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Explain why The angle of contact of mer...

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

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When a drop of liquid is poured on a solid surface, three interfaces namely liquid-air, solid-air and solid-liquid are formed. If `S_(la),S_(sa)` and `S_(sl)` be surface tensions corresponding to liquid-air solid-air, and solid-liquid respectively and `(theta)` be angle of contact between the liquid and the solid, then `S_(la)costheta+S_(sl)=S_(sa)`
`costheta=(S_(sa)-S_(sl))/(S_(la))`

For mercury glass, `S_(sa)ltS_(sl)` therefore `costheta` is negative. Thus `theta` is an obtuse angle. In case water-glass, `S_(sa)gtS_(sl)` therefore coso is positive. Therefore `theta` is acute angle.
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