A solution with a concentration of hydrogen ions higher than 10-7mol/L is acidic, and a solution with a lower concentration is alkaline (another way to say basic). Using the formula, pH=-log[H+], a pH of 7 is neutral, a pH less than 7 is acidic, and a pH greater than 7 is basic.
The pH scale is a method to measure if a substance is an acid or base. The amount of hydronium and hydroxide ions there are in a solution determines how acidic or basic it is. Water is the most neutral on this scale, with an equal number of hydronium and hydroxide ions.
When a substance has more hydronium ions, it is more acidic.
When a substance has more hydroxide ions, it is more basic.
