Final answer:
The pH measures H+ concentration in a solution, with a pH of 7 being neutral for water. Solutions with pH less than 7 are acidic, and those with pH greater than 7 are basic.
Step-by-step explanation:
The measure of H+ in a solution is called the pH. The value for pure water, which is considered neutral, is 7. Solutions are acidic if the pH is less than 7, because they have a higher concentration of hydronium ions ([H3O+]) compared to water. On the other hand, solutions with a pH higher than 7 are basic (or alkaline) because they have a lower concentration of hydronium ions.
A pH value is the negative logarithm of the hydronium ion concentration (-log[H3O+]). For example, with a neutral solution where [H3O+] = 1 × 10-7 M, the pH is 7. If [H3O+] is greater than 1 × 10-7 M, the solution is acidic, and if it is less, the solution is basic.