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When we speak of the 'surface' of a liquid (the region responsible for surface tension), we mean

1. a purely geometric plane such as x = 0
2. every molecule that is in contact with the atmosphere
3. a thin layer whose thickness is of the order of 10⁻⁸ m
4. a thin layer whose thickness is of the order of 10⁻⁴ m

Correct answer: a thin layer whose thickness is of the order of 10⁻⁸ m

Solution

Surface tension originates from the unbalanced intermolecular attractions on molecules lying within the range of molecular forces of the free surface. This region is a layer a few molecular diameters thick, of order 10⁻⁸ m (a few nanometres), not a mathematical plane.

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