# Why is water inside a vacuum chamber not boiling?

I have a vacuum that shows a max vacuum gauge reading of 23"Hg when connected to a chamber. When placing room temperature water inside vacuum, the water doesn't boil. I have seen this chart before https://www.engineersedge.com/h2o_boil_pressure.htm which indicates that my vacuum pressure is not enough to boil the water at room temp. The chart states that I would need a vacuum pressure of about 29.12 "Hg to boil water at room temp. I understand this. However, I have seen multiple videos of people using water inside a syringe and being able to adjust the pressure (by adjusting the plunger) enough for the water to boil without the need of a powerful vacuum. How is this possible?

Thanks

• Does the vacuum gauge reading of 23"Hg mean that your chamber is at a pressure of 23"Hg, or does it mean that your chamber is at a pressure that is 23"Hg below atmospheric pressure? Commented Sep 5, 2018 at 21:27
• Are you sure that the syringes in the videos didn't contain $warm$ water? Commented Sep 5, 2018 at 21:43
• @PhilipWood I always use alcohol. It's much more dramatic.
– JEB
Commented Sep 6, 2018 at 0:44
• I'm going to guess that you have only a mechanical pump, and that you don't have a high precision gauge to tell you how good your vacuum is. Commented Sep 6, 2018 at 14:17
• @probably_someone The chamber is at a pressure of 23"Hg below atmospheric pressure. Commented Sep 10, 2018 at 14:38

One has to look at the phase diagramme to see when what happens - and what happens during boiling.

Boiling is the transition from liquid to gas form - so that requires independent of temperature a pressure and temperature above the triple point, that is at least 611Pa. So when you put a glass of water into a vacuum chamber at room temperature and lower the pressure, what will happen?

The water will indeed boil briefly - but boiling means that the most energetic molecules do leave the liquid, in essence cooling it. And as you do not add energy like on your stove, the energy is removed from the remaining water. As such the boiling at constant ambient temperature will in essence reduce the temperature of the remaining water, so that finally it will pass the transition to the freezing / solidification line at around 0°C. As a result you will most likely have a clump of ice and a bit of water vapour (which you pump away in your vacuum chamber). If you don't pump or stop pumping you will reach an equilibrium point between the condensation and evaporation.

As some of the comments notes: The simple experiments with water in a syringe or similar will mostly show that the already existing gases inside the water will be outgas and fill the void to establish more of a pressure equilibrium. So what you see in those cases is less boiling but more the action as happening in a glass of sparking water.

Remember that boiling is a phenomena in which the liquid is no longer in equilibrium with its vapor above the surface. If there is already an equilibrium, you won't see the common occurrence of boiling.

• This is true, but without discussing the relationship between the quality of the vacuum and the vapor pressure of water it is rather incomplete and probably dosen't help the OP a lot. Commented Sep 6, 2018 at 14:15

Depends on the water, to a point. Depends how long the pump runs after all the air seams to escape the bag. I have noticed this, so make sure the bag is flattened such that I've squeezed most of the loose(?) air out of the bag and the tide is closing on the seal. Further I find that I sum times run out of timer, so this needs to be adjusted to give 5 or more seconds pumping at max with the bag pretty much devoid of 'non liquid' space. This I find gives me sealed products that are bubble free. Or were you seeking a more scientific answer?