Question

Oil spreads on cold water but may remain as a drop on hot water because:

Answer 1

Hint : When a liquid comes in contact with a surface (solid or liquid), two forces come into action one is cohesive forces and the other is adhesive forces. These forces are responsible for the shape of liquid on that surface. Cohesive forces are generic for the collective intermolecular forces (hydrogen bonding and van der Waals forces) which are responsible for bulk properties of liquids resisting separation.

Complete Step By Step Answer:
To know about why oil spreads on cold water and remains as a drop on hot water, first we need to understand the two types of forces which are responsible for the shape of liquid on that surface, one is cohesive forces and the other is adhesive forces.
So, cohesive forces are generic for the collective intermolecular forces which are responsible for the bulk property of liquids resisting separation while the adhesive forces are the attractive forces between substances.
Another phenomenon responsible for shape is surface tension. It can be defined as the property of the liquid surface which allows it to resist an external force because of the cohesive nature of the water molecules.
Now, the difference of shape in hot and cold water can be explained by surface tension because Surface tension of oil is less than that of cold water, so the oil spreads over cold water. However, the surface tension of oil is greater than hot water so, its shape remains as a drop.

Note :
Surface tension is the tendency of a liquid surface to cover the minimum surface area possible. For example, the rain drops are spherical due to the surface tension because at liquid–air interfaces the surface tension results from the greater cohesion of liquid molecules to each other than to the adhesion.