A steel blade floats on the surface of pure water but sinks when detergent is added to the water.

1. Physics in day to day Life – Part II
A steel blade floats on the surface of pure water but sinks
when detergent is added to the water.
A steel razor blade (or a sewing needle) lowered horizontally on
the surface of pure water is found to float. (First drawing the dry
blade or needle through the fingers deposits a thin film of grease
on its surface which makes it non-wetting and easier to float.)
The free surface of water becomes depressed until the floating

2. body is supported by a combination of forces due to buoyancy
and surface tension. Very small floating bodies (like a razor
blade or needle here) are supported principally by surface
tension rather than buoyancy. Adding detergent (or any
surfactant) to the water greatly lowers the surface tension and
consequently the ability of the surface film to support the weight
of the blade (or needle). Thus, the blade sinks.
Hot soap water is used for washing clothes.
To remove dirt from clothes thoroughly, water must be forced
through the tiny spaces between the weaves. This requires an
increase in the surface area of water, which is precluded by its
large surface tension. Increasing the temperature of the water
and adding soap both decrease the surface tension of water and
make washing clothes easier.
Stay with us for more information.
- Team Ednexa