# Does the dew point of a certain amount of vapor in a confined space change with temperature?

I know that dew point is temperature independent and is pressure dependent.But my textbook states that in a confined space dew point changes with temperature as the volume is constant pressure is proportional to temperature. Apparently it seems to be correct as pressure is changing with temperature but I am really confused about this.

My textbook also states that vapor pressure at a particular temperature is exactly equal to vapor pressure at dew point. And based on that relative humidity is formulated as the ratio of vapor pressures at dew point to the saturated vapor pressure at that temperature. This seems wrong too.

Can anyone please verify whether these two statements are right or not?

Judging by that question and the post time, I think it's fair to assume that you're a fellow candidate. Best of luck with your exam. I tried to answer your question to the best of my abilities, but not everything I say is completely set in stone, so feel free to take it with a grain of salt.

in a confined space dew point changes with temperature as the volume is constant pressure is proportional to temperature

This is correct, because key word - confined space. SVP at dew point really is just a measure of the vapour pressure at a certain place. $$\frac{Amount \,of \,vapour \,present(vapour\, pressure)}{Amount\, of \,vapour\, needed (SVP \,at \,\theta ))} = \frac{SVP \,at \,dew \,point}{SVP \,at \,\theta }$$

A temperature decrease results in a decrease in molecular activity, subsequently decreasing pressure, and in a confined space the amount of vapour molecules can't increase to mitigate that (this is what happens in an open space, and why we consider dew point to be independent of temperature there) and that's why while the amount of vapour in that place stays constant, the pressure exerted by that amount goes down. Now it's imporatnt to remember that dew point is the temperature at which a set amount of vapour already present in an environment will saturate it. Since the amount of vapour doesn't change, dew point should not change either. However, when the pressure exerted by the same amount of molecules goes down, it seems as if the amount of molecules was decreased. This superficial change in the number of molecules is what changes the dew point.

• >$$\frac{Amount \,of \,vapour \,present(vapour\, pressure)}{Amount\, of \,vapour\, needed (SVP \,at \,\theta ))} = \frac{SVP \,at \,dew \,point}{SVP \,at \,\theta }$$ Didn't get this portion. How vapor pressure at any temperatue be equal to vapor pressure at dew point? As far as I know unsaturated vapor acts more like an ideal gas so considering that and taking pressure law in account, decreasing temperature should decrease pressure as well.
– MSKB
Commented Nov 30, 2021 at 15:26
• Does that superficial change cause the pressure exerted by vapor in saturated air to decrease too under the same condition without changing the amount of vapor?
– MSKB
Commented Nov 30, 2021 at 15:31
• >How vapor pressure at any temperatue be equal to vapor pressure at dew point? the capor pressure is equal to the SATURATED vapour pressure at dew point. Dew point is defined such that if the temperature is decreased while keeping the amount of vapour constant (which it will be at normal circumstances), the temperature at which it will become saturated is the dew point. >Does that superficial change cause the pressure exerted by vapor in saturated air to decrease too under the same condition without changing the amount of vapor? I believe it does. Commented Nov 30, 2021 at 16:04
• as for your answer to the 1st question(comment), I used to know otherwise since the pressure that is applied by the vapor is due to the motion of the particle which is a function of temperature( referring to kinetic energy actually) not only due to the amount of vapor. I might be wrong here.
– MSKB
Commented Nov 30, 2021 at 16:16
• And to the second answer if both the pressure of saturated vapor and unsaturated vapor decrease due to the decrement of temperature then how subtle is that change in dew point? I suppose it is extremely small, that is at a scale of 10^-3 within a range of 10-20°C (just guessing it). Is it so?
– MSKB
Commented Nov 30, 2021 at 16:18