Which term describes gas absorbed into a solid porous material with an internal pressure less than 101.3 kPa at 20°C?

• A. Gas
• B. Dissolved Gas
• C. Liquefied Gas
• D. Absorbed Gas

Answer: (D)

Gas stored inside the pores of a solid material at a pressure below atmospheric is described as absorbed gas. In porous solids, the gas is held on and inside the internal surfaces of the material by adsorption, rather than simply being free in a space or dissolved in a liquid or turned into a liquid. This makes absorbed gas the best descriptor for gas retained within the solid’s pore structure under sub-atmospheric pressure. The other terms don’t fit: gas is too general, dissolved gas implies a liquid medium, liquefied gas means the gas has been turned into a liquid, and absorbed gas specifically calls out the storage within the solid’s pores.