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Chemical Engineering Basics - Engineering

Q1:

The pressure outside a bubble/droplet of liquid is __________ the internal pressure.

A greater than

B less than

C equal to

D unpredictable ; depends on the bubble size

ANS:B - less than

The pressure outside a bubble/droplet of liquid is less than the internal pressure. Explanation: When a bubble or droplet of liquid is immersed in another fluid (such as air or another liquid), it experiences pressure from the surrounding fluid. This external pressure is less than the internal pressure within the bubble or droplet. This pressure difference arises due to the curvature of the surface of the bubble or droplet. The pressure inside the bubble or droplet is higher than the pressure outside because the surface tension of the liquid-air interface tends to minimize the surface area of the bubble or droplet. As a result, the pressure inside the bubble or droplet is greater than the pressure outside. Mathematically, this pressure difference is described by the Laplace pressure equation: ΔP=r2γ​ Where:

  • ΔP is the pressure difference,
  • γ is the surface tension of the liquid-air interface,
  • r is the radius of the bubble or droplet.
Since the surface tension γ is always positive and the radius r is positive, the pressure difference ΔP is positive. This indicates that the pressure inside the bubble or droplet is greater than the pressure outside.