Acids, Bases & Salts
Acids, bases and salts hub: aqueous ions, pH, core reactions, solubility rules, salt preparation, ammonia and exam practice.
Learning goals
- describe the meanings of the terms acid and alkali in terms of the ions they produce in aqueous solution and their effects on Universal Indicator
- describe neutrality and relative acidity and alkalinity, in terms of — relative H+ and OH– ion concentrations,
- describe neutrality and relative acidity and alkalinity, in terms of — colour in Universal Indicator, and
- describe neutrality and relative acidity and alkalinity, in terms of — the pH scale (calculation of pH from hydrogen ion concentration is not required)
- describe qualitatively the difference between strong and weak acids in terms of the extent of ionisation
- describe the characteristic properties of acids as in reactions with metals, bases and carbonates to form salts
- describe the reaction between hydrogen ions and hydroxide ions to produce water, H+ + OH– → H2O, as neutralisation
- describe the importance of controlling the pH in soils and how excess acidity can be treated using calcium hydroxide
- describe the characteristic properties of bases in reactions with acids and with ammonium salts
- classify oxides as acidic, basic, amphoteric or neutral based on metallic/non-metallic character.
- describe the techniques used in the preparation, separation and purification of salts as examples of some of the techniques specified in Section 1.2(a) (methods for preparation should include precipitation and titration together with reactions of acids with metals, insoluble bases and insoluble carbonates)
- describe the general rules of solubility for common salts to include nitrates, chlorides (including those of silver and lead), sulfates (including those of barium, calcium and lead), carbonates, hydroxides, salts of Group 1 cations and ammonium salts
- suggest a method of preparing a given salt from suitable starting materials, given appropriate information.
- describe the use of nitrogen, from air, and hydrogen, from the cracking of crude oil, in the manufacture of ammonia
- state that some chemical reactions are reversible, e.g. manufacture of ammonia
- interpret data relating to the conditions used in industry for processes involving reversible reactions, e.g. manufacture of ammonia by the Haber Process (knowledge of Le Chatelier’s Principle is not required).
Acids, bases and salts are among the most frequently tested ideas in O Level Chemistry: aqueous ions, pH and indicators, characteristic reactions, oxide classification and choosing a salt-preparation method. The registered lessons also connect these ideas to ammonia manufacture, the ammonium-ion test and sulfur dioxide pollution.
Jump to: Prerequisites · Quick Reference · Core knowledge · Sub-topics · Quiz
Prerequisites
Before you start
- Ions and ionic bonding
- Writing and balancing chemical equations
- Basic measurement and lab techniques
Useful links:
Beginner path (recommended order):
- Introduction to Acids
- Physical & Chemical Properties of Acids
- Introduction to Bases
- pH Scale & Indicators
- Salts
The topic is not one long prerequisite chain. Acids and bases are two foundations; pH and oxide classification use both, while salts branch from the characteristic reactions. Haber-process data interpretation is a separate industrial branch.
Revision: jump to Quick Reference, Core knowledge, then Common mistakes.
Paper 3 (Practical):
- Use the K324 / 6092 Chemistry Practical Hub.
- Practical-heavy lessons: pH Scale & Indicators, Preparation of Soluble & Insoluble Salts.
Quick Reference
| Item | Quick rule / reminder |
|---|---|
| Acid (aqueous) | produces H + (aq) ions in water |
| Alkali (aqueous) | soluble base that produces OH⁻(aq) ions in water |
| Neutralisation | acid + base → salt + water; net ionic: H + (aq) + OH⁻(aq) → H₂O(l) |
| Acid + metal | acid + metal → salt + H₂ |
| Acid + oxide/hydroxide | acid + metal oxide/hydroxide → salt + water |
| Acid + carbonate | acid + carbonate → salt + water + CO₂ |
| pH rule | pH < 7 acidic; pH = 7 neutral; pH > 7 alkaline |
| Strong vs concentrated | strong = ionises fully; concentrated = higher amount per volume |
| Litmus for gases | litmus must be damp (dry paper can give “no change”) |
| Titration indicator | use a few drops to locate the end point; repeat the reacting volumes without indicator when preparing crystals |
| Titration purity rule | use indicator to find volumes, then repeat without indicator for crystals |
| Sulfur dioxide test | acidified potassium manganate(VII): purple → colourless |
| Insoluble salt prep | precipitation → filter → wash → dry |
| Soluble salt prep | acid + insoluble solid: excess solid, filter, crystallise; acid + alkali: titration, crystallise |
Core knowledge to remember
- Acid: produces H + (aq) ions in aqueous solution.
- Base: neutralises an acid by accepting H + (aq) ions.
- Alkali: soluble base that produces OH⁻(aq) ions in water.
- Neutralisation: reaction between H + (aq) and OH⁻(aq) to form water.
- Indicator: substance that changes colour with pH.
- Salt: ionic compound formed when acid H⁺ is replaced by a metal or NH₄ +.
- Amphoteric oxide: reacts with both acids and alkalis.
- Strong acid / weak acid: complete / partial ionisation in water.
Sub-topics
Core Definitions and pH
Introduction to Acids
Aqueous hydrogen ions and strong versus weak acids.
Physical & Chemical Properties of Acids
Reactions with metals, bases, and carbonates.
Introduction to Bases
Bases, alkalis, and common examples.
Physical & Chemical Properties of Alkalis
Neutralisation and the ammonium-ion reaction with alkali.
pH Scale & Indicators
pH interpretation and indicator colour changes.
Oxides
Acidic, basic, amphoteric, and neutral oxides.
Salts and Preparation
Salts
Solubility rules and salt names.
Preparation of Soluble & Insoluble Salts
Titration, excess solid, and precipitation methods.
Ammonia and Environmental Links
Manufacturing Ammonia (Haber Process)
Raw materials, reversible manufacture, separation, recycling and supplied industrial data.
Ammonia & Ammonium Salts (Properties, Tests, Uses)
The ammonium-ion test and identification of ammonia gas.
Reversible Reactions
Dynamic reversibility, equilibrium conditions and the Haber-process connection.
Sulphuric Acid & Sulphur Dioxide
The prescribed sulfur dioxide test, acid rain and flue-gas desulfurisation.
Common mistakes
- Base vs alkali: all alkalis are bases, but not all bases are soluble.
- Strength vs concentration: strong acids ionise fully; concentrated means “lots per volume”.
- Carbonate reaction products: acid + carbonate → salt + water + CO₂.
- Salt prep method choice: use titration for soluble acid + alkali salts.
- Solubility before method: decide whether the target salt is soluble before choosing titration, excess solid or precipitation.
- Indicator contamination: use the first titration to find volumes, then repeat without indicator for pure crystals.
- Filtrate vs residue: a soluble salt is in the filtrate before crystallisation; an insoluble precipitate is the residue.
- Precipitate purity: wash the residue with distilled water before drying it.
- Salt formula: write correct ion charges first, then balance to get the formula (e.g. SO₄²⁻ needs two Na⁺).
- Ammonia test: ammonia turns damp red litmus paper blue; an ammonium salt releases ammonia when warmed with aqueous sodium hydroxide.