Ph- Measures the acidity of a substance. Neutral pH is considered to be 7.0 (the pH of pure water). Substances with a pH of less than 7.0 are acids; pHs higher than 7.0 are bases. The lower the pH the stronger the acid. The higher the pH, the stronger the base. Very high pHs or low pHs are irritating to the skin and may cause acid or chemical burns.