The pH scale
pH just measures how many H⁺ ions are floating about. Slide it — or tap a substance — and watch. Below 7 is acidic, 7 is neutral, above 7 is basic, and each step is a tenfold change.
Simplified mixing estimate — this converts each pH to [H⁺], takes the volume-weighted average of the two, and converts back to a pH. Real mixing also depends on the acid/base strength and buffering, not just the pH values.
The ideas you're seeing
pH = how acidic
The pH scale runs 0 to 14. Below 7 is acidic, 7 is neutral, above 7 is basic. More H⁺ ions means a lower pH.
Each step is ×10
A drop of one pH unit means ten times more acidic. So pH 3 is 100× more acidic than pH 5 — the scale is a shortcut for very big changes.
Indicators
Universal indicator shows the whole range by colour. Litmus is red in acid, blue in base; phenolphthalein is pink only in base.
pH in daily life
Tooth decay starts when the mouth drops below pH 5.5; an antacid (a base) neutralises excess stomach acid; acid rain has a pH below 5.6.
Part of the Acids, Bases and Salts chapter. One of Priodemy for School, free with every EduSuite school.
What the pH number really counts
A scale built on powers of ten
pH measures the concentration of hydrogen ions in a solution, but it does so logarithmically: pH = −log[H⁺]. The reason is practical. Hydrogen ion concentrations in everyday solutions range from about 1 mol/L down to 0.00000000000001 mol/L, and writing those numbers out is unmanageable. Taking the negative logarithm compresses that whole range onto a scale from 0 to 14.
The consequence is the one students most often underestimate: each step of 1 on the scale is a factor of 10 in concentration. A solution at pH 3 is not slightly more acidic than one at pH 5 — it is a hundred times more acidic. Lemon juice at pH 2 has roughly a thousand times the hydrogen ion concentration of black coffee at pH 5. Step through the scale here and watch the concentration readout change by an order of magnitude each time.
Why 7 is the middle
Pure water splits very slightly into H⁺ and OH⁻ ions, and at 25 °C this gives a hydrogen ion concentration of 10⁻⁷ mol/L. Taking the negative logarithm gives exactly 7, so neutral sits at 7 not because someone chose it, but because that is what water does. Below 7 the H⁺ ions outnumber OH⁻ and the solution is acidic; above 7 the reverse holds and it is basic.
The two are linked by pH + pOH = 14, so knowing one gives the other immediately. Note that the neutral point is temperature-dependent — water ionises more at higher temperatures — which is why the value 7 is always quoted at a stated temperature.
Where it matters
Blood is held between about 7.35 and 7.45, a remarkably narrow window, and moving outside it is life-threatening. Tooth enamel begins to dissolve below roughly pH 5.5, which is why bacteria producing acid from sugar cause decay and why brushing counteracts it. Most crops grow best in soil near neutral, so farmers add lime to raise the pH of acidic soil. Acid rain, rain made acidic by dissolved oxides of sulphur and nitrogen, damages both crops and stonework for the same reason.
Mistakes that cost marks
Treating the scale as linear. Describing pH 4 as "twice as acidic" as pH 8 is wrong by a factor of ten thousand. Any comparison question is testing whether you have understood that the scale is logarithmic.
Confusing strength with concentration. A strong acid ionises completely; a concentrated acid simply has a lot of solute. A dilute solution of a strong acid can easily have a higher pH than a concentrated solution of a weak one. The two words are not interchangeable, and questions deliberately test the difference.
Assuming the scale stops at 0 and 14. Those are the usual limits for ordinary solutions, not hard boundaries. A sufficiently concentrated strong acid can have a negative pH.