pH measures how acidic something is, from 0 (fierce acid) through 7 (plain water) to 14 (fierce alkali). The catch: every step is times ten. A pH 5 coffee isn't a bit more acidic than pH 7 water — it's a hundred times more.
Most people read pH as a simple linear scale. In fact every step is a factor of ten, so pH 5 coffee is a hundred times more acidic than pH 7 water, and a tiny ocean shift from 8.2 to 8.1 is a 26% jump in acidity.
What's actually happening
The active ingredient of acidity is the loose hydrogen ion, H⁺ — a bare proton, the smallest chemical actor there is. Acids are molecules that shed these protons into water; the more protons loose per litre, the more acidic the solution, the more aggressively it attacks metals, teeth, and tastebuds (sourness is your tongue's proton detector). Bases are the opposite: proton sponges that soak them up.
The concentrations involved span an absurd range — battery acid carries about 100 trillion times more loose protons per litre than drain cleaner leaves around. No linear scale can hold that, so in 1909 the chemist Søren Sørensen (working quality control at the Carlsberg brewery) compressed it with a logarithm: pH is the power of ten, sign-flipped. Pure water sits at 10⁻⁷ moles of H⁺ per litre — pH 7. Each step down multiplies the proton count by ten. The scale is tidy; the reality it compresses is violently exponential.
That exponential character is why pH numbers that look close are chemically far apart. Ocean water drifting from pH 8.2 toward 8.1 sounds like a rounding error — it's a ~26% jump in acidity, enough to thin the shells of sea creatures. Your blood is held between 7.35 and 7.45 by frantic buffer chemistry, because a drift of half a unit is a medical emergency. On a times-ten ladder, half a rung is a long way to fall.
pH counts loose hydrogen ions on a times-ten ladder, so numbers that look close are chemically far apart — half a rung is a long way to fall.
- 1Chop a quarter of a red cabbage, cover with boiling water, wait ten minutes, and strain off the purple liquid — a genuine pH indicator.
- 2Split it between glasses and add a splash of test liquid to each: lemon juice, vinegar, fizzy water, soap solution, baking-soda water.
- 3Watch the colours fan out — pink/red for acids, blue-green for bases. You've just built the chemistry-lab classic from a vegetable.
Common questions
Acid concentrations span an absurd range — battery acid carries about 100 trillion times more loose protons per litre than drain cleaner. A logarithm compresses that into a tidy 0–14 scale, so small pH shifts mean large chemical ones.
Because the scale is exponential, a drift from pH 8.2 toward 8.1 is roughly a 26% jump in acidity — enough to thin the shells of sea creatures, despite sounding like a rounding error.
Cola sits near pH 2.5 to 3, about a thousand times more acidic than the pH 5.5 at which tooth enamel starts to dissolve.