The acidity or alkalinity of a solution is determined by the concentration of hydrogen ions (H+) present in the solution. According to the pH scale, which ranges from 0 to 14, a solution with a pH value of 7 is considered neutral, indicating an equal concentration of hydrogen ions and hydroxide ions (OH-). Solutions with a pH value below 7 are acidic, signifying a higher concentration of hydrogen ions, while those with a pH value above 7 are basic or alkaline, indicating a lower concentration of hydrogen ions and a higher concentration of hydroxide ions.

The free hydrogen ions in solution are responsible for giving acidic solutions their characteristic properties, such as sour taste, ability to react with metals and bases, and the ability to turn certain indicators (like litmus) red. The higher the concentration of free hydrogen ions, the stronger the acidity of the solution.