Question

A steel blade floats on the surface of pure water, when detergent is added it sinks.Why ?

Answer 1

When a steel blade is placed on water horizontally$,$ the surface tension and the buoyancy will be able to balance the wight of the blade and it will float on the water surface$.$ You know the buoyancy is not sufficient to make the steel blade float$.$ Now$.$ if you add detergent to water$,$ the surface tension of water immediately decreases$.$ The surface tension no longer be able to balance the blade and it will sink$.$