7.1 The characteristic properties of acids and bases
- Syllabus
- 0620–2026–2027
- Topic
- 7.1
- Level
- —
Learning objectives
7.1.1Characteristic properties of acids in• Describe characteristic properties of acids in terms of their reactions with: (a) metals (b) bases (c) carbonates7.1.2Acids in terms of their effect on: (a)• Describe acids in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange7.1.3Bases are oxides or hydroxides of• State: bases are oxides or hydroxides of metals and that alkalis are soluble bases7.1.4Characteristic properties of bases in• Describe characteristic properties of bases in terms of their reactions with: (a) acids (b) ammonium salts7.1.5Alkalis in terms of their effect on• Describe alkalis in terms of their effect on: (a) litmus (b) thymolphthalein (c) methyl orange7.1.6Aqueous solutions of acids contain H+• State: aqueous solutions of acids contain H+ ions and aqueous solutions of alkalis contain OH– ions7.1.7To compare hydrogen ion concentration• Describe how to compare hydrogen ion concentration, neutrality, relative acidity and relative alkalinity in terms of colour and pH using universal indicator paper7.1.8Neutralisation reaction between an• Describe neutralisation reaction between an acid and an alkali to produce water, H+ (aq) + OH– (aq) → H2O (l)7.1.9Acids as proton donors and bases as• Define acids as proton donors and bases as proton acceptors7.1.10Strong acid as an acid that is• Define a strong acid as an acid that is completely dissociated in aqueous solution and a weak acid as an acid that is partially dissociated in aqueous solution7.1.11Hydrochloric acid is a strong acid, as• State: hydrochloric acid is a strong acid, as shown by the symbol equation, HCl (aq) → H+(aq) + Cl –(aq)7.1.12Ethanoic acid is a weak acid, as shown• State: ethanoic acid is a weak acid, as shown by the symbol equation, CH3COOH(aq) ⇌ H+(aq) + CH3COO–(aq)
Describe how acids react
| Acid reacts with | Products | Typical observation |
|---|---|---|
| a reactive metal | salt + hydrogen | effervescence; hydrogen gives a squeaky pop with a lighted splint |
| a base | salt + water | base dissolves; neutralisation occurs |
| a carbonate | salt + water + carbon dioxide | effervescence; CO₂ turns limewater milky |
Mg + H₂SO₄ → MgSO₄ + H₂; CuO + 2HNO₃ → Cu(NO₃)₂ + H₂O; CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
Choose the salt from the acid anion and the metal or ammonium ion supplied by the other reactant, then balance the equation.
Acids do not produce hydrogen with every metal. A metal must be reactive enough, and nitric acid may not behave like a typical acid with metals.
Recall indicator colours in acids
| Indicator | Colour in an acid |
|---|---|
| litmus | red; blue litmus turns red |
| thymolphthalein | colourless |
| methyl orange | red |
Add only a small amount of indicator and compare the resulting colour with its known acidic colour.
These indicators show an acidic or alkaline range but do not give a precise pH or acid strength.
Do not confuse methyl orange with universal indicator: methyl orange is red in acid, while universal indicator can show several acidic colours.
Distinguish bases and alkalis
Bases are oxides or hydroxides of metals. An alkali is a base that is soluble in water.
| Substance | Base? | Alkali? |
|---|---|---|
| copper(II) oxide | yes | no; insoluble |
| magnesium hydroxide | yes | no; only slightly soluble |
| sodium hydroxide | yes | yes; soluble |
| potassium hydroxide | yes | yes; soluble |
Every alkali is a base, but not every base is an alkali. Solubility is the deciding distinction.
Do not define every base as soluble. Insoluble metal oxides can still neutralise acids.
Describe how bases react
| Base reacts with | Products | Observation |
|---|---|---|
| acid | salt + water | neutralisation; an insoluble base may dissolve |
| ammonium salt, on warming | salt + water + ammonia | pungent alkaline gas released; damp red litmus turns blue |
CuO + 2HCl → CuCl₂ + H₂O. For an alkali and ammonium salt: NH₄⁺ + OH⁻ → NH₃ + H₂O.
Warm the mixture gently to release ammonia. Test the gas with damp indicator paper because ammonia must dissolve in water before showing alkalinity.
An ammonium salt does not release ammonia merely on its own; it must react with a base or alkali, usually on warming.
