A tall narrow glass tube is stood upright in a dish of mercury, and mercury makes a contact angle of 140 degrees with glass. What does the mercury in the tube do?
AIt rises well above the dish level
BIt is suppressed below the dish level
CIt stays exactly level with the dish
DIt rises until the tube is quite full
Answer & Solution
Correct answer: B. It is suppressed below the dish level
1. Capillary action raises a liquid when the contact angle is less than 90 degrees and suppresses it when the angle is greater than 90 degrees.
2. Mercury on glass has a contact angle of 140 degrees, which is greater than 90 degrees.
3. Its surface curves downward like a drop, and surface tension flattening that curve pulls the column down.
4. The mercury is therefore suppressed below the level in the dish.
5. Water in glass behaves the opposite way, with a contact angle near zero, so it is drawn up the tube instead.
_Source: OpenStax College Physics (CC BY 4.0), Ch 11 "Fluid Statics", section 11.8 Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action_
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