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Explain why (a) The angle of contact of mercury with glass is obtuse, while that of water with glass is acute.

Q 10.2  Explain why

(a) The angle of contact of mercury with glass is obtuse, while that of water with glass is acute.

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S Sayak

TSL = Surface tension corresponding to the solid-liquid layer

TLA = Surface tension corresponding to liquid-air layer

TSA = Surface tension corresponding to solid-air layer

The angle of contact is \Theta

Since the liquid is not flowing over the solid surface the components of TSL, TLA and TSA along the solid surface must cancel out each other.

\\T_{SL}+T_{LA}cos\Theta =T_{SA}\\ cos\Theta =\frac{T_{SA}-T_{SL}}{T_{LA}}

In case of mercury TSA < TSL and therefore cos\Theta <0 and therefore \Theta >\frac{\pi }{2} i.e the angle of contact of mercury with glass is obtuse.

 

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