AcidsEdexcel GCSE Chemistry: Revision notes
Section 1
What are acids, alkalis and pH?
Acids in solution are sources of hydrogen ions (H+). Alkalis in solution are sources of hydroxide ions (OH−).
The pH scale measures acidity/alkalinity:
- A neutral solution has a pH of 7
- Acidic solutions have pH values lower than 7
- Alkaline solutions have pH values higher than 7
Indicators change colour depending on pH: litmus (red in acid, blue in alkali), methyl orange (red in acid, yellow in alkali), phenolphthalein (colourless in acid, pink in alkali).
(Higher tier) The higher the concentration of H+ ions, the lower the pH; the higher the concentration of OH− ions, the higher the pH. As H+ ion concentration increases by a factor of 10, pH decreases by 1.
State indicator colours precisely — 'red' alone isn't enough if the indicator has three possible colours; always link colour to acid/neutral/alkali.
Section 2
What is the difference between dilute/concentrated and weak/strong acids? (Higher tier)
Dilute and concentrated describe the amount of substance dissolved in a given volume of solution — a concentrated solution has a large amount of acid dissolved, a dilute solution has a small amount.
Weak and strong describe the degree of dissociation into ions:
- A strong acid fully dissociates (ionises) into ions in solution
- A weak acid only partially dissociates, so most of the acid remains as molecules
Students often confuse 'concentrated' with 'strong' — they describe different things: amount dissolved vs degree of ionisation.
Section 3
How do acids react with metals, oxides, hydroxides and carbonates?
A base is any substance that reacts with an acid to form a salt and water only. Alkalis are soluble bases.
General reactions of acids:
- acid + metal → salt + hydrogen
- acid + metal oxide → salt + water
- acid + metal hydroxide → salt + water
- acid + metal carbonate → salt + water + carbon dioxide
Neutralisation is the reaction between an acid and a base. In an acid-alkali neutralisation, H+ ions from the acid react with OH− ions from the alkali to form water: H+ + OH− → H2O.
Chemical tests: hydrogen gas is tested with a lit splash — it makes a 'squeaky pop'. Carbon dioxide gas is tested by bubbling through limewater, which turns cloudy/milky.
Hydrochloric acid + sodium hydroxide → sodium chloride + water (a neutralisation reaction).
Section 4
How are soluble and insoluble salts prepared?
Preparing a soluble salt from an insoluble base: excess insoluble reactant (e.g. metal oxide) is added to the acid so all the acid reacts; the excess unreacted solid is then filtered off, leaving a solution of only salt and water, which is evaporated/crystallised to obtain the salt.
Preparing a soluble salt from a soluble reactant (e.g. an alkali): since both reactants are soluble, you cannot simply filter off excess — titration must be used, adding acid and alkali together using a burette, pipette and a suitable indicator, in the exact volumes needed to reach neutralisation, to make sure the acid and base are mixed in the correct proportions.
Core practical: pure, dry hydrated copper sulfate crystals are prepared from copper oxide (an insoluble base) and dilute sulfuric acid, warming the mixture in a water bath, filtering off excess copper oxide, then evaporating and crystallising the solution.
Preparing an insoluble salt: mix two suitable soluble solutions so they form a precipitate of the insoluble salt, then filter, wash and dry the precipitate.
For the copper sulfate practical, always mention using excess copper oxide to ensure all the acid reacts, then filtering — this is a common mark-scheme point.
Section 5
How do you predict solubility and precipitates?
General solubility rules:
- All common sodium, potassium and ammonium salts are soluble
- All nitrates are soluble
- Common chlorides are soluble, except silver and lead
- Common sulfates are soluble, except lead, barium and calcium
- Common carbonates and hydroxides are insoluble, except those of sodium, potassium and ammonium
Using these rules, you can predict whether mixing two named solutions will form a precipitate and name it.
Mixing lead nitrate solution and potassium iodide solution forms a precipitate of lead iodide, since lead salts (other than nitrates) are generally insoluble.
Must Know
- Acids release H+ ions; alkalis release OH− ions; pH 7 neutral, below 7 acidic, above 7 alkaline
- Indicator colours: litmus (red/blue), methyl orange (red/yellow), phenolphthalein (colourless/pink)
- (Higher) dilute/concentrated = amount dissolved; weak/strong = degree of dissociation into ions
- Acid + metal → salt + hydrogen; + oxide/hydroxide → salt + water; + carbonate → salt + water + CO2
- Neutralisation: H+ + OH− → H2O
- Soluble salt from insoluble base = excess + filter; from soluble reactants = titration
- Solubility rules: all Na/K/NH4 salts and nitrates soluble; most chlorides/sulfates soluble (exceptions); most carbonates/hydroxides insoluble (exceptions)
That's the notes covered.
Carry on to the next subtopic.