My textbook says a pH of 4 is 10 times more acidic than pH 5. That seems like a big jump for "just one number". Why is it like that?
Yes. pH is minus the base-10 logarithm of the hydrogen-ion concentration, so each step down the scale means ten times more H⁺ ions. A solution at pH 3 is ten times more acidic than one at pH 4 and a hundred times more than pH 5. That is how one short scale can cover everything from battery acid to drain cleaner.
Accepted answer
Yes - pH is defined as pH = -log₁₀[H⁺], the negative base-10 log of the hydrogen ion concentration. Because it’s a log scale, each whole-number drop in pH means the H⁺ concentration (and acidity) increases by a factor of 10, not just "a bit more".