Temp & Melting Point of Water: Pressure Effects Explained

[wave525]

assuming water at atmospheric conditions, now if the pressure is reduced, at some lower point of pressure, water starts to boil.at this point, will the temp of water change and why?

 

[davenn]

assuming water at atmospheric conditions, now if the pressure is reduced, at some lower point of pressure, water starts to boil.at this point, will the temp of water change and why?

hi wave535
welcome to PF

The boiling point of any liquid is the temperature at which its vapor pressure becomes equal to the atmospheric pressure. So if the atmospheric pressure is lower, it will take a lower temperature to make the vapor pressure equal to that of atmospheric pressure.
Therefore the lower air pressure will cause the boiling point temperature to be lower than 100C

here's a couple of examples I found...

In Denver, Colorado, USA (1700 m above sea level), the boiling point of water is around 95 degrees Celsius (203 degrees Fahrenheit). At the top of Mt. Everest (9500 m above sea level), water boils at 75 degrees Celsius (167 degrees Fahrenheit).

cheers
Dave

 

[mfb]

will the temp of water change and why?

If no heat is exchanged with the environment in other ways, the boiling will cool the water.

 

[wave525]

can you explain why

 

[mfb]

You need some energy to overcome the attraction between water molecules - molecules can boil only when they have more than the average energy (otherwise your water would simply explode). If you remove those molecules with high energy, the average energy of the remaining molecules gets lower.

 

[wave525]

yes but the same energy is given by lowering the pressure, lowering the pressure is same as increasing the temperature of the water to boiling point, and if temp of water decreases due to lowering of pressure then temp of water should also drop on increasing temperature at atmospheric pressure.but this doesn't seem to happen.

 

[mikeph]

I was never great at thermodynamics, but I think this question might need more detail. How do you propose to lower the pressure? If it's without any change in energy, then it's an adiabatic free expansion and the gas temperature is unchanged. This is not the same as increasing the water temperature.

In the former case, my answer would be that since the overall heat is constant, when the boiling point of the liquid drops to the gas/liquid temperature, the liquid will begin to boil. However, a vapour molecule at this temperature has more energy than a liquid molecule at this temperature (due to the latent heat), and since energy is constant, the liquid must lose some energy, dropping its temperature.

Maybe this is right, if so it's probably another way of saying what mfb already said.

 

[nasu]

yes but the same energy is given by lowering the pressure, lowering the pressure is same as increasing the temperature of the water to boiling point, and if temp of water decreases due to lowering of pressure then temp of water should also drop on increasing temperature at atmospheric pressure.but this doesn't seem to happen.

I am not sure what you mean.
But if, at atmospheric pressure, you increase the temperature of water (by transferring heat from an external source) until it starts boiling and then remove the source of heat, the water will cool below boiling point and will stop boiling.
Same thing will happen when you reduce the pressure. It will boil for a while until it cools below the boiling point at that pressure. Unless the environment is warm enough to provide heat.

 

[mfb]

yes but the same energy is given by lowering the pressure, lowering the pressure is same as increasing the temperature of the water to boiling point, and if temp of water decreases due to lowering of pressure then temp of water should also drop on increasing temperature at atmospheric pressure.but this doesn't seem to happen.

I don't understand what you mean there, sorry. If you increase the temperature, the temperature should decrease?
The boiling itself (molecules leaving the liquid) needs energy, this is independent of expansion/compression/heating of gases or liquids.

 

[wave525]

I don't understand what you mean there, sorry. If you increase the temperature, the temperature should decrease?
The boiling itself (molecules leaving the liquid) needs energy, this is independent of expansion/compression/heating of gases or liquids.

i mean that the case of water boiling at "atmospheric pressure and 100°" is same as that of water boiling at "room temperature below atmospheric pressure".

∴, since you said boiling below atmospheric pressure causes decrease of water temperature, the same effect i.e. lowering of temperature of water should be seen when heating at atmospheric pressure (which doesn't happen)

 

[jbriggs444]

Raising the temperature of water does not lower the temperature of water. That much is true.

But heating water and having it boil DOES result in a lesser temperature rise than if the same amount of heat had been added without boiling (e.g. by using a pressure cooker).

 

[Khashishi]

If you slowly heat water, the temperature will steadily rise until it reaches the boiling point, and then the temperature will stop rising. As you put more heat into the water, the temperature doesn't increase past the boiling point, as long as you heat slow enough to stay in equilibrium. The extra heat goes into converting liquid water to vapor. Once all the water is vapor, then additional heat will increase the temperature.

If you reach the boiling point by decreasing the pressure, then since you aren't adding heat, the heat of vaporization comes from the thermal energy of the water, and the water cools down.

 

[mfb]

∴, since you said boiling below atmospheric pressure causes decrease of water temperature

This happens at all pressures, unless you constantly add heat.