The expansion of a material from liquid to gas involves comparing which volumes?

• A. The volume of the gas form to the volume of the liquid form
• B. The mass of liquid to the mass of the gas
• C. The volume of the liquid form to the volume of the gas form
• D. The pressure of liquid to gas

Answer: (C)

When a material changes from liquid to gas, the key thing that changes is how much space it occupies. The useful measure is the volume in each phase for the same amount of substance, so you compare the volume of the liquid form to the volume of the gas form. Mass stays the same during vaporization, so comparing mass wouldn’t show how much the volume expands. Pressure influences the actual volume of the gas, but it isn’t the measure of expansion itself. Because gas molecules are far more spread out and highly compressible, the gas volume is vastly larger than the liquid volume, illustrating the dramatic expansion.