Question

when water containing washing powder is stirred vigorously, the foam floats above water surface?why?

Answer 1

Hey here is your answer
When water is mixed with washing powder, white mass of bubbles are formed called lather which separates out and floats above the water surface because it is less denser than the water. 
Hope it helps u

Answer 2

When water containing washing powder is stirred vigorously, the foam floats above the water surface because, when we add washing powder in water and stir it vigorously the washing powder particles start to dissolve in water.

It results in the formation of lather. Lather is foam or lump of bubbles produced when a detergent (such as soap) is stirred in water. The foam formed always floats on water surface as it is less thick than water because of presence of air bubbles.