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Question
The sufrace tension and vapour pressure of water at 20°C is 7.28 × 10–2 Nm–1 and 2.33 × 103 Pa, respectively. What is the radius of the smallest spherical water droplet which can form without evaporating at 20°C?
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Solution
According to the problem, the surface tension of water, T = 7.28 × 10–2 Nm–1 Vapour pressure P = 2.33 × 103 Pa. Let r = radius of drop, which formed without evaporating.
The excess pressure (2T/r) should be greater than the vapour pressure. Then, the drop will evaporate.
Vapour pressure = Excess pressure in drop
`(2T)/r = P`
⇒ `r = (2T)/P`
⇒ `r = (2 xx 7.28 xx 10^-2)/(2.33 xx 10^3)` = 6.25 × 10–5 m
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