pH Scale Explained: Acids, Bases and Indicators

A scale for H⁺

Some liquids taste sour, others feel slippery. Chemists measure how acidic a liquid is with pH. In water, a few molecules split into H⁺ and OH⁻ ions, and pH = −log₁₀[H⁺], where [H⁺] is the concentration of hydrogen ions in moles per litre. In pure water at 25 °C it is 10⁻⁷, so pH = 7, which is neutral. Each step down the scale means ten times more H⁺, so pH 3 has 100 times more than pH 5. The picture shows a scale from 0 to 14 with a beaker whose colour ticks along it. Slide it and watch the ions appear.

Acids

An acid releases H⁺ ions in water. Lemon juice has a pH of about 2 because of citric acid, vinegar about 2.5 to 3 because of acetic acid, and stomach acid, which is hydrochloric acid, between about 1.5 and 3.5. That strong acid kills many germs and helps digest food, while a layer of mucus protects the stomach wall. The acids in food are weak and safe to eat. Strong acids in laboratories and some cleaners can burn skin, so never taste or sniff unknown liquids and always follow the label. In the picture, a lemon slice drops into the beaker and the pH marker slides to the left.

Bases

A base takes up H⁺ ions or releases OH⁻ ions. Seawater is slightly basic, around 8.1. A baking-soda solution is about 8.3, and soap is about 9 to 10; its slippery feel is typical of bases. Drain and oven cleaners can come close to 13 or 14 and burn skin, which is why labels tell you to wear gloves. A base that dissolves in water is also called an alkali. Choose liquids in the sandbox under the picture, such as lemon juice, vinegar, coffee, pure water, seawater, baking-soda solution and soap, and read off their pH.

Acid plus base

Put an acid together with a base and they cancel each other out. This is neutralisation: acid plus base gives a salt plus water. With hydrochloric acid and sodium hydroxide, HCl + NaOH → NaCl + H₂O. The heart of it is H⁺ + OH⁻ → H₂O; the Na⁺ and Cl⁻ ions are only spectators and stay dissolved as ordinary salt. Worked example: 25 mL of 0.10 mol/L HCl holds 0.025 L × 0.10 mol/L = 0.0025 mol of H⁺. It needs exactly 0.0025 mol of OH⁻, which is 25 mL of 0.10 mol/L NaOH. Antacid tablets use a mild base in just this way against heartburn.

A colour that reads pH

Some plant pigments change colour with pH. Red cabbage juice contains anthocyanins, which turn red-pink in acids, purple near neutral, and blue, green and finally yellow as the liquid becomes more basic. That makes it a natural indicator and a safe kitchen experiment with lemon juice and baking soda. Pick a liquid in the sandbox and see its pH and its indicator colour. A calm reminder: some cleaning products, bleach above all, must never be mixed with other cleaners, because they can release toxic gas. Use one at a time, with fresh air.

Acids, bases and pH