Question

The oil drops float on the water surface due to _______________ .
1. Viscosity
2. Surface tension
3. friction
4. Stress

Answer 1

Correct Answer - Option 2 : Surface tension

CONCEPT:

• Surface tension: The property by virtue of which liquid tries to minimize its free surface area is called surface tension.
  • In a spherical shape, the surface area is minimum and for this reason, the raindrops are spherical.
  • Surface tension is measured as the force acting per unit length of an imaginary line drawn on the liquid surface.

\(Surface\;tension = \frac{{Force}}{{length}}\)

• When Oil or any other non-polar liquid float over water it decreases the number of hydrogen bonds to form with other surfaces.

EXPLANATION:

• The adhesive force between the water-oil molecules is more than the cohesive force between the oil-oil molecules.
• That's why the oil drops spread on the water surface and it floats on it.
• Hence option 2 is correct.

Cohesive force	Adhesive force
The force of attraction between molecules of the same substance is called the force of cohesion. This force is lesser in liquids and least in gases.	The force of attraction between the molecules of the different substances is called the force of adhesion.
Real-life example: It is difficult to break a drop of mercury into small droplets because of the large cohesive force between the mercury molecules.	Real-life example: Adhesive force enables us to write on the blackboard with chalk.