Home › Learning Hub › IGCSE Chemistry › 7 Acids, bases and salts
Topic 7 · 7.1–7.3

Acids, bases and salts

Core and Extended · Papers 1–6

🎯What you need to be able to do

  • Describe the reactions of acids with metals, bases and carbonates, and of bases with acids and ammonium salts.
  • Describe the colours of litmus, thymolphthalein and methyl orange in acids and alkalis, and use universal indicator and the pH scale.
  • State that acids contain H+(aq) and alkalis OH−(aq); write the ionic equation for neutralisation.
  • Define acids as proton donors and bases as proton acceptors; strong and weak acids EXTENDED.
  • Classify oxides as acidic or basic; amphoteric oxides EXTENDED.
  • Describe how to prepare, separate and purify soluble salts; use the solubility rules; define hydrated and anhydrous.
  • Prepare insoluble salts by precipitation, and define water of crystallisation EXTENDED.

📚The chemistry

Acids and bases

Aqueous acids contain hydrogen ions, H+(aq). A base is a metal oxide or hydroxide; an alkali is a soluble base, and its solution contains hydroxide ions, OH−(aq). The reactions to know:

acid + metal → salt + hydrogen
acid + base → salt + water
acid + carbonate → salt + water + carbon dioxide
base + ammonium salt → salt + water + ammonia

Neutralisation between an acid and an alkali is always the same reaction between the ions:

\[ \mathrm{H^{+}(aq) + OH^{-}(aq) \rightarrow H_2O(l)} \]

The name of the salt comes from the acid: hydrochloric acid gives chlorides, sulfuric acid sulfates, nitric acid nitrates.

Indicators and pH

The pH scale from 0 to 14 in universal indicator colours: red at 0 to 2, orange and yellow at 3 to 6, green at 7, blue at 8 to 11, purple at 12 to 14. Examples: dilute hydrochloric acid at 1, vinegar at 3, pure water at 7, ammonia solution at 11, dilute sodium hydroxide at 14. Below, three indicators: litmus is red in acid and blue in alkali; methyl orange is red in acid and yellow in alkali; thymolphthalein is colourless in acid and blue in alkali.
The lower the pH, the higher the concentration of H+ ions. pH 7 is neutral; below 7 acidic; above 7 alkaline.

Strong and weak acids EXTENDED

An acid is a proton (H+) donor; a base is a proton acceptor. A strong acid is completely dissociated in aqueous solution; a weak acid is only partially dissociated:

\[ \mathrm{HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq)} \]
\[ \mathrm{CH_3COOH(aq) \rightleftharpoons H^{+}(aq) + CH_3COO^{-}(aq)} \]
Two beakers. Hydrochloric acid contains only separate hydrogen ions and chloride ions: it is completely dissociated. Ethanoic acid contains mostly whole ethanoic acid molecules, with just one hydrogen ion and one ethanoate ion: it is only partly dissociated.
At the same concentration the strong acid has far more H+ ions, so it has a lower pH and reacts faster with magnesium or a carbonate. EXTENDED

“Strong” is about dissociation, not concentration: a dilute solution of a strong acid is still a strong acid.

Oxides

Acidic oxides: oxides of non-metals, e.g. SO2, CO2. They react with bases.
Basic oxides: oxides of metals, e.g. CuO, CaO. They react with acids.
EXTENDED Amphoteric oxides: react with acids and with bases to give a salt and water: Al2O3, ZnO.

Solubility rules

  • All sodium, potassium and ammonium salts are soluble, and all nitrates.
  • Chlorides are soluble, except silver and lead(II) chloride.
  • Sulfates are soluble, except barium, calcium and lead(II) sulfate.
  • Carbonates are insoluble, except sodium, potassium and ammonium carbonate.
  • Hydroxides are insoluble, except sodium, potassium, ammonium and (partially) calcium hydroxide.

Preparing a soluble salt

Choose the method by what the other reactant is:

  • Insoluble metal, base or carbonate: add it in excess to the acid, so that all the acid is used up; filter off the excess.
  • Soluble alkali (or a sodium, potassium or ammonium carbonate): the excess could not be filtered out, so use a titration to find the exact volumes (topic 12a), then repeat the mixing without the indicator.
Four steps. One: excess solid is added to warm acid in a beaker and stirred, leaving unreacted solid at the bottom. Two: the mixture is filtered, the excess solid stays in the filter paper and the salt solution drips into a beaker. Three: the filtrate is heated in an evaporating basin on a tripod over a Bunsen burner until it reaches the crystallisation point. Four: the solution cools in the basin and crystals form, which are filtered and dried.
The excess-solid method. Heating only until crystals start to form (a drop on a cold glass rod crystallises) keeps the water of crystallisation in the product.

