Oxides and Preparation of Salts
ChemistryΒ· 7.2, 7.3Β· 25 min read
1. Classification of Oxides (Core)β β ββββ± 7 min
Basic oxide
Metal oxide that reacts with acids to form salt and water. Most are insoluble in water, except group 1 metal oxides and calcium oxide which react with water to form alkalis.
Example:
Copper(II) oxide (CuO), magnesium oxide (MgO), sodium oxide (NaβO)
Acidic oxide
Non-metal oxide that reacts with bases/alkalis to form salt and water. Most dissolve in water to form acidic solutions.
Example:
Carbon dioxide (COβ), sulfur dioxide (SOβ), phosphorus(V) oxide (PβOββ)
Neutral oxide
Non-metal oxide that does not react with acids or bases, forms neutral solutions when dissolved in water.
Example:
Carbon monoxide (CO), water (HβO), dinitrogen oxide (NβO)
You can classify any oxide by testing its reaction with dilute acid and dilute sodium hydroxide solution: if it reacts with acid only, it is basic; if it reacts with alkali only, it is acidic; if it reacts with neither, it is neutral.
Classify the following oxides as acidic, basic or neutral: a) magnesium oxide, b) carbon dioxide, c) carbon monoxide
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Step 1: Identify if each oxide is metal or non-metal, and check its reaction properties.
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Step 2a: Magnesium is a metal, magnesium oxide reacts with dilute hydrochloric acid to form magnesium chloride and water: it is basic.
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Step 2b: Carbon is a non-metal, carbon dioxide reacts with sodium hydroxide solution to form sodium carbonate and water: it is acidic.
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Step 2c: Carbon monoxide does not react with either dilute acids or alkalis: it is neutral.
Exam tip:
Core candidates do not need to learn amphoteric oxides for this section, save that for the Extended part of the guide.
2. Preparation of Soluble Salts (Core)β β β βββ± 8 min
Soluble salts are made using either the excess insoluble reactant method (if the base/carbonate/metal is insoluble) or the titration method (if the reactants are all soluble, e.g. group 1 salts or ammonium salts).
Excess insoluble reactant method
Method for making soluble salts where you add excess insoluble base/carbonate/metal to warm dilute acid, filter off unreacted solid, then evaporate the filtrate to get salt crystals.
Example:
Making copper(II) sulfate from copper(II) oxide and dilute sulfuric acid.
Describe the practical steps to make pure, dry copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid.
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Step 1: Warm 50 cmΒ³ of dilute sulfuric acid in a beaker over a Bunsen burner (do not boil).
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Step 2: Add copper(II) oxide a little at a time, stirring, until no more dissolves (copper oxide is in excess, all acid has reacted).
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Step 3: Filter the mixture to remove unreacted copper(II) oxide, collect the blue copper sulfate filtrate in an evaporating dish.
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Step 4: Heat the filtrate gently to evaporate half the water to make a concentrated solution.
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Step 5: Leave the concentrated solution to cool and crystallise, filter off the crystals, wash with a small amount of cold distilled water, then pat dry between filter papers.
Exam tip:
Always mention that you add excess insoluble reactant to make sure all acid is used up, so your salt is not contaminated with unreacted acid.
3. Preparation of Insoluble Salts (Core)β β ββββ± 5 min
Insoluble salts are made using the precipitation method, which involves mixing two soluble salt solutions that contain the ions needed to form the insoluble salt.
Precipitation reaction
Reaction where two soluble reactants in solution form an insoluble solid product (precipitate) that can be separated by filtration.
Example:
Making lead(II) iodide from lead(II) nitrate solution and potassium iodide solution.
Write the word equation and balanced symbol equation for the formation of insoluble barium sulfate from barium nitrate and sodium sulfate solutions.
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Step 1: Write the word equation: barium nitrate + sodium sulfate β barium sulfate + sodium nitrate
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Exam tip:
Always include state symbols in your equations for precipitation reactions: (aq) for soluble reactants, (s) for the insoluble precipitate product.
4. Extended Only: Amphoteric Oxides & Advanced Salt Preparationβ β β β βExtended onlyβ± 7 min
Amphoteric oxide
Metal oxide that reacts with both strong acids and strong bases to form salt and water.
Example:
Aluminium oxide (AlβOβ), zinc oxide (ZnO), lead(II) oxide (PbO)
For example, zinc oxide reacts with dilute hydrochloric acid (acid) to form zinc chloride and water, and also reacts with concentrated sodium hydroxide (strong base) to form sodium zincate and water.
