Question

	Why does water form into beads on a freshly washed or freshly waxed car. What characteristics of water and the car's finish or wax cause this?

Answer 1

Due to hydrogen bonding between water molecules, water has two unique properties :cohesion and adhesion.Cohesion refers to the fact that water sticks to itself very easily.Adhesion means that water also sticks very well to other things, which is why it spreads out in a thin film on certain surfaces, like glass.When water come in contact with these surfaces, the adhesive forces are stronger than the cohesive forces.

Water form into beads on a freshly washed or freshly waxed car. Water is a typical polar molecule. It can have adhesive forces only with other polar molecules in solid form like glass.With non polar molecules, water has no adhesive forces.And wax is nonpolar.

Water also has high level of surface tension.This means that the molecules on the surface of the water are not surrounded by similar molecules on all sides, so they are being pulled only by cohesion from other molecules deep inside.These molecules cohere to each other strongly but adhere to the other medium weakly.This leads to water to beads up on waxy surfaces.Because there is no attractions between polar water molecules and non polar wax.Surface tension makes these water droplets round so they cover the smallest possible surface area.