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The pH scale is a numerical scale used to measure the acidity or basicity of a solution. The scale ranges from 0 to 14, where 7 is considered neutral. This means that pure water, which is neither acidic nor basic, has a pH of 7.
Values less than 7 on the pH scale indicate that the solution is acidic, while values greater than 7 indicate that the solution is basic or alkaline.
The pH scale is logarithmic, meaning each whole pH value below 7 is ten times more acidic than the next higher value. For example, a pH of 5 is ten times more acidic than a pH of 6. This logarithmic nature helps in differentiating between solutions with vastly different hydrogen ion concentrations.

Hydrogen ion concentration is a key factor in determining the pH of a solution. It is usually represented as \[ { \text{H}^+ } \].[br] The concentration of hydrogen ions in a solution is inversely related to its pH. A high concentration of hydrogen ions results in a low pH, making the solution acidic, while a low concentration of hydrogen ions results in a high pH, making the solution basic.
Mathematically, pH can be expressed as pH = -log \[ { \text{H}^+ } \], where \[ { \text{H}^+ } \] is the molar concentration of hydrogen ions. This formula shows that as the concentration of hydrogen ions increases, the pH value decreases.

Acidic solutions have a pH value lower than 7. These solutions contain a higher concentration of hydrogen ions (\[ { \text{H}^+ } \]) compared to neutral or basic solutions. Common examples of acidic solutions include lemon juice, vinegar, and stomach acid.
When an acid is added to a neutral solution, it increases the number of \[ { \text{H}^+ } \] ions present. This increase in hydrogen ion concentration results in a decrease in pH, making the solution more acidic. For example, if you add hydrochloric acid (HCl) to water, it dissociates into H\text{ }^+\ and Cl\text{ }^-\ ions. The additional H\text{ }^+\ ions lower the pH of the solution.
Knowing how to manipulate and measure pH is crucial in many fields, such as chemistry, biology, medicine, and environmental science.