7.1Acids, Bases and Salts
The characteristic properties of acids and bases — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 10 parts Lesson 1 of 3 2 quizzes
Lesson summary
7.1 The characteristic properties of acids and bases — Notes — IGCSE Chemistry notes: Describe acids, bases and alkalis, their reactions, indicators, the pH scale, acid strength and titration.Acid: a substance whose aqueous solution contains H+ ions, with pH below 7.HCl(aq) → H+(…
- Neutralisation
- Strong acid
- Acid: a substance whose aqueous solution contains H+ ions, with pH below 7.HCl(aq) → H+(aq) + Cl−(aq)
- Acids taste sour when edible and are corrosive
- Common acids are hydrochloric (HCl), sulfuric (H2SO4) and nitric (HNO3)
- Acid solutions conduct because they contain mobile H+ ions
Section 1 of 10Acids, bases and alkalis
Acids, bases and alkalis
Acid: a substance whose aqueous solution contains H+ ions, with pH below 7.
- HCl(aq) → H+(aq) + Cl−(aq).
- Acids taste sour when edible and are corrosive.
- Common acids are hydrochloric (HCl), sulfuric (H2SO4) and nitric (HNO3).
- Acid solutions conduct because they contain mobile H+ ions.
Base: an oxide or hydroxide of a metal that neutralises an acid to form a salt and water, with pH above 7.
Alkali: a soluble base whose aqueous solution contains OH− ions, such as NaOH → Na+ + OH−.
- Common alkalis are sodium hydroxide, potassium hydroxide and ammonia solution.
| Feature | Base | Alkali |
|---|---|---|
| What it is | Metal oxide or hydroxide | A base that is soluble in water |
| Ions in water | Not necessarily dissolved | Produces OH−(aq) |
| With an acid | Salt and water | Salt and water |
| pH | Above 7 | Above 7 |
- Brønsted-Lowry (Supplement)acids are proton (H+) donors and bases are proton acceptors.
- In HCl + NH3 → NH4Cl, HCl donates H+ and NH3 accepts it.
Section 2 of 10Reactions of acids
| Reaction | Products | Example |
|---|---|---|
| Acid + metal | Salt + hydrogen | Mg + 2HCl → MgCl2 + H2 |
| Acid + base (oxide or hydroxide) | Salt + water | CuO + H2SO4 → CuSO4 + H2O |
| Acid + alkali | Salt + water | HCl + NaOH → NaCl + H2O |
| Acid + carbonate | Salt + water + carbon dioxide | CaCO3 + 2HCl → CaCl2 + H2O + CO2 |
| Base + ammonium salt (warm) | Salt + water + ammonia | NH4Cl + NaOH → NaCl + H2O + NH3 |
1
Only metals above hydrogen in the reactivity series react with dilute acids.
2
Salt names: hydrochloric acid gives chlorides, sulfuric acid gives sulfates and nitric acid gives nitrates.
3
Ammonia turns damp red litmus paper blue, which is the test for an ammonium salt warmed with an alkali.
4
Hydrogen gives a squeaky pop and carbon dioxide turns limewater milky.
Section 3 of 10Neutralisation and its uses
1
Neutralisation: an acid reacts with a base to form a salt and water, H+(aq) + OH−(aq) → H2O(l).
2
A metal reacting with an acid forms a salt but no water, so it is not neutralisation.
3
Soil: farmers add lime (calcium hydroxide) or calcium oxide to neutralise acidic soil.
4
Indigestion tablets contain bases that neutralise excess stomach acid.
5
Toothpaste is mildly alkaline.
6
Factory waste can be neutralised before release.
7
Acidic bee stings are treated with baking soda.
Section 4 of 10Indicators
| Indicator | Colour in acid | Colour in alkali |
|---|---|---|
| Litmus | Red | Blue |
| Methyl orange | Red | Yellow |
| Thymolphthalein | Colourless | Blue |
| Universal indicator | Red, orange, yellow (pH 0–6) | Blue, purple (pH 8–14); green at pH 7 |
1
Universal indicator is a mixture of indicators matched to a colour chart to find the pH.
2
Universal indicator paper compares hydrogen ion concentration, neutrality, relative acidity and relative alkalinity by colour and pH.
3
Methyl orange and thymolphthalein change sharply, so they suit titrations, whereas litmus changes gradually through purple.
4
Litmus paper is still useful for testing solutions and damp gases.
5
Litmus comes from lichens, and synthetic indicators are organic compounds.