Write balanced symbol equations for the reaction of zinc oxide with a) dilute hydrochloric acid, b) concentrated sodium hydroxide solution.
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Step a: Reaction with acid: zinc oxide + hydrochloric acid β zinc chloride + water
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Step b: Reaction with base: zinc oxide + sodium hydroxide β sodium zincate + water
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Exam tip:
Extended candidates are expected to recall the three common amphoteric oxides: aluminium oxide, zinc oxide and lead(II) oxide, you do not need to learn others for this syllabus.
5. Common Pitfalls
Wrong move:
Classifying all metal oxides as basic, including zinc oxide for Extended candidates.
Why:
Zinc, aluminium and lead oxides are amphoteric, not basic, so they react with both acids and bases.
Correct move:
For Core, metal oxides are basic; for Extended, memorise the 3 amphoteric metal oxides as exceptions.
Wrong move:
Using the titration method to make copper sulfate from copper oxide and sulfuric acid.
Why:
Copper oxide is insoluble, so titration (used for all-soluble reactants) is unnecessary and inefficient.
Correct move:
Use the excess insoluble reactant method for salts made from insoluble bases/carbonates/metals.
Wrong move:
Forgetting to add excess insoluble reactant when making soluble salts.
Why:
If you do not add excess, the resulting salt will be contaminated with unreacted acid.
Correct move:
Add insoluble reactant until no more dissolves, then filter off the excess to get pure salt solution.
Wrong move:
Using solid reactants for precipitation reactions to make insoluble salts.
Why:
Precipitation requires ions to be free in solution to react, solid reactants will not form the precipitate effectively.
Correct move:
Dissolve both reactants in distilled water first to make aqueous solutions before mixing.
Wrong move:
Classifying water as an acidic or basic oxide.
Why:
Water is a neutral oxide, it does not react with either acids or bases to form salts.
Correct move:
Classify water, carbon monoxide and dinitrogen oxide as neutral oxides in all exam questions.
6. Quick Reference Cheatsheet
Category | Key Properties | Examples | Reaction/Preparation Steps |
|---|---|---|---|
Acidic oxide | Non-metal, reacts with bases/alkalis, forms acidic solution in water | COβ, SOβ, PβOββ | Forms salt + water when reacted with base |
Basic oxide | Metal, reacts with acids, most insoluble in water | CuO, MgO, NaβO | Forms salt + water when reacted with acid |
Neutral oxide | Non-metal, no reaction with acids/bases, neutral solution | CO, HβO, NβO | No reaction with acids or bases |
Amphoteric oxide (Extended) | Metal, reacts with both acids and strong bases | AlβOβ, ZnO, PbO | Forms salt + water with both acid and base |
Soluble salt (insoluble reactant method) | Made from insoluble base/carbonate/metal | CuSOβ, ZnClβ | Add excess reactant, filter, evaporate filtrate, crystallise |
Insoluble salt (precipitation method) | Made from two soluble reactants | BaSOβ, AgCl | Mix aqueous reactants, filter precipitate, wash, dry |
7. Frequently Asked
Which salt preparation method should I use for insoluble salts?
Use precipitation: mix two soluble salt solutions that contain the ions of the insoluble salt you want to make, filter, wash and dry the residue. This method is assessed in both Core and Extended papers.
Do I need to memorise solubility rules for this topic?
Yes, you are expected to recall common solubility rules for IGCSE 0620: all nitrates are soluble, common chlorides are soluble except silver and lead chloride, common sulfates are soluble except barium, lead and calcium sulfate, all group 1 salts and ammonium salts are soluble.
Are amphoteric oxides tested in Core papers?
No, amphoteric oxides are only part of the Extended (Supplement) syllabus, so Core candidates do not need to learn their properties or examples.
Going deeper
What's Next
Now that you have mastered oxide classification and salt preparation methods, you are ready to advance through the Acids, Bases and Salts unit for CIE IGCSE Chemistry 0620. First, practice writing balanced symbol equations for all reactions covered, including state symbols, as these are frequently assessed in both Paper 2 (Core) and Paper 4 (Extended) theory exams. Next, practice structured practical questions that ask you to outline steps for making a given salt, as these carry high marks. Extended candidates should also learn to write ionic equations for precipitation reactions, which are commonly tested in higher-tier papers. Regularly revise solubility rules to quickly select the correct preparation method for any salt in exams.