Worksheet: Contact Angles

In this worksheet, we will practice relating the contact angles of drops on a surface with the solid–liquid, liquid–gas, and solid–gas surface tensions.

Q1:

How is it possible for a steel needle to be supported on the surface of water?

  • AThe interactions between water molecules are stronger than those between water and steel, so the system has a lower potential energy if the needle does not sink.
  • BSteel is less dense than water.
  • CSinking of the needle is energetically costly because air–water interactions are much weaker than air–steel interactions.
  • DThe high surface tension of water supports the steel needle.
  • ESinking of the needle is energetically costly because it increases the surface area of the water per unit volume.

Q2:

When a glass capillary tube is placed into a flask of mercury, the meniscus of the mercury in the tube is lower than the surface of the mercury outside. What can be concluded from this observation?

  • AThe contact angle between mercury and glass is less than 90.
  • BThe contact angle between mercury and glass is negative.
  • CThe surface tension between mercury and glass is zero.
  • DThe contact angle between mercury and glass is greater than 90.
  • EThe surface tension between mercury and glass is negative.

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