Why Coloured Soaps Produce White Foam

We use soaps in our day-to-day life for bathing and washing our hands. With the advent of technology and advancement in science, the soaps which were usually white and round are now being sold in different colours and shapes. Liquid soap is one of the evolutionary examples in this sector. However, it has been observed that irrespective of the colour, the foam of the soap is always white whenever it comes in contact with water.

Netizens have started wondering about the reason for the same. Some of the curious users posted this query on the online public interactive platform Quora, in an attempt to seek answers to this surprising phenomenon. The replies have gone viral now. As per the responses, it is believed that white foam from coloured soaps is the result of the light refraction phenomena which is also known as Mie Scattering. The same principle can also be seen in the clouds, as they also appear to be white when watched from a distance.

One of the users on Quora responded to this query and listed the basic constituents of the soap to explain this phenomenon. He said a typical coloured soap contains a surfactant (cleaning agent), a dye (to create colour), a perfume (for aroma) and other additives. It is done to maintain the solid shape of the soap and to induce the moisturising agents so that they are gentle on the skin. When the soap comes in contact with water, its surfactant with its organic chain and ionic heads comes in contact with water molecules to create foam.

So, when these ionic heads get preferentially attracted to water (which is ionic) they detach from the dye molecules in the soap. The bubbles of the soap then reflect light from all wavelengths, hence resulting in a white appearance. Another user wrote that colour soaps produce white foam because the colourants used in the soap don’t affect the colour of the foam.

According to the report of Science ABC, the layer formed when soap is dissolved in water is very thin. It is completely transparent. The prime reason for it is light. When light enters a soap solution, it has to pass through many small soap bubbles, that is, many surfaces. These countless surfaces scatter light in different directions, due to which the amount of light inside becomes very high and foam becomes visible. The low quantity of dye used in the soap is also one of the major reasons for the production of white foam.