Acid-base chemistry
Ions in solution, pH and indicators, acid reactions, solubility rules, salt preparation and ammonia for G3 Pure and O-Level Chemistry.
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Before you begin
Acids, bases and salts covers aqueous ions, pH and indicators, the characteristic reactions of acids and bases, oxide classification and choosing how to prepare a salt. Acids and bases are the two foundations: pH and oxides use both, salts branch from the characteristic reactions, and ammonia manufacture and sulfur dioxide are a separate industrial branch.
Be comfortable with: ions and ionic bonding, writing and balancing chemical equations and basic measurement and lab techniques.
Learning goals
- Describe acids and alkalis by the ions they form in water and their effect on Universal Indicator
- Compare neutral, acidic and alkaline solutions by their relative H⁺ and OH⁻ ion concentrations
- Compare neutral, acidic and alkaline solutions by their colour in Universal Indicator
- Compare neutral, acidic and alkaline solutions using the pH scale
- Describe the difference between strong and weak acids by how fully they ionise
- Describe how acids react with metals, bases and carbonates to form salts
- Describe neutralisation as hydrogen ions reacting with hydroxide ions to form water, H⁺ + OH⁻ → H₂O
- Describe why soil pH must be controlled and how calcium hydroxide treats acidic soil
- Describe how bases react with acids and with ammonium salts
- Classify oxides as acidic, basic, amphoteric or neutral from whether the element is a metal or non-metal
- Describe how to prepare, separate and purify salts by precipitation, by titration and by reacting acids with metals, insoluble bases or insoluble carbonates
- Describe the solubility rules for nitrates, chlorides (including silver and lead chloride), sulfates (including barium, calcium and lead sulfate), carbonates, hydroxides, Group 1 salts and ammonium salts
- Suggest how to prepare a given salt from suitable starting materials
- Describe how ammonia is made from nitrogen in air and hydrogen from cracking crude oil
- State that some reactions, such as making ammonia, are reversible
- Interpret data on the industrial conditions for reversible reactions, such as the Haber process
Syllabus statements covered
- 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).
Lessons
Work through them in order.
Acids, bases and pH
- Introduction to acidsExplain acidity with aqueous hydrogen ions, and tell strong and weak acids apart from concentration.
- Reactions of acids with metals, bases and carbonatesPredict the products when acids react with metals, bases and carbonates, and test for the gases.
- Bases and alkalisDistinguish bases from alkalis, and explain how alkalis produce hydroxide ions in water.
- Properties and reactions of alkalisDescribe neutralisation, the release of ammonia from ammonium salts and metal hydroxide precipitates.
- pH and indicatorsInterpret pH values and indicator colours, and explain how calcium hydroxide treats acidic soil.
- OxidesClassify oxides as acidic, basic, amphoteric or neutral from their reactions with acids and alkalis.
Salts
- SaltsName salts, write their formulae and use the solubility rules to predict precipitates.
- Preparing soluble saltsPrepare a soluble salt using an excess solid or titration, then crystallise the product.
- Preparing insoluble saltsPrepare an insoluble salt by precipitation, then filter, wash and dry it.
Industrial chemistry and gas tests
- Reversible reactionsRead the reversible-reaction arrow and interpret industrial data on rate and yield.
- Making ammonia: the Haber processExplain how ammonia is made, separated and recycled in the Haber process, using supplied data.
- Ammonia and ammonium-ion testsTest for ammonia gas with damp red litmus, and for ammonium ions by warming with sodium hydroxide.
- Sulfur dioxide and sulfuric acidTest for sulfur dioxide, and explain acid rain and flue-gas desulfurisation.
Practise and check
Or choose
Topic reference
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 | an acid reacts with a base; for a hydroxide base: H⁺(aq) + OH⁻(aq) → H₂O(l) |
| Acid + metal | a suitable metal + dilute hydrochloric or sulfuric acid → salt + H₂ |
| Acid + oxide/hydroxide | acid + metal oxide/hydroxide → salt + water |
| Acid + carbonate | acid + carbonate → salt + water + CO₂ |
| pH rule | near room temperature: pH < 7 acidic; pH = 7 neutral; pH > 7 alkaline |
| Strong vs concentrated | strength describes the extent of ionisation; concentration describes the amount of solute 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 |
| 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: an acid reacts with a base. With a hydroxide base, H⁺(aq) and OH⁻(aq) form water; with ammonia, H⁺(aq) + NH₃(aq) → NH₄⁺(aq).
- Indicator: substance that changes colour with pH.
- Salt: an electrically neutral ionic compound. In the acid–salt model, the acid’s replaceable hydrogen is replaced by a metal or ammonium ion.
- Amphoteric oxide: reacts with both acids and alkalis.
- Strong acid / weak acid: ionises completely / partially in water. Sulfuric acid is strong because it releases its first hydrogen ion completely.
Common mistakes
- Base vs alkali: all alkalis are bases, but not all bases are soluble.
- Strength vs concentration: ionisation and amount of solute per volume are different ideas; a strong acid can be dilute.
- Carbonate reaction products: acid + carbonate → salt + water + CO₂.
- Choosing a salt method: decide first whether the salt is soluble. Use titration when both reactants are solutions, and repeat it without indicator before crystallising.
- Filtrate vs residue: a soluble salt is in the filtrate before crystallisation; an insoluble precipitate is the residue, washed with a little cold distilled water and dried.
- Salt formula: write the ion charges first, then balance them (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.