Acidic

• (environment) The condition of water or soil that contains a sufficient amount of acid substances to lower the pH below 7.0.
  Source: Terms of the Environment
• (geology)

1. A descriptive term applied to those igneous rocks that contain more than 60% silica. Acidic is one of four subdivisions of a widely used system for classifying igneous rocks based on their silica content: acidic, intermediate, basic, and ultrabasic. AGI
2. Applied loosely to any igneous rock composed predominantly of light-colored minerals having a relatively low specific gravity. CF: felsic
   Source: Dictionary of Mining, Mineral, and Related Terms