Measuring acidity of rainwater. Electronic pH meter being used to measure the acidity of rainwater that has been collected in a jar. The meter is showing a pH of 5.5, which indicates that the water is slightly acidic. Pure water, which is neutral, has a pH of 7. The pH scale runs from 1 (strong acid) to 14 (strong alkali). Rainwater can be acidified by atmospheric pollution, such as that from car exhausts, factories and refineries. Strongly acidic rain can damage buildings and kill plants and animals.