Acids, Bases & Salts
Acids, bases and salts hub: aqueous ions, pH, core reactions, solubility rules, salt preparation, ammonia and exam practice.
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 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 distinguish strong acids from weak ones.
- Physical & Chemical Properties of AcidsPredict the products when acids react with metals, bases and carbonates.
- Introduction to BasesDistinguish bases from alkalis, and recognise common examples of each.
- Physical & Chemical Properties of AlkalisDescribe neutralisation and the reaction of alkalis with ammonium salts.
- pH Scale & IndicatorsInterpret pH values and the colour changes of indicators.
- OxidesClassify oxides as acidic, basic, amphoteric or neutral from how they react.
Salts
- SaltsUse the solubility rules to predict which salts dissolve, and name salts.
- Preparing Soluble SaltsPrepare a soluble salt using an excess solid or titration, then crystallise the product.
- Preparing Insoluble SaltsPrepare an insoluble salt by precipitation: choose soluble ion sources, collect the residue, wash away dissolved impurities and dry the product.
Industrial and retained chemistry
- Manufacturing Ammonia (Haber Process)Explain how ammonia is made, separated and recycled in the Haber process, using supplied data.
- Ammonia and Ammonium SaltsTest for ammonium ions and identify ammonia gas.
- Sulfur Dioxide and Sulfuric AcidTest for sulfur dioxide, and explain acid rain and flue-gas desulfurisation.
Practise and check
Recommended nextAcid-Base Chemistry topic check
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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 |
| 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: 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: complete / partial ionisation of the proton being considered in water. For sulfuric acid, the strong-acid description refers to its first proton.
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.
- Salt prep method choice: use titration when both reacting solutions and the target salt are soluble.
- 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 small portions of cold distilled water to remove soluble impurities, then dry 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.