Water spreads over a clean glass plate but mercury does not. Why?
Let us consider a liquid drop on the surface in equilibrium.
For water the surface tension force between glass plate and air is greater than the surface tension between liquid and solid, therefore angle of contact is less than 90° and it will spread over the glass plate. For mercury the surface tension force between glass plate and mercury is less than the surface tension between liquid and solid, therefore angle of contact is greater than 90° and hence mercury will not spread over the glass plate.
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