A pH difference of 1 indicates that the H+ concentration in the lower pH solution is 10 times greater than in the higher pH one. Therefore, a pH of 3 has ten times more hydrogen ions and is ten times more acidic than a pH of 4, making it a stronger acid.
How does a difference in 1 pH unit change in terms of H+ concentration? A pH of 3 differs from a pH of 4 in that the H+ concentration in the pH 3 solution is 10 times higher than that in the pH 4 solution.
This is because the pH scale is logarithmic; a decrease in pH by one unit reflects a 10-fold increase in hydrogen ion concentration.
Therefore, a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4. The pH of 3 is considered stronger because it has a higher concentration of hydrogen ions compared to the pH of 4.