Acids, bases and salts
Chemistry · GCSE · The School
The four general equations
Acid plus METAL gives a salt plus hydrogen. Acid plus METAL OXIDE gives a salt plus water. Acid plus METAL HYDROXIDE gives a salt plus water. Acid plus METAL CARBONATE gives a salt plus water plus carbon dioxide. Learn these four and most salt questions answer themselves — the only remaining work is naming the salt.
Naming the salt
The metal comes from the base and the rest comes from the acid. Hydrochloric acid gives CHLORIDES, sulfuric acid gives SULFATES, nitric acid gives NITRATES. So copper oxide plus sulfuric acid gives copper sulfate; magnesium plus hydrochloric acid gives magnesium chloride. Say the two halves out loud in that order and the name assembles itself.
pH and what neutralisation actually is
The pH scale measures hydrogen ion concentration, and each step of one is a factor of TEN — pH 3 is ten times more acidic than pH 4, not a third more. Neutralisation is one reaction underneath all four equations above: hydrogen ions from the acid meet hydroxide ions from the base and make water. Everything else in the equation is a spectator.
This is a required practical and the order is the marks. ONE: warm dilute sulfuric acid gently. TWO: add copper oxide a little at a time, stirring, until no more dissolves — the EXCESS is what guarantees all the acid has reacted, which is what makes the product pure. THREE: filter to remove the unreacted copper oxide. FOUR: heat the filtrate to the point of crystallisation, not to dryness — boiling it dry gives a powder rather than crystals. FIVE: leave to cool so crystals form, then pat dry between filter papers. Ask yourself at step two why excess is added and not a measured amount: because measuring exactly would leave acid behind if you were slightly under, and the excess solid is easy to filter out while dissolved acid is not.
Neutralisation written as an ion equation
Strip away the spectator ions and every acid-base neutralisation is the same reaction: H⁺(aq) + OH⁻(aq) → H₂O(l). In hydrochloric acid plus sodium hydroxide, the sodium and chloride ions stay in solution unchanged on both sides — they are SPECTATORS, present but not reacting, so they cancel out of the equation. Writing the ionic equation, rather than the full one, is what shows an examiner you understand what neutralisation actually is rather than just naming the products.
A titration calculation, step by step
25.0 cm³ of sodium hydroxide solution exactly neutralises 22.50 cm³ of 0.100 mol/dm³ hydrochloric acid, using NaOH + HCl → NaCl + H₂O — one mole of each. Moles of acid = 0.02250 × 0.100 = 0.00225 mol, so moles of NaOH are the same. Concentration of NaOH = moles ÷ volume in dm³ = 0.00225 ÷ 0.0250 = 0.090 mol/dm³. The method is always the same three steps: moles of the known, the ratio from the equation, then moles ÷ volume for the concentration you want.