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KVPY Sample Paper KVPY Stream-SX Model Paper-7

  • question_answer
    Surface tension of a liquid at room temperature is \[6.99\times {{10}^{-2}}N{{m}^{-1}}\]. If vapour pressure of the liquid is \[2.33\times {{10}^{3}}\] Pa, then radius of smallest drop which can be formed without evaporating at room temperature is

    A) 0.06 mm                      

    B)  0.4 mm

    C) 0.6 mm                        

    D) 0.32 mm

    Correct Answer: A

    Solution :

    Drop evaporates if pressure inside drop is greater than its vapour pressure. So, to avoid evaporation,
    \[{{P}_{excess}}\le {{p}_{vapour}}\Rightarrow \frac{2S}{R}={{p}_{vapour}}\]
    \[R=\frac{2S}{P}\]
    \[=\frac{2\times 6.99\times {{10}^{-2}}}{2.33\times {{10}^{3}}}\]
    \[=6\times {{10}^{-5}}m=0.06mm\]


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