The pH Scale: Counting in Powers of Ten
- Chemistry
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Acids release hydrogen ions in water, and pH measures their concentration on a logarithmic scale.
- Each step of one pH unit is a tenfold change in hydrogen ion concentration.
- Strong and weak acids differ in how completely they ionise, not in how concentrated they are.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
A scale of powers of ten
Acids release hydrogen ions, written H+, when they dissolve in water. The more hydrogen ions a solution has, the more acidic it is. The amounts vary hugely, so in 1909 a Danish chemist, Søren Sørensen, invented the pH scale to measure them. Each step on the pH scale is ten times bigger than the next. A solution at pH 1 has ten times more hydrogen ions than one at pH 2, and a hundred times more than one at pH 3. So lemon juice at pH 2 is about a thousand times more acidic than coffee at pH 5. Pure water is neutral, at pH 7. Solutions above 7 are alkaline, which means they have more hydroxide ions, OH−, than hydrogen ions. Bleach is about pH 12 or 13. Universal indicator turns red in strong acid, green when neutral and violet in strong alkali.
Diluting, neutralising, and strong versus weak
Adding water makes an acid weaker, but more slowly than you might think. Dilute a pH 1 acid ten times and it only reaches pH 2. However much water you add, it will never go above pH 7. To really cancel an acid, you add an alkali. The hydroxide ions join with hydrogen ions to make water. This is called neutralisation. Chemists do it drop by drop in a titration. Near the end, a single drop can change the pH by several steps. Acids also differ in strength. Hydrochloric acid is strong: almost all of it breaks into ions in water. Ethanoic acid, the acid in vinegar, is weak: only about one molecule in a hundred does. So at the same concentration, hydrochloric acid has a lower pH. Strong and weak are not the same as concentrated and dilute.
Curriculum links: NGSS HS-PS1-6 · CCSS HSF-LE.A.1