What is scum and how is it formed?

Hard water is water that contains a high amount of dissolved minerals, especially calcium and magnesium ions, unlike soft water.
It is usually formed when rainwater or groundwater flows through rocks like limestone, chalk, or gypsum that are rich in these minerals.

The hardness of water is of two types:

1. Temporary hardness – caused by bicarbonates of calcium and magnesium, which can be removed by boiling.
2. Permanent hardness – caused mainly by salts like calcium sulphate and magnesium sulphate, which do not get removed by boiling because they do not precipitate on heating.

Normally, soaps form micelles (tiny radial structures) around dirt or oil particles, helping wash them away.
But in hard water, this process is disturbed.

When soap is used in hard water, the calcium and magnesium ions react with the soap molecules to form an insoluble substance called scum.

• This scum is sticky and gets deposited on clothes, floors, and sinks, making cleaning less effective.