Answer: surface tension of oil is much smaller than that of water
Notes: Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in the floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e. g. water striders) to run on the water surface. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids. Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. Water has the greatest surface tension, due to greater forces between the molecules of water compared to oil (mainly due to hydrogen bonding of water molecules to each other). So an oil droplet spreads over water.
