The pH Scale: Acids, Alkalis and Neutral

The pH scale shows how acidic or alkaline a solution is. It usually runs from 0 to 14:

• pH 7 is neutral, like pure water at 25 °C.
• Below 7 is acidic. The lower the number, the more acidic the solution.
• Above 7 is alkaline (basic). The higher the number, the more alkaline.

pH depends on the concentration of hydrogen ions in the solution. Acids release hydrogen ions in water; alkalis release hydroxide ions. Very concentrated solutions can go slightly beyond 0 or 14, but the 0–14 range covers nearly everything met in school and at home.

The pH scale is not like a ruler. Each step of one pH unit is a tenfold change in hydrogen ion concentration. A solution at pH 3 is 10 times more acidic than one at pH 4, and 100 times more acidic than one at pH 5. So lemon juice, at about pH 2, is roughly 1,000 times more acidic than coffee at about pH 5. This is why small changes in pH matter, for example in rivers, oceans and the human body.

The values on the poster are typical, not exact; real samples vary. Gastric juice in the stomach is strongly acidic, at about pH 1.5, and lemon and lime juice are about pH 2. Coffee is weakly acidic, at about pH 5, and even clean rain is slightly acidic, at about pH 5.6. Pure water is neutral. Seawater is slightly alkaline, at about pH 8, while milk of magnesia, an antacid, is about pH 10.5 and household ammonia is about pH 12. At the two ends of the scale, laboratory hydrochloric acid and sodium hydroxide are corrosive and are used only with eye protection.

An indicator changes colour depending on pH. Litmus is red in acid and blue in alkali, so it shows only whether a solution is acidic or alkaline. Universal indicator is a mixture of indicators that runs from red for strong acids, through green at neutral, to purple for strong alkalis, so it gives an approximate pH. Red cabbage juice works as a homemade indicator. A pH meter gives a more precise number and works with coloured liquids that would hide an indicator's colour.

When an acid reacts with a base, they cancel each other out: acid + base → salt + water, and the pH moves towards 7. Antacids such as milk of magnesia use this reaction to neutralise excess stomach acid. Hydrochloric acid and sodium hydroxide react to make sodium chloride and water.

Print the poster on A4 or US Letter for handouts, or on A3 or Tabloid for a classroom wall. It pairs with the LearnBySlides lesson on acids, bases and pH, which includes a safe universal indicator practical.