Question

What is scum and how is it formed?

Answer 1

Understand the action of soaps when introduced in water. Now differentiate hard water from soft water because scum formation takes place in hard water only. With this you will understand the ions present which are different from soft water and thus can answer the question given above.

Complete step by step answer:
Hard water is the water having high mineral content in contrast to soft water. Hard water is formed when water percolates through deposits of ores like limestone, chalk or gypsum which contain the ions of calcium and magnesium.
Hardness of water can be of two types namely, Temporary and permanent hardness.
Permanent hardness contains mainly salts like calcium sulphate and magnesium sulphate. These salts cannot be removed by simple boiling. This is because these salts do not precipitate when the solution is heated.
Ideally soaps are supposed to form a radial structure(called micelle) around the dirt or oil particle and easily wash away. However, this is not the case when soaps are used in hard water.
When a soap is used in hard water, it leads to the formation of a solid substance. This substance is commonly called scum.

The formation of scum is due to the calcium and magnesium ions present in water which react with the soap molecule to form an insoluble and undesirable product. Scum builds up on clothes, floor and sinks.

Note: To remove permanent hardness from water, we use Calgon. Calgon is chemical compound with the composition sodium hexametaphosphate.
The chemical reaction taking place is given below:
${{[N{{a}_{4}}{{P}_{6}}{{O}_{18}}]}^{2-}}\text{ + }C{{a}^{2+}}\text{ }\to \text{ }\\!\\![\\!\\!\text{ }N{{a}_{2}}Ca{{P}_{6}}{{O}_{18}}{{]}^{2-}}$
In this way calcium ions precipitate out, making the water suitable for the soaps function.