Acids & Bases: The pH Scale
Sour foods, stinging cleaners, and your own stomach share one particle: H⁺. Counting it takes a logarithm.
Builds on: 3.3 Solutions & Concentration
What acids and bases really are
Dissolve an acid in water and it releases H⁺ ions (bare protons, immediately grabbed by water to ride as H₃O⁺). Hydrochloric acid: HCl → H⁺ + Cl⁻. A base does the mirror image: it releases OH⁻ (like NaOH → Na⁺ + OH⁻) or swallows H⁺ directly (like ammonia). Sourness, the sting of citrus in a cut, limescale dissolving in vinegar — all of it is H⁺ at work; the slippery feel of soap is OH⁻ attacking the oils of your skin.
The two are made for each other. H⁺ meets OH⁻ and they vanish into the most harmless substance there is:
pH: a logarithmic ruler
H⁺ concentrations span an absurd range — from ~1 mol/L in battery acid to 10⁻¹⁴ mol/L in drain cleaner. Writing fourteen zeros is no way to live, so chemistry compresses the range with a logarithm:
The factor of ten is the part everyone forgets. Cola (pH 2.5) isn’t “a bit” more acidic than coffee (pH 5) — it carries about 300 times the H⁺ concentration. And pure water isn’t H⁺-free: water itself splits ever so slightly, giving 10⁻⁷ mol/L of each ion — that is the definition of neutral.
Dilution and its limit
Diluting an acid tenfold raises its pH by one step — but you can never dilute your way past 7. Add water to vinegar forever and you approach water, not a base. The scale’s two halves can only be crossed by chemistry (neutralization), never by plumbing.
A strong acid (HCl) releases every H⁺ it has; a weak acid (vinegar’s acetic acid) releases only a small fraction. Concentration says how much acid is in the bottle. Dilute HCl is strong but harmless-ish; glacial acetic acid is weak but will burn you. Chemistry’s vocabulary is precise where everyday language is sloppy.
⚗️ Lab — The pH Playground
Twelve everyday liquids on a 14-step ruler, with a dilution tap.
- Compare cola and coffee — read the ×10-per-step fine print below the scale.
- Dilute lemon juice ×10 repeatedly. Where does the pH get stuck, and why?
- Note where blood sits — your body holds it within ±0.05 of pH 7.4.
Work it by hand — 0 / 4
No multiple choice here: compute the value and type it. Suffixes like 2.5k, 20m or 100µ are understood; answers within ±2% count.