Acids, Alkalis and Useful Products
Acids, Bases and the pH Scale
An acid is a substance that releases hydrogen ions (\(H^+\)) in water. A base is a substance that neutralises an acid. An alkali is a soluble base — it produces hydroxide ions (\(OH^-\)) in solution. All alkalis are bases, but not all bases are alkalis (some bases are insoluble, such as copper oxide).
The pH scale runs from 0 to 14. A pH of 7 is neutral; below 7 is acidic; above 7 is alkaline. pH is measured using universal indicator (colour matched to a chart) or a digital pH probe (more precise). The scale is logarithmic — each unit change represents a tenfold change in hydrogen ion concentration (Higher tier only).
Acids produce \(H^+\) ions in water; alkalis produce \(OH^-\) ions. Neutralisation: \(H^+ + OH^- \rightarrow H_2O\). pH 0–6 = acidic; 7 = neutral; 8–14 = alkaline.
Neutralisation Reactions
Neutralisation is the reaction between an acid and a base. The general ionic equation is:
$$H^+ + OH^- \rightarrow H_2O$$
The products are a salt and water. The key reactions of acids that produce salts are:
- Acid + metal → salt + hydrogen. E.g. \(Mg + 2HCl \rightarrow MgCl_2 + H_2\)
- Acid + metal oxide → salt + water. E.g. \(CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O\)
- Acid + metal hydroxide → salt + water. E.g. \(NaOH + HCl \rightarrow NaCl + H_2O\)
- Acid + metal carbonate → salt + water + carbon dioxide. E.g. \(CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\)
Naming Salts and Making Them
A salt takes its name from the metal and the acid. Hydrochloric acid (\(HCl\)) produces chlorides; sulfuric acid (\(H_2SO_4\)) produces sulfates; nitric acid (\(HNO_3\)) produces nitrates. To predict a salt, name the metal first, then the acid type — e.g. zinc + hydrochloric acid gives zinc chloride.
To prepare a pure, dry salt: add an excess of insoluble base (e.g. copper oxide) to a warm acid (e.g. sulfuric acid) and stir. The excess solid ensures all acid is used up. Filter off the unreacted solid. Gently heat the filtrate to evaporate some water, then leave to cool so crystals form. Filter the crystals and dry at low temperature. The excess base guarantees no acid remains in the product — this is the key step that distinguishes this method from a titration.
HCl → chlorides. \(H_2SO_4\) → sulfates. \(HNO_3\) → nitrates. Predict any salt: metal name + acid type. Example: magnesium + nitric acid = magnesium nitrate.
Titration
Titration is used to find the exact volume of acid needed to neutralise a known volume of alkali (or vice versa). The method uses a burette to deliver acid accurately, a pipette to measure a fixed volume of alkali into a conical flask, and an indicator to detect the end point (when the solution changes colour, showing neutralisation is complete).
Set up a burette filled with acid and record the initial reading. Using a pipette and filler, transfer a fixed volume of alkali into a conical flask. Add two or three drops of indicator (e.g. phenolphthalein or methyl orange). Slowly add acid from the burette, swirling the flask. Add dropwise near the end point — the indicator will change colour permanently when neutralisation is complete. Record the final burette reading. The titre is the volume of acid added (final reading minus initial reading). Repeat until two concordant results are obtained (within 0.10 cm3 of each other). Average the concordant titres for accuracy.
Strong acids (hydrochloric, sulfuric, nitric) fully ionise in water — every molecule releases its \(H^+\) ions. Weak acids (ethanoic, citric) only partially ionise, establishing an equilibrium. At the same concentration, a strong acid has a lower pH than a weak acid because it produces more \(H^+\) ions. Concentration and strength are different properties — a dilute strong acid can have the same pH as a concentrated weak acid.
Indicators and Measuring pH
An indicator is a substance whose colour depends on pH. Universal indicator is a mixture of dyes that shows a range of colours: red and orange for strongly acidic solutions, green for neutral, blue and violet for alkaline solutions. However, it gives only an approximate pH reading. For more accurate measurement, a pH probe is used — these give a numerical reading to several decimal places and are essential in research and industry.
Phenolphthalein is a specific indicator that is colourless in acid and turns pink in alkali. Methyl orange is yellow in alkali and red in acid. The choice of indicator in a titration depends on the strength of the acid and alkali being used. For a strong acid reacting with a strong alkali, either indicator works well. The end point shows a permanent colour change that means exactly enough acid has been added to neutralise all the alkali.
Everyday Examples of Neutralisation
Neutralisation reactions are not just laboratory techniques — they are at work in agriculture, medicine, and industry. Farmers add powdered limestone (calcium carbonate) or quicklime (calcium oxide) to acidic soils to raise the pH, making conditions better for crops. Indigestion remedies work by neutralising excess stomach acid (hydrochloric acid) with a mild alkali such as magnesium hydroxide or calcium carbonate. Toothpastes often contain mild alkalis to neutralise the acids produced by bacteria in the mouth, which cause tooth decay. Understanding which acid is present and which base will neutralise it efficiently is a practical application of this chemistry.