Acids, bases, salts and qualitative analysis form one of the most predictable scoring areas in O-Level / SEC Chemistry — once you know the reactions of acids, the right salt-preparation method, and the standard ion tests, whole questions become routine. Many students lose marks here not because the chemistry is hard, but because they have not drilled the patterns. This guide is from Ancourage Academy, whose secondary Chemistry tuition teaches these reactions pattern-first in small groups of 3–6 at Bishan and Woodlands.
This is a single-topic deep-dive that complements our O-Level / SEC Chemistry guide and builds on our mole concept and stoichiometry guide, since titration calculations rest on the mole. See also our combined vs pure science guide.
If acids, salts or qualitative analysis are where marks slip, Ancourage Academy's Sec 4 Chemistry programme drills these reaction patterns directly — book a trial class (usually $18) for a diagnostic assessment.
What Do Acids, Bases and Salts Cover?
This part of the syllabus covers the properties and reactions of acids, bases and alkalis; the pH scale and indicators; oxides; how to prepare soluble and insoluble salts; and qualitative analysis to identify unknown ions. The SEAB Chemistry syllabus (6092) sets the requirements, and from 2027 the same content carries into the SEC G3 Chemistry syllabus (K324). Together these topics reward memorising clear patterns rather than open-ended reasoning.
What Are the Properties of Acids, Bases and Alkalis?
Acids are substances that produce hydrogen ions in water, while bases neutralise acids; a base that dissolves in water is called an alkali and produces hydroxide ions.
- Acids react with metals to give a salt and hydrogen gas.
- Acids react with carbonates to give a salt, water and carbon dioxide.
- Acids react with bases or alkalis in neutralisation to give a salt and water.
- Alkalis feel soapy, turn red litmus blue, and react with ammonium salts to release ammonia.
Learning these three acid reactions as a set covers most short-answer questions, because nearly every acid problem is one of these patterns applied to a different reactant.
How Does the pH Scale and Indicators Work?
The pH scale runs from below 0 to about 14, where values below 7 are acidic, 7 is neutral and above 7 is alkaline, and indicators show pH by changing colour. The lower the pH, the higher the concentration of hydrogen ions.
| Indicator | In acid | In alkali |
|---|---|---|
| Litmus | Red | Blue |
| Methyl orange | Red | Yellow |
| Universal indicator | Red to orange | Blue to purple |
Universal indicator gives an approximate pH from its colour, while a pH meter gives a precise value. Remember the distinction between strong and weak: a strong acid is fully ionised in water and so gives a lower pH at the same concentration than a weak acid, which only partly ionises.
What Are the Four Types of Oxide?
Oxides are classified by how they react: acidic oxides react with bases, basic oxides react with acids, amphoteric oxides react with both, and neutral oxides react with neither.
- Acidic oxides: usually non-metal oxides, such as carbon dioxide and sulfur dioxide.
- Basic oxides: usually metal oxides, such as sodium oxide and copper(II) oxide.
- Amphoteric oxides: react with both acids and bases — aluminium oxide and zinc oxide are the standard examples.
- Neutral oxides: a small group, such as water, carbon monoxide and nitrogen monoxide.
Amphoteric behaviour is the most commonly tested point, so commit aluminium oxide and zinc oxide to memory as the examples to quote.
How Do You Prepare Different Salts?
The preparation method depends on the solubility of the salt: soluble salts are made by titration or by reacting acid with excess solid, while insoluble salts are made by precipitation. Choosing the method correctly is the key marking point, so the solubility rules below come first.
| Salt type | Generally |
|---|---|
| Sodium, potassium, ammonium salts | All soluble |
| Nitrates | All soluble |
| Chlorides | Soluble except silver and lead |
| Sulfates | Soluble except barium, lead and calcium |
| Carbonates | Insoluble except sodium, potassium and ammonium |
| Hydroxides | Insoluble except sodium, potassium and ammonium; calcium hydroxide is slightly soluble |
For a soluble salt of a reactive metal use titration when one reactant is a soluble base or alkali, or add excess insoluble solid (metal, base or carbonate) to the acid and filter off the excess. For an insoluble salt, mix two soluble solutions so the salt precipitates, then filter, wash and dry.
What Is Qualitative Analysis in O-Level Chemistry?
Qualitative analysis is the set of standard tests used to identify the ions and gases present in an unknown sample, using aqueous sodium hydroxide and aqueous ammonia for cations, specific reagents for anions, and confirmatory tests for gases. The aim is to recognise the result, not to reason from scratch, so the test patterns must be memorised exactly.
How Do You Test for Cations, Anions and Gases?
Cations are identified by adding aqueous sodium hydroxide and aqueous ammonia and observing the precipitate, anions by their own reagent tests, and gases by their characteristic confirmatory tests.
