In areas where water comes into contact with limestone, there may be high levels of calcium and magnesium present. These chemicals make the water "hard", producing scale and soap scum. Hardness is removed by a process called softening. Two chemicals (lime, CaO and soda ash, Na₂CO₃ ) are added to the water, causing the calcium and magnesium to form a precipitate. This solid substance is then removed with the other particles by sedimentation and filtration.

World's largest limestone quarry
Rogers City, Michigan

Hardness leads to deposits in pipes
which can greatly reduce water flow.