Ch 3 · Preparation of Salts · IGCSE Chemistry 0620
Topic 3 of 3 in Acids, Bases and Salts — IGCSE Chemistry 0620 — 44 lessons.
Introduction
A salt is the compound formed when the hydrogen of an acid is replaced by a metal or ammonium ion. Making a pure, dry sample of a named salt is a favourite Paper 6 practical and Paper 4 six-marker. The method depends on one question: is the salt soluble, and if so, are its starting materials soluble too? Learn the solubility rules first, then the three methods.
Key ideas — Solubility rules
An insoluble salt is made by precipitation. A soluble salt is made from an acid by titration (if the base is soluble) or by adding an excess of an insoluble solid.
🧑🏫 Teacher's explanation
Think of solubility like a club membership. Some ions, like Sodium (Na^+) and Potassium (K^+), are 'VIPs'—they are so friendly that they will dissolve with almost any partner you give them. Nitrates (NO_3^-) are also VIPs. If you see these in a formula, the salt is definitely soluble.
📖 Definitions
🔍 Worked example — Easy
Predict whether Silver Chloride (AgCl) is soluble or insoluble based on the solubility rules.
🔍 Worked example — Standard
A student wants to prepare a pure sample of Barium Sulfate (BaSO_4). Which method should they use: titration or precipitation?
🔍 Worked example — Exam-style
Calculate the mass of Sodium Nitrate (NaNO_3) produced when 50.0 g of NaOH reacts completely with excess HNO_3. Assume M_r of NaOH = 40, HNO_3 = 63, NaNO_3 = 85.
⚠️ Common errors
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🎯 Exam question
(a) State whether the salt is soluble or insoluble in water. [1] (b) Describe the steps the student should take to prepare this salt, including the method of formation and the steps to obtain the pure dry solid. [4] [5 marks]
Key ideas — Soluble salt from acid and an excess insoluble solid
Used when the salt is soluble but the metal, base or carbonate is insoluble, for example copper(II) sulfate from copper(II) oxide or zinc chloride from zinc.
🧑🏫 Teacher's explanation
Imagine you are making a sugary syrup. If you want a very concentrated syrup, you add sugar to hot water until no more will dissolve. In chemistry, we do something similar to make a salt solution. We use an acid as our 'solvent' and add a solid (like a metal or an oxide) that cannot dissolve.
📖 Definitions
⚠️ Common errors
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🎯 Exam question
Key ideas — Soluble salt from acid and alkali by titration
Used for sodium, potassium and ammonium salts, because the alkali and the salt are both soluble, so an excess could not be filtered off. Instead the exact volume of acid needed is found by titration.
🧑🏫 Teacher's explanation
Imagine you are making a perfectly flavored drink by mixing syrup and water. If you add too much syrup, it's too sweet; too little, and it's too watery. In chemistry, when we want to make a salt from an acid and an alkali, we face a problem: both the reactants and the resulting salt are soluble in water. If we just added 'excess' alkali like we do with insoluble metals, we would end up with a messy mixture of salt and leftover alkali that we can't easily remove by filtration.
📖 Definitions
🔍 Worked example — Easy
A student uses 25.0 cm³ of 0.10 mol/dm³ NaOH and finds it requires 20.0 cm³ of 0.10 mol/dm³ HCl to reach the end point. Calculate the moles of NaCl produced.
🔍 Worked example — Standard
Calculate the mass of NaCl produced in the reaction above. (Ar: Na=23, Cl=35.5)
🔍 Worked example — Exam-style
A student performs a titration. They use 25.0 cm³ of 0.50 mol/dm³ KOH and find it requires 30.0 cm³ of H_2SO_4. Write the balanced equation for the reaction.
⚠️ Common errors
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🎯 Exam question
Key ideas — Insoluble salts, hydrated and anhydrous substances
🧑🏫 Teacher's explanation
Think of making an insoluble salt like making jelly from powder. When you mix two clear liquids together, if the product isn't soluble, it 'crashes out' of the solution as a solid cloud or precipitate. You can't use titration for this because there is no color change to watch; you simply mix the ions and watch the solid appear.
📖 Definitions
🔍 Worked example — Easy
Write a balanced net ionic equation for the formation of silver chloride (s) from silver nitrate (aq) and sodium chloride (aq).
🔍 Worked example — Standard
A student heats 25.0 g of hydrated copper(II) sulfate, ceCuSO4 · 5H2O, until all water of crystallisation is removed. Calculate the mass of the anhydrous ceCuSO4 produced.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. A student prepares hydrated zinc sulfate crystals, ZnSO_4 · 7H_2O, by adding zinc carbonate to 25.0 cm³ of 1.00 mol/dm³ sulfuric acid.
🔍 Worked example — Easy
Calculate the number of moles in 30.0 cm³ of 0.500 mol/dm³ HCl.
🔍 Worked example — Standard
A student reacts 2.00 g of magnesium carbonate (M_r = 84) with excess sulfuric acid. Calculate the mass of magnesium sulfate (M_r = 120) produced. Equation: MgCO_3 + H_2SO_4 arrow MgSO_4 + H_2O + CO_2
🔍 Worked example — Exam-style
A student prepares hydrated copper(II) sulfate, CuSO_4 · 5H_2O (M_r = 249.5), by reacting 25.0 cm³ of 2.00 mol/dm³ H_2SO_4 with excess copper(II) oxide. Calculate the maximum mass of the hydrated salt obtained.
⚠️ Common errors
Common mistakes
Practice
Answers Warm the acid and add magnesium oxide until in excess [M1]; filter to remove unreacted oxide [M1]; heat the filtrate to the point of crystallisation, shown by crystals forming on a glass rod [M1]; cool, filter and dry the crystals [M1]; MgO + H_2SO_4 arrow MgSO_4 + H_2O [B1]. Potassium hydroxide and potassium carbonate are soluble, so any excess would remain dissolved and could not be filtered off [B1]; titration [B1]. Lead(II) nitrate and sodium chloride (or any soluble chloride) [B1]; mathrmPb^2+(aq) + 2Cl^-(aq) arrow PbCl_2(s) [B1]; filter off the precipitate [B1]; wash with distilled water and dry [B1].