Sir/madam I have a question. How can we say that water spreads on glass easily inspite of its low surface tension. Also how can we say that the interface of mercury-glass does not spread on glass easily inspite of its high surface tension?????plz reply quickly!!!

How easily a liquid drop spreads on a solid surface depends on contact angle.

 

A contact angle less than 90° (low contact angle) usually indicates that wetting of the surface is very favorable, and the fluid will spread over a large area of the surface. Contact angles greater than 90° (high contact angle) generally means that wetting of the surface is unfavorable so the fluid will minimize contact with the surface and form a compact liquid droplet.

The angle of contact between water and glass typically lies in the range to , whereas that between mercury and glass is about

Therefore, mercury does not spread easily on glass surface while water spreads easily on it.

 

Contact angle	Degree of wetting
θ = 0	Perfect wetting
0 < θ < 90°	high wettability
90° ≤ θ < 180°	low wettability
θ = 180°	perfectly non-wetting