The pH scale and neutralisation — AQA GCSE Chemistry

When an acid dissolves in water, it ionises to release hydrogen ions, H⁺. These are the species responsible for acidic properties such as turning universal indicator red or orange, reacting with metals to produce hydrogen gas, and reacting with bases to form salts and water. For example, hydrogen chloride gas dissolves in water to form hydrochloric acid, which contains H⁺(aq) and Cl⁻(aq); the H⁺ ions are produced because the polar water molecules pull the HCl molecule apart. The subscript (aq) shows that the ions are dissolved in water. In aqueous solution, H⁺ ions are strongly attracted to water molecules, but for GCSE it is sufficient to represent them as H⁺(aq). The concentration of H⁺ ions determines the pH: a higher concentration of H⁺ gives a lower pH value.

Alkalis are bases that dissolve in water to produce hydroxide ions, OH⁻(aq). These ions are responsible for alkaline properties such as turning universal indicator blue or purple, feeling soapy, and neutralising acids to form a salt and water. For example, sodium hydroxide dissolves in water to form Na⁺(aq) and OH⁻(aq); the OH⁻ ions make the solution alkaline. The concentration of OH⁻ ions determines the pH: a higher concentration of OH⁻ gives a higher pH value. In neutralisation, H⁺(aq) from the acid reacts with OH⁻(aq) from the alkali to form water, H₂O(l). This ionic equation, H⁺(aq) + OH⁻(aq) → H₂O(l), is the essential neutralisation reaction and applies to any acid reacting with any alkali.

The pH scale runs from 0 to 14 and tells you how acidic or alkaline a solution is. A pH below 7 is acidic, pH 7 is neutral, and a pH above 7 is alkaline. The scale is logarithmic, so each change of one pH unit represents a tenfold change in the concentration of H⁺ ions; for example, pH 3 is ten times more acidic in terms of H⁺ concentration than pH 4. Universal indicator is a mixture of dyes that gives a colour for each pH, which you compare with a colour chart; it is quick but only gives an approximate value. A pH probe is an electronic sensor connected to a meter; it gives a numerical reading and is more precise and objective. For example, testing a solution with universal indicator might show orange, suggesting pH 4, while a pH probe could give a reading of 4.2.

The pH scale runs from 0 to 14 and tells you how acidic or alkaline an aqueous solution is. A solution with pH 7 is neutral, for example pure water at room temperature. Aqueous solutions of acids have pH values below 7: the lower the number, the more acidic the solution, so pH 1 is more acidic than pH 5. Aqueous solutions of alkalis have pH values above 7: the higher the number, the more alkaline the solution, so pH 13 is more alkaline than pH 9. The scale is continuous, so a solution with pH 6.5 is weakly acidic. You can measure pH approximately using universal indicator, which changes colour across the range, or more precisely with a pH meter. Knowing the pH allows you to classify a solution as acidic, neutral or alkaline.

When an acid reacts with an alkali, the reaction is called neutralisation. In aqueous solution, acids produce hydrogen ions, H⁺, and alkalis produce hydroxide ions, OH⁻. The essential change is that a hydrogen ion reacts with a hydroxide ion to form water: H⁺ + OH⁻ → H₂O. This ionic equation shows the reacting particles rather than the whole compounds. For example, when hydrochloric acid reacts with sodium hydroxide solution, the sodium ions and chloride ions remain in solution as spectator ions, while H⁺ and OH⁻ combine to make water. The resulting solution is closer to neutral because the hydrogen and hydroxide ions have been removed. The same ionic change happens whenever any acid neutralises any alkali, which is why it is called the ionic equation for neutralisation.

The neutralisation reaction between an acid and an alkali can be represented by the ionic equation H⁺ + OH⁻ → H₂O. To measure the approximate pH of a solution, you can use universal indicator or a wide range indicator. These indicators contain a mixture of dyes and change through a range of colours as pH changes. A few drops are added to the solution, or a piece of indicator paper is dipped into it, and the colour is compared with a colour chart. The chart links each colour to an approximate pH value. Red and orange colours indicate acidic solutions with pH below 7; green indicates a neutral solution around pH 7; blue and purple indicate alkaline solutions with pH above 7. The indicator gives an approximate value, not an exact one, so it is used to identify whether a solution is acidic or alkaline and to estimate its pH.