# How to know if a liquid is boiling?

Unless we know the boiling temperature of a liquid, how can we know when a liquid is boiling ( I have this doubt because at temperatures nearing boiling point evaporation and boiling cannot be distinguished easily) I know that the boiling point can be found in a closed system( using its vapor pressure which equals the atmospheric pressure at the boiling point) but is there any method to check if a liquid is boiling or if it is still evaporating in an open system?

• look for bubbles? – Bobak Hashemi Apr 7 '19 at 4:35
• I'd be happy to answer your question, but can you clarify it a little bit? You seem to be asking about boiling point elevation (the change in boiling point due to changes in vapor pressure), but as the question stands right now I'm not sure what exactly you're asking. – the_photon Apr 7 '19 at 4:48
• It stops making noise when it boils. Noise is caused by bubbles collapsing in colder water at the top. When they stop collapsing, the temperature has reached the boiling point. – safesphere Apr 7 '19 at 7:35
• Do you understand the distinction between evaporation and boiling? Do you understand that a liquid can always evaporate as long as the partial pressure of the substance in the air above the liquid is less than the equilibrium vapor pressure at the liquid temperature? – Chet Miller Apr 7 '19 at 11:07

## 2 Answers

How do we know that pure water at atmospheric pressure boils at 100 °C ? The thermodynamic answer is to detect the temperature where two phenomena happen:

1. bubbles form in the bulk of the liquid;
2. temperature remains constant until there is liquid water.

Evidence n.1 is related to the coexistence of liquid and vapor at the boiling point. Evidence n.2 is the fingerprint of a first order phase transition.

From a practical point of view, in order to identify with enough precision the boiling point temperature, a few shrewd technical details are in order like starting with a pure sample, avoiding large difference of temperature at the surface and in the bulk of the sample.

• The phenomenon of overheating makes that the boiling point was not very suitable for calibrating thermometers. The better purity and cleanliness, the worse this problem was. The solution was to measure the temperature of a thermometer above the liquid, in saturated steam. – Pieter Apr 7 '19 at 9:50
• @GiorgioP is it the same for any liquid ? That is what I was asking i'm sorry my misplaced example suggested otherwise. – ark22 Apr 7 '19 at 10:27
• @Pieter Overheating is indeed a problem with pure samples. However, it is a non-equilibrium condition. Usually, mechanical agitation should be enough to enhance enough homogeneous nucleation of vapor. – GiorgioP Apr 7 '19 at 10:52
• @ark22 Yes it is the same for any liquid, although liquids with very high viscosity may present additional practical problems. – GiorgioP Apr 7 '19 at 10:54

To do it in a scientific way:

First, you need to make sure the bubble is actually water vapor, but not some gas solution in the water. This can be done by chemical detection after the bubbles got out of the water, but the most accurate way should be imaging the focused laser pulse scattering in multiple dimension and read from spectroscopy. Or simply estimate the gas density by estimate the velocity of how fast the vapor is going up(but the bubble do shrink though, so it's going to be a nasty integration).

If that's not already hard enough, the next thing you need to do in an open system is to build up profile of the local density. Especially, in order to determine the exact pressure, you might need to take into account of the velocity flow as well. I suggest to dope the water by salt(some non vaporize medium such as NaCl), or replace some hydrogen atom in the chemical bound with isotope such as deuteron or triton(which probably won't work too well, because it's molecular not some simple atoms), thus tag fluid velocity motion in molecular level. (I was thinking, if you use two salts who's ions had large atomic mass difference, you might actually be able to measure density and velocity simultaneously.)

There, you have density(and thus atom number), pressure, chemical potential, e.t.c. you should be able to assume the temperature gradient was $$0$$. If not, use an isotropic heating sources and large plate, and approximate the the temperature in thermal camera.

Then, use statistical physics, you should be able to get a basic ideal.

But mainly, in high school level physics: vapor is on the surface, boiling is happening inside the density.