Acids, alkalis and pH

The pH scale, from 0 to 14, measures how acidic or alkaline a solution is. Acids have a pH below 7, alkalis above 7, and neutral solutions exactly 7. Indicators change colour to show which it is: universal indicator shows the whole scale. An acid and an alkali cancel each other out in neutralisation, making a salt and water.

Key facts

  • pH below 7: acidic (the lower, the more acidic). pH 7: neutral. pH above 7: alkaline (the higher, the more alkaline).
  • Universal indicator: red (pH 0–2), orange (3–4), yellow (5–6), green (7), blue (8–10), purple (11–14).
  • Litmus is red in acids and blue in alkalis. Phenolphthalein is colourless in acids and pink in alkalis. Methyl orange is red in acids and yellow in alkalis.
  • Acids release hydrogen ions, H⁺, in water. Alkalis release hydroxide ions, OH⁻. A base neutralises an acid; an alkali is a base that dissolves in water.
  • acid + alkali → salt + water. Hydrochloric acid makes chlorides, sulfuric acid sulfates and nitric acid nitrates. In every neutralisation: H⁺ + OH⁻ → H2O.
  • Everyday acids: stomach acid (hydrochloric), vinegar (ethanoic), citrus fruit (citric). Everyday alkalis: soap, oven cleaner, bleach.
  • A strong acid is fully ionised in water; a weak acid only partly. Each fall of 1 in pH means 10 times as many hydrogen ions.

Worked examples

Example 1

Lemon juice has pH 2, pure water pH 7 and oven cleaner pH 13. Describe each one and give the colour of universal indicator.

  1. pH 2 is well below 7: strongly acidic. Universal indicator turns red.
  2. pH 7 is neutral: universal indicator turns green.
  3. pH 13 is well above 7: strongly alkaline. Universal indicator turns purple.

Answer: lemon juice: strongly acidic, red; water: neutral, green; oven cleaner: strongly alkaline, purple

Example 2

Name the salt made when potassium hydroxide neutralises sulfuric acid, and write the word equation.

  1. An acid and an alkali make a salt and water.
  2. The metal comes from the alkali (potassium); sulfuric acid makes sulfates.
  3. Write it out: potassium hydroxide + sulfuric acid → potassium sulfate + water. The ions that react: H⁺ + OH⁻ → H2O

Answer: potassium sulfate; potassium hydroxide + sulfuric acid → potassium sulfate + water

Example 3

An acid has pH 2. Another has pH 4. How many times more hydrogen ions per cm³ does the first have?

  1. Each fall of 1 in pH means 10 times as many hydrogen ions.
  2. From pH 4 to pH 2 is a fall of 2.
  3. Multiply by 10 twice. 10 × 10 = 100

Answer: 100 times