Section 5 of 10The pH scale
| pH | Type |
|---|---|
| 0–2 | Strong acid (can be below 1) |
| 3–6 | Weak acid |
| 7 | Neutral |
| 8–11 | Weak alkali |
| 12–14 | Strong alkali |
1
pH 0 is strongly acidic, pH 7 is neutral and pH 14 is strongly alkaline.
2
Logarithmic scale (Supplement): the lower the pH, the higher the H+ concentration, and each pH unit is a tenfold change.
- pH 3 has 10 times the H+ of pH 4, and pH 2 has 100 times that of pH 4.
3
Measuring pH
- Take a sample of the solution in a spotting tile or test tube.
- Add a few drops of universal indicator or dip universal indicator paper.
- Compare the colour with the pH chart and record the pH.
4
A pH meter gives more precision.
5
To compare acids, measure the pH of each at the same concentration.
Section 6 of 10Strong and weak acids (Supplement)
| Feature | Strong acid | Weak acid |
|---|---|---|
| Dissociation in water | Complete | Partial |
| Equilibrium | None (single arrow) | Reversible arrow, lies to the left |
| H+ concentration | High | Low |
| pH at the same concentration | Very low | Higher, between 3 and 6 |
| Reaction with metal | Vigorous, H2 gas | Slow, H2 gas |
| Examples | HCl, H2SO4, HNO3 | Ethanoic acid, propanoic acid |
1
Strong acid: HCl(aq) → H+(aq) + Cl−(aq).
2
Weak acid: CH3COOH(aq) ⇌ H+(aq) + CH3COO−(aq).
3
Strong alkalis such as NaOH are completely ionised and weak alkalis such as NH3 are partly ionised, and neither reacts with metals.
4
At the same concentration strong acids produce more H+, so they react more vigorously with metals and carbonates.
5
Strength is how fully an acid dissociates, whereas concentration is the amount of solute per dm3.
6
Dilute hydrochloric acid (strong) can have a lower pH than concentrated ethanoic acid (weak).
Section 7 of 10Titration
Titration
Purpose: finds the exact volume of one solution needed to neutralise another, and so the concentration of an unknown acid or alkali.
Method
- Use a pipette to transfer 25.0 cm3 of alkali into a conical flask and add a few drops of indicator.
- Fill a burette with acid and note the start reading.
- Add acid slowly, swirling, until the indicator just changes colour (the end-point).
- Record the final reading and the volume added.
- Repeat until two results agree within 0.10 cm3.
Section 8 of 10More from the specification
More from the specification
Characteristic Reactions of Aqueous Acids
- Acids react with metals to form a salt and hydrogen gas (e.g. Mg + 2HCl → MgCl2 + H2), with metal oxides to form a salt and water,
- and with carbonates to form a salt, water,
- and carbon dioxide.
Classification of Oxides by Acid-Base Behaviour
- Non-metal oxides (like SO2 and CO2) are acidic and dissolve in water to form acidic solutions; basic metal oxides react with acids to form salts.
- (like MgO and CaO)
Amphoteric Oxides Reactivity
- Amphoteric oxides, such as aluminium oxide (Al2O3) and zinc oxide (ZnO), react with both strong acids and strong bases to form salts and water.
Acidic oxide
- An oxide that reacts with bases to form a salt and water.
Amphoteric oxide
- An oxide that reacts with both acids and bases.
Basic oxide
- An oxide that reacts with acids to form a salt and water.
Section 9 of 10Key terms
| Term | Meaning |
|---|---|
| Acid | a substance that produces H+ ions in aqueous solution, a proton donor. |
| Base | a substance that neutralises an acid, a proton acceptor. |
| Alkali | a soluble base that produces OH− ions in water. |
| Neutralisation | the reaction of an acid with a base to form a salt and water. |
| pH | a scale from 0 to 14 that measures acidity or alkalinity. |
| Strong acid | an acid that is completely dissociated in water. |
| Weak acid | an acid that is only partially dissociated in water. |
| End-point | the point in a titration where the indicator just changes colour. |
Section 10 of 10Exam technique
1
Saying acid + metal gives salt + waterit gives salt + hydrogen.
2
Saying a strong acid is a concentrated acid: strength depends on the degree of dissociation.
3
Saying an alkali is any basean alkali is a soluble base that produces OH− ions.
4
Saying weak acids do not ionisethey ionise partially.
5
Saying neutral means pH 0 or that acids simply contain hydrogen: neutral is pH 7 and acids produce H+ in aqueous solution.
Marked practiceQuizzes for this lesson
Last updated: