PH value is a numerical value used to describe the acidity and alkalinity of a substance, which is expressed by 0~ 14, where 0 is a strong acid and 14 is a strong base. When the pH value is 7, the substance is neutral. The pH value of pure water is between 6.5 and 8.5, because water in nature is usually mixed with other substances, and it is these substances that change the pH value of water.