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**1. Homework Statement**

The partial pressure of water vapor in air at 20

^{0}is 10[mm] mercury. according to the table of partial pressures we have to cool the air to 11.4

^{0}in order to bring the air to saturation, that is100% relative humidity. this according to the book.

But when we cool the pressure and\or volume decrease according to ##\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}##, so why is P left constant? is it because V, the volume shrinks only? the atmosphere shrinks? and why doesn't P reduce, only V?

**2. Homework Equations**

From the equation of state of the ideal gases: ##\frac{P_1V_1}{T_1}=\frac{P_2V_2}{T_2}##

**3. The Attempt at a Solution**

Maybe, because we talk about free air, the volume isn't fixed and we can reduce V. but i am not sure we talk about atmospheric air only, it's not explicitly stated in the book.