid in the same straightforward way as zinc. Choose starting materials that truly react.

Worked case 3 — sodium sulfate using acid and alkali

Target: a pure soluble sodium sulfate salt.

Sodium hydroxide and sulfuric acid are both soluble in water. Mixing them in uncontrolled excess does not leave a solid reactant that can simply be filtered out. A neutralisation or titration method can identify appropriate amounts of acid and alkali.

Balanced equation: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.

In the typical school method, an initial titration with an indicator identifies the volume of one solution required to neutralise a measured volume of the other. To prepare uncontaminated salt solution for crystallisation, repeat the neutralisation using the known corresponding volumes without indicator, then concentrate and crystallise appropriately.

The indicator is useful for detecting the endpoint in the preliminary titration, but leaving an indicator in a preparation intended to produce a pure salt would be undesirable.

The important understanding is why we cannot filter excess sodium hydroxide out of the final solution: it is soluble. A different separation logic is required.

Why does the balanced equation matter here?

The equation tells us that two moles of NaOH react with one mole of H₂SO₄. Equal concentrations do not therefore imply equal reacting volumes, because the stoichiometric ratio is 2:1. If the concentrations and volumes are known, the student can use n = cV to calculate amounts and check the neutralisation ratio.

It is the same Chemistry that appears in mole calculations and later titration questions, not an isolated laboratory trick.

Worked case 4 — barium sulfate by precipitation

Target: insoluble barium sulfate, BaSO₄.

Select two suitable soluble starting salts that provide Ba²⁺ and SO₄²⁻ ions in solution, for example barium nitrate solution and sodium sulfate solution.

Balanced equation:

Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq).

The ionic equation is:

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).

A white insoluble precipitate forms. In the standard school procedure, filtration recovers the solid precipitate, washing removes soluble impurities, and appropriate drying gives the final sample.

Notice how filtration plays a different role from the copper(II) sulfate example. There, filtration removes an unwanted insoluble reactant while the desired product passes through in solution. Here, filtration keeps the insoluble product on the filter paper.

That is exactly why students should reason from solubility and the location of the desired substance.

Barium compounds require proper laboratory control because soluble barium salts are hazardous. This example is for classroom explanation, not home experimentation.

A decision route that works in unfamiliar questions

When the exam names a salt you have not memorised, follow the same order each time.

First identify the ions in the salt and write its correct formula. Check that the overall ionic charge balances.

Second check the target salt’s solubility. If it is insoluble, consider precipitation from two suitable soluble solutions. If it is soluble, consider which acid and metal, insoluble oxide, hydroxide or carbonate can provide the needed ions.

Third decide whether an excess insoluble solid can be removed by filtration. If both reactants are soluble, consider whether a titration-style neutralisation is appropriate.

Fourth explain purification. A dissolved product may require controlled concentration, cooling and crystallisation; a precipitate may require filtration, washing and drying. A pure dry salt is not automatically obtained merely because a reaction took place.

Finally, write a balanced equation and check that it matches the stated reactants. A correct method with the wrong formula is not a correct answer.

Five common questions that expose conceptual gaps

Why use an excess insoluble reactant?

It can ensure that the intended soluble acid is consumed; the remaining solid is separable by filtration.

Why not use an excess soluble alkali?

Any excess remains dissolved alongside the soluble salt and may contaminate the product; ordinary filtration will not remove it.

Why does cooling matter in crystallisation?

A solution made more concentrated by carefully removing solvent may produce crystals as it cools, depending on the substance’s solubility behaviour.

Why wash a precipitate?

Washing with an appropriate amount of suitable solvent removes dissolved contaminants from the surface of the precipitated solid. Using too much of an unsuitable solvent could introduce losses or other problems.

Why are solubility rules part of salt preparation?

They tell us whether our chosen route can produce an insoluble solid to collect, or whether we must instead isolate a soluble product from solution.

Those five answers form the causal chain of the whole topic.

