The escape of a gas through a small opening is called what?

• A. Diffusion
• B. Effusion
• C. Evaporation
• D. Adsorption

Answer: (B)

Effusion is the process of gas molecules escaping through a very small opening into a region of lower pressure. Because the hole is tiny, molecules pass through one by one, rather than mixing through the surrounding space all at once. The rate of effusion depends on how fast the gas molecules move, so lighter gases with higher average speeds effuse more quickly than heavier ones (Graham’s law). This is different from diffusion, which is the spreading of gas molecules through space due to concentration differences, not specifically through a small aperture. Evaporation is liquid turning into gas at a surface, and adsorption is gas molecules sticking to a surface.