Preparing an insoluble salt EXTENDED

Mix solutions of two soluble salts, one containing each ion of the product. The insoluble salt forms as a precipitate; filter it off, wash it with distilled water to remove the soluble salts left on it, and dry it.

\[ \mathrm{Ba^{2+}(aq) + SO_4^{2-}(aq) \rightarrow BaSO_4(s)} \]
A beaker in which solutions of barium chloride and sodium sulfate have been mixed, with a white precipitate of barium sulfate settling at the bottom. An arrow leads to a filter funnel holding the white residue of barium sulfate, with sodium chloride solution as the filtrate in a beaker below. The residue is washed with distilled water and dried.
Precipitation: the product is the residue, and the unwanted salt stays in the filtrate. EXTENDED

Hydrated and anhydrous

A hydrated substance is chemically combined with water; an anhydrous substance contains no water. EXTENDED The water of crystallisation is the water molecules present in hydrated crystals, such as CuSO4·5H2O and CoCl2·6H2O. The dot means the water is part of the formula: add it when you work out Mr.

✏️Worked example

Pure, dry crystals of copper(II) chloride, CuCl2·2H2O, are made from copper(II) carbonate (an insoluble green powder) and dilute hydrochloric acid. (a) Write the balanced equation. [2] (b) Describe how to make the crystals. [4] (c) How can you tell when the acid has been used up? [1] (d) EXTENDED Calculate the percentage by mass of water in CuCl2·2H2O. (Ar: H 1, O 16, Cl 35.5, Cu 64) [2]

(a) \( \mathrm{CuCO_3(s) + 2HCl(aq) \rightarrow CuCl_2(aq) + H_2O(l) + CO_2(g)} \)

(b) Warm the acid; add copper(II) carbonate a little at a time, stirring, until some is left over (excess). Filter to remove the excess solid. Heat the filtrate in an evaporating basin until it is saturated (crystals form on a cold glass rod dipped in it). Leave to cool and crystallise; filter off the crystals and dry them between filter papers.

(c) The fizzing stops, and solid remains undissolved however much you stir.

(d) Mr \( = 64 + 2(35.5) + 2(18) = 64 + 71 + 36 = 171 \). Water: \( \tfrac{36}{171} \times 100 = 21.1\% \).

Check it. About a fifth of the mass of these crystals is water. Heating them to dryness (rather than to the crystallisation point) drives it off and gives a different, anhydrous substance with a different colour and Mr.
“Evaporate all the water.” That destroys the hydrated crystals and can decompose the salt. The method is: heat to the crystallisation point, then let the rest crystallise as it cools.

📝Practise

In the style of the multiple-choice, theory and practical papers. EXTENDED marks Supplement content.

1. (Multiple choice.) A solution turns methyl orange yellow and thymolphthalein blue. What is its pH likely to be? A: 1. B: 5. C: 7. D: 12.
D. Both colours are the alkaline ones, so the pH is above 7.
2. (Theory.) Write a word equation for the reaction of nitric acid with calcium carbonate, and state one observation. [2]
nitric acid + calcium carbonate → calcium nitrate + water + carbon dioxide. Fizzing (effervescence), and the solid disappears.
3. (Multiple choice.) Which salt is insoluble in water? A: ammonium carbonate. B: lead(II) nitrate. C: silver chloride. D: calcium chloride.
C. Chlorides are soluble except silver and lead. All ammonium salts and all nitrates are soluble.
4. (Theory.) Ammonium chloride is warmed with sodium hydroxide solution. Name the gas given off and give a test for it. [2]
Ammonia. It turns damp red litmus paper blue.
5. (Theory.) Explain why sodium sulfate is made by titration rather than by adding excess sodium hydroxide to sulfuric acid. [2]
Sodium hydroxide is soluble, so any excess could not be filtered off and would contaminate the salt. A titration finds the exact volume of alkali that just neutralises the acid.
6. (Theory.) EXTENDED Solutions of hydrochloric acid and ethanoic acid have the same concentration. Explain why the hydrochloric acid has the lower pH. [2]
Hydrochloric acid is a strong acid, completely dissociated into ions; ethanoic acid is weak and only partially dissociated. So the hydrochloric acid has a higher concentration of H+ ions.
7. (Theory.) EXTENDED In \( \mathrm{NH_3 + HCl \rightarrow NH_4Cl} \), identify the acid and the base, and explain your choice. [2]
HCl is the acid: it donates a proton (H+). NH3 is the base: it accepts the proton to form NH4+.
8. (Theory.) EXTENDED Silver chloride is made by precipitation. Name two solutions you could mix, write the ionic equation with state symbols, and explain why the solid is washed. [4]
Silver nitrate and sodium chloride (any soluble silver salt and any soluble chloride). \( \mathrm{Ag^{+}(aq) + Cl^{-}(aq) \rightarrow AgCl(s)} \). Washing with distilled water removes the soluble sodium nitrate left on the solid.
9. (Theory.) EXTENDED Zinc oxide reacts with dilute hydrochloric acid and with aqueous sodium hydroxide. What type of oxide is it? [1]
Amphoteric: it reacts with both acids and bases to form a salt and water.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • PhET “Acid-Base Solutions” — compare strong and weak acids at the particle level
  • Royal Society of Chemistry — the copper(II) sulfate crystals practical, step by step