Recall indicator colours in alkalis
| Indicator | Colour in an alkali |
|---|---|
| litmus | blue; red litmus turns blue |
| thymolphthalein | blue |
| methyl orange | yellow |
Use a clean sample and a small amount of indicator so contamination does not shift the colour.
Indicator colour identifies an alkaline range, but universal indicator or a pH meter is needed to compare relative alkalinity.
Thymolphthalein is blue in alkali and colourless in acid; methyl orange is yellow in alkali and red in acid.
Link acids and alkalis to their ions
| Aqueous solution | Characteristic ion |
|---|---|
| acid | H⁺(aq) |
| alkali | OH⁻(aq) |
Acidic properties arise from mobile H⁺ ions in aqueous solution; alkaline properties arise from mobile OH⁻ ions.
HCl(aq) supplies H⁺ ions, while NaOH(aq) supplies OH⁻ ions. The state symbol (aq) matters because these ions are present in water.
A compound containing hydrogen is not automatically an acid, and a compound containing oxygen is not automatically an alkali.
Compare acidity and alkalinity using pH
| Approximate pH | Universal indicator colour | Interpretation |
|---|---|---|
| 0–2 | red | strongly acidic; highest H⁺ concentration |
| 3–4 | orange | acidic |
| 5–6 | yellow | weakly acidic |
| 7 | green | neutral |
| 8–10 | blue | alkaline |
| 11–14 | purple | strongly alkaline |
Touch universal indicator paper with a clean drop of solution and compare the colour with the chart. Lower pH means greater hydrogen ion concentration and greater acidity; higher pH means greater alkalinity.
Use equal measurement conditions when comparing samples. Neutrality is pH 7, not merely a pale or colourless solution.
Universal indicator gives an approximate pH range. It does not by itself distinguish acid strength from concentration unless concentrations are controlled.
Describe acid–alkali neutralisation
In neutralisation, H⁺ ions from an acid react with OH⁻ ions from an alkali to form water: H⁺(aq) + OH⁻(aq) → H₂O(l).
The remaining ions form the salt. They are spectator ions in the net ionic equation because they do not change.
HCl + NaOH → NaCl + H₂O. The full equation contains the salt; the ionic equation shows the chemical change common to every acid–alkali neutralisation.
Neutralisation describes acid reacting with base, but a neutral final pH occurs only when suitable reacting amounts are used; excess acid or alkali leaves the mixture non-neutral.
Define acids and bases by proton transfer
| Species role | Proton transfer |
|---|---|
| acid | donates H⁺ |
| base | accepts H⁺ |
In HCl + H₂O → H₃O⁺ + Cl⁻, HCl donates a proton and acts as the acid; water accepts it and acts as the base.
Compare each species before and after the reaction. The acid has lost H⁺; the base has gained H⁺.
Proton transfer is not electron transfer. A proton is H⁺, so acid–base roles are separate from oxidation and reduction roles.
Distinguish strong and weak acids
| Acid type | Dissociation in water | Particle picture |
|---|---|---|
| strong acid | complete | essentially all acid particles form ions |
| weak acid | partial | molecules and ions coexist at equilibrium |
At the same concentration, a strong acid has a greater H⁺ concentration, lower pH, greater electrical conductivity, and usually reacts faster than a weak acid.
Use a one-way arrow for complete dissociation and a reversible arrow for partial dissociation.
Strength is the extent of dissociation; concentration is the amount of acid per unit volume. A weak acid can be concentrated and a strong acid can be dilute.
Show that hydrochloric acid is strong
HCl(aq) → H⁺(aq) + Cl⁻(aq)
The one-way arrow shows that hydrochloric acid is completely dissociated into hydrogen ions and chloride ions in aqueous solution.
For this syllabus model, the aqueous solution is represented by H⁺ and Cl⁻ ions rather than an equilibrium containing a substantial amount of undissociated HCl.
Strong does not mean concentrated or dangerous by definition. It refers to complete dissociation in water.
Show that ethanoic acid is weak
CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)
The reversible arrow shows partial dissociation: the aqueous mixture contains undissociated ethanoic acid molecules as well as H⁺ and ethanoate ions.
At the same total acid concentration, ethanoic acid produces fewer H⁺ ions and therefore has a higher pH than hydrochloric acid.
Weak does not mean that ethanoic acid cannot react with metals, bases, or carbonates. It describes partial dissociation, not absence of acidic reactions.