Observation is not identification

A white precipitate is an observation. It may be consistent with barium sulfate in the right experimental context, but many white precipitates exist. The scientific conclusion depends on the reagents, the ion chemistry and, when necessary, confirmatory tests.

Likewise, fizzing when a carbonate reacts with acid is an observation. To support identification of carbon dioxide, the appropriate school gas test uses limewater. Students should avoid writing “carbon dioxide was observed” unless the experimental evidence actually supports it.

For a focused companion, use Hougang Chemistry Qualitative Analysis: six worked cases. That guide strengthens the crucial difference between something visible and a justified chemical identification.

What does SEAB K324 expect?

For the 2027 SEC G3 pure Chemistry subject code K324, SEAB explicitly covers acid–base reactions, solubility rules and preparation, separation and purification of salts. The methods include precipitation and titration, as well as reactions of acids with metals, insoluble bases and insoluble carbonates. Students must be able to suggest methods from suitable starting materials.

These are the relevant syllabus boundaries: SEAB 2027 G3 Chemistry directory and K324 Chemistry official syllabus. For 2026 GCE O-Level students, check the relevant 6092 syllabus; for combined routes under the SEC, use the correct G2/G3 paired-Science syllabus.

There is no benefit in making a learner master every imaginable industrial salt-preparation technology when the immediate problem is choosing between precipitation, titration and excess-solid methods.

A useful four-session revision plan

Session 1: sort ten named salts by solubility. Explain the exceptions and identify the ions needed for each salt.

Session 2: practise the copper(II) sulfate and zinc sulfate examples. For every step, name the substance being kept and removed.

Session 3: work through sodium sulfate neutralisation and barium sulfate precipitation. Explain why filtration has different purposes in the two cases.

Session 4: solve mixed questions without seeing the chapter heading. One question should ask students to critique a wrong method such as “filter sodium chloride out of a clear sodium chloride solution”.

Parents should ask for the reasoning rather than an impressive number of worksheets completed. The student has mastered the route when a new salt name does not cause panic.

What a three-student Chemistry lesson can do

In a carefully structured three-student lesson, one learner might struggle to identify ions, another with solubility, and a third with purification. The tutor can address each missing link while letting students compare and defend proposed methods.

Ask for evidence of independence. Can the student choose a route for an unfamiliar but syllabus-appropriate salt, explain the separation, and write the correct equation without the answer beside them? That is a far more useful signal than how many pages were covered.

For programme questions and parent consultations, see Hougang Chemistry tuition guide and Hougang Secondary 4 Chemistry support. The Hougang Science Learning Hub relates laboratory learning to matter, water, materials and the real world. Confirm actual class availability and location before making travel or timetable decisions.

Frequently asked questions

Can I prepare every soluble salt by titration?

No. Titration is particularly useful when reacting soluble acids and alkalis in controlled proportions. Some salts are more suitably prepared by reacting a dilute acid with an excess insoluble oxide, base, carbonate or suitable metal.

Do I filter before or after crystallising?

It depends on which substance is being separated. Filter off unwanted excess insoluble reactant before concentrating a soluble salt solution. For an insoluble precipitate, filtration collects the desired solid.

Are all sulfates soluble?

No. There are important insoluble or sparingly soluble examples, including barium sulfate, lead(II) sulfate and calcium sulfate. The rule must be learned with its exceptions.

How do I know if a salt is pure?

School-level techniques can include appropriate separation and purification, followed by suitable tests depending on the substance and question. A product merely looking crystalline does not prove absolute purity.

Where should a Hougang student continue?

Move from acid–base chemistry to ionic equations, mole calculations, qualitative analysis and practical technique. Read Chemistry Qualitative Analysis in Hougang, How to Study Chemistry and eduKate Sengkang Science Hub. Once a student can say what remains dissolved, what becomes a solid and how each stage changes the sample, salt preparation becomes a sensible plan rather than a memorised laboratory script.