High School Why is the volume of a gas bigger during a phase change

[Richie Smash]

Hello, I would like to know, why is the volume of gas produced during phase change significantly more than the original volume of the liquid being used? Say water changing phase at 1atm.

 

[BvU]

The molecules have a hefty lot more kinetic energy
https://chem.libretexts.org/Textboo...s,_Liquids,_and_Gases:_A_Molecular_Comparison

 

[Khashishi]

Gas molecules don't necessarily have more kinetic energy than liquid -- it depends on the temperature. If you raise the temperature past the boiling point, it will change to gas. But below the boiling point, you can have gas (called vapor) and liquid in equilibrium at the same temperature.

The volume of a gas is greater since the molecules spread out to fill the container. In a liquid, the molecules are stuck to each other, so they don't fill the whole container (just the bottom, assuming gravity is pulling them down).

 

[BvU]

Gas molecules don't necessarily have more kinetic energy than liquid

Is there a way to show that ?

 

[Richie Smash]

Ok thank you, both sound correct to me

 

[Khashishi]

Is there a way to show that ?

The equipartition theorem states that the average energy in a mode is $\frac{1}{2} kT$. So, the energy per degree of freedom is the same for a gas and liquid at equilibrium. But, if you want a complete answer, you need to know how many modes there are.

Wikipedia has a table here
https://en.wikipedia.org/wiki/Heat_capacity
which shows that liquid water has a higher heat capacity than gaseous water at 100C. I think this means the liquid water has more degrees of freedom than gaseous water, and therefore contains more kinetic energy per molecule. Of course, gaseous water has more potential energy than liquid water.