# From the equation u = q + w

1. Oct 13, 2011

### jackcie

when a liquid vaporises to a gas,
q = m x lv

this energy is lost???

also, what does this mean for when a gas turns to a liquid is q added to the system, or lost (added i presume??)
cheers
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2. Oct 13, 2011

### Andrew Mason

The first law of thermodynamics says that it cannot be lost: $\Delta Q = \Delta U + W$. The heat flow of vaporization has to be equal to the work done by the gas plus the change in internal energy of the gas.

When water, for example, vaporizes, it expands doing work on the surrounding atmosphere. Also internal energy increases. Although the temperature of the water does not change in vaporization, the bonds between water molecules are broken in the vaporization, thereby increasing the potential energy of the gas.
It depends on the gas. When water vapor condenses heat flows out of the gas with the loss of molecular potential energy and work is done on the gas by the surroundings as the volume decreases.

AM

3. Oct 14, 2011

### Staff: Mentor

(quoting the OP)