- Cation tests: on separate portions, add aqueous sodium hydroxide (dropwise then in excess) and aqueous ammonia (dropwise then in excess), noting the colour of any precipitate and whether it dissolves in excess — this distinguishes ions such as iron(II), iron(III), copper(II), zinc, aluminium and calcium.
- Ammonium: warm with aqueous sodium hydroxide; ammonia gas is released, turning damp red litmus blue.
- Carbonate: add dilute acid; effervescence of carbon dioxide turns limewater milky.
- Chloride: add dilute nitric acid then silver nitrate to give a white precipitate.
- Iodide: add dilute nitric acid then silver nitrate to give a yellow precipitate.
- Sulfate: add dilute nitric acid then barium nitrate to give a white precipitate.
- Nitrate: add sodium hydroxide and aluminium foil, warm, and test for ammonia gas.
- Gas tests: hydrogen pops with a lit splint, oxygen relights a glowing splint, carbon dioxide turns limewater milky, ammonia turns damp red litmus blue, chlorine bleaches damp litmus paper, and sulfur dioxide turns acidified potassium manganate(VII) from purple to colourless.
Practical questions reward precise observation language, so describe both what you add and exactly what you see. See our science practical exam guide for how observations are marked.
The Most Common Acids, Salts and QA Mistakes
In our Chemistry classes at Ancourage Academy, a handful of recurring errors cause most avoidable mark loss in these topics.
| Mistake | Why it happens | How to fix it |
|---|---|---|
| Wrong salt-preparation method | Not checking solubility first | Decide soluble or insoluble, then pick titration, excess solid or precipitation |
| Confusing strong and concentrated | Treating them as the same idea | Strong means fully ionised; concentrated means a lot dissolved |
| Forgetting the acid in anion tests | Adding silver or barium nitrate alone | Always add dilute nitric acid first to remove carbonate interference |
| Vague QA observations | Writing only the conclusion | State the reagent added and the exact colour or change seen |
| Mixing up amphoteric oxides | Memorising the wrong examples | Quote aluminium oxide and zinc oxide as the amphoteric pair |
A Study Plan for Acids, Bases, Salts and QA
Work this topic in order: acid reactions and pH, then oxides and salts, then qualitative analysis.
- Week 1 — acids and pH: drill the three acid reactions, indicators, and strong versus weak.
- Week 2 — oxides and salts: learn the four oxide types, the solubility rules, and the three salt-preparation methods.
- Week 3 — qualitative analysis: memorise the cation, anion and gas tests as exact patterns.
- Week 4 — mixed practice: work past-paper QA and salt-preparation questions under timed conditions.
Ancourage Academy's Sec 3 and Sec 4 Chemistry programmes work through these topics on this progression in small groups of 3–6. Book a trial class (usually $18) for a diagnostic, or WhatsApp us with any questions.
Common Questions About O-Level / SEC Chemistry Acids and Salts
What is the difference between a strong and a weak acid?
A strong acid is fully ionised in water, releasing all of its hydrogen ions, so at the same concentration it gives a lower pH and reacts faster. A weak acid only partly ionises, so fewer hydrogen ions are present and the pH is higher. Strong and weak describe the degree of ionisation, which is different from concentrated and dilute — those describe how much acid is dissolved. A solution can be a dilute strong acid or a concentrated weak acid.
How do you decide which salt-preparation method to use?
First decide whether the salt is soluble or insoluble using the solubility rules. For a soluble salt, use titration when both reactants are in solution (for example acid with a soluble alkali), or add excess insoluble solid such as a metal, base or carbonate to the acid and filter off the excess. For an insoluble salt, mix two soluble solutions so the salt precipitates, then filter, wash and dry the residue.
How do you test for a sulfate ion?
To test for a sulfate ion, add dilute nitric acid to the solution first, then add aqueous barium nitrate. A white precipitate of barium sulfate confirms the presence of a sulfate. Adding the nitric acid first is important because it removes carbonate ions, which would otherwise also give a white precipitate and lead to a false positive. Describing both the reagent and the white precipitate earns the observation marks.
What are amphoteric oxides?
Amphoteric oxides are oxides that react with both acids and bases to form a salt and water, behaving as either acidic or basic depending on what they meet. The two examples to quote at O-Level / SEC are aluminium oxide and zinc oxide. This contrasts with basic oxides (metal oxides that react only with acids), acidic oxides (non-metal oxides that react only with bases), and neutral oxides such as water and carbon monoxide that react with neither.
Related: O-Level / SEC Chemistry Guide · Metals and Electrolysis · Combined vs Pure Science · H2 Chemistry Inorganic · O-Level / SEC Chemistry · O-Level / SEC Chemistry