pH is a number that tells you how acid or alkaline a substance is, usually a liquid. Learner books give the scale as 0 to 14. 7 is the middle — like pure water — neither acid nor alkaline. Below 7 is acid (lemon juice, vinegar, the acid in your stomach). Above 7 is alkaline (soap, baking soda in water, bleach). Each whole step is a big jump: pH 3 is much more acid than pH 4. Scientists use it, but so do pool cleaners, farmers, and people with fish tanks. You say it as two letters, 'p' 'H', never like the 'ph' in 'phone'. The H is the international sign for hydrogen; pH is about hydrogen ions in the mix. The scale was introduced in 1909 by S. P. L. Sørensen, who spoke of a 'hydrogen ion exponent'. He may have chosen p as a simple label; others guess it came from words for 'power' (French puissance, German Potenz, Danish potens) or from 'potential'. Though actually, in a serious lab the number can sit a little outside 0–14 when a mix is extremely strong — but the everyday scale you will be tested on is still that 0-to-14 line with 7 in the middle.