The pH scale: acids, alkalis and neutral solutions

Reading the scale

Tap a number on the scale to see what that pH tells you. There are three zones:

• pH 0 up to just under 7: acidic
• pH 7: neutral
• just over 7 up to pH 14: alkaline

An acid makes hydrogen ions, written H⁺, when it dissolves in water. An alkali in water gives hydroxide ions, written OH⁻. A base is any substance that reacts with an acid to give a salt and water only. Edexcel defines an alkali as a base that dissolves in water.

Each step on the scale is ten times bigger

Each whole number on the scale is a big jump in acidity. When the hydrogen ion concentration goes up ten times, the pH goes down by 1. So a solution at pH 3 has ten times the hydrogen ion concentration of one at pH 4, and 100 times that of one at pH 5.

Edexcel and both OCR courses state this rule too. AQA marks it as Higher tier content. AQA and OCR Gateway both keep pH to whole numbers for this work. Pick two points on the scale widget to see the factor between them.

Measuring pH

Universal indicator changes colour to show roughly where a solution sits on the scale. AQA also calls it a wide range indicator, and says it gives an approximate pH. A pH probe or pH meter gives a reading as a number. OCR Gateway names both indicator and meter.

Edexcel also names three single indicators: litmus, methyl orange and phenolphthalein. It lists the effect of acids and alkalis on each one. KS3 science covers the pH scale and indicators without naming any.

Neutralisation

An acid and an alkali react to make a salt and water. This is neutralisation. The hydrogen ions from the acid join the hydroxide ions from the alkali to make water: H⁺(aq) + OH⁻(aq) → H₂O(l). Writing ionic equations like this one yourself is Higher tier on AQA, Edexcel and OCR Gateway.

The acid decides the family of salt. AQA gives the pattern: hydrochloric acid makes chlorides, nitric acid makes nitrates and sulfuric acid makes sulfates. Acids also react with some metals to make a salt and hydrogen, so the reactivity series matters here too. With a metal carbonate, AQA says you get a salt, water and carbon dioxide. The page on tests for gases shows how to check for both gases.

Strong, weak, dilute and concentrated

These four words sound alike, and they describe two different things. Strong and weak are about how much of the acid splits into ions in water. A strong acid splits up completely. A weak acid only partly splits up. Dilute and concentrated are about how much acid is in a set volume of water.

AQA gives hydrochloric, nitric and sulfuric acids as strong acids, and ethanoic, citric and carbonic acids as weak ones. At the same concentration, the stronger acid has the lower pH. AQA places this in Higher tier. OCR Twenty First Century adds that a weak acid reacts less than a strong acid of the same concentration.