Question

soaps are of different colour but all the different soap give white colour when they wash

Answer 1

Lather or foam is nothing but a large collection of small soap bubbles. A soap bubble is , in turn, a very thin film of soap solution enclosing some air. Because of the low surface tension of soap solution, the film can stretch and spread and form innumerable bubbles with a very large total surface area.

Due to this, whatever slight tint is present in the thin film of the colored soap solution gets subdued. Although a soap film is more or less transparent, the lather or foam looks white because the light striking this large collection of bubbles gets scattered. That is the reason why all kinds of lather or foam appear white.

Answer 2

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Why does a colored soap produce white foam?

7 April, 2014 Blog

How does soap clean?
The secret lies in its synthesis and the “special” characteristics of the soap’s molecules, which consist of a hydrophilic and a hydrophobic part. The water molecules alone are unable to pass through a greasy stain, since oil and water do not mix. On the contrary, soap molecules, through their hydrophobic side, pass through the dirt, surround it and disrupt it, while their hydrophilic side helps to dissolve it. So when we wash our hands with soap, the soap attracts the dirt and when the water comes next it washes away the dirty soap.

Why does a colored soap produce white foam?
When we wash our hands, apart from mixing soap with water, we also mix air. In its effort to avoid contact with air and in order to isolate it, the water creates bubbles, in other words foam. This explains the round shape of the bubbles. In addition the color of the foam always tends to be white, even in the case of colored soaps. This is due to the fact that bubbles store within them relatively more air than soap material. Besides, the color of the natural light is white. What we actually see is the reflection of the natural light in the bubble, which is white. However, if we for instance happen to be in a room with red lighting the foam might look reddish.