The pressure of trapped ammonia liquid increases when temperature rises; this is described as?

• A. Vapor expansion
• B. Hydrostatic pressure increase
• C. Thermal conduction
• D. Pneumatic pressure

Answer: (B)

When ammonia is trapped in a fixed-volume vessel, heating increases the molecules’ kinetic energy and, more importantly, raises the vapor pressure of ammonia. More ammonia enters the vapor phase and the gas in the headspace exerts greater pressure on the container. This rise in pressure due to higher vapor pressure is best described as vapor expansion from temperature increase. Hydrostatic pressure, on the other hand, depends on the height of a liquid column and gravity, not on temperature, so it doesn’t explain the pressure rise here. Thermal conduction is about transferring heat, not pressure, and pneumatic pressure refers to air/gas systems rather than the liquid’s behavior.