# Surface tension and definition of boiling point

The boiling point of a substance is defined as the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. Does the pressure surrounding includes the surface tension? If yes then e.g. the boiling point of water should be greater at 1atm if we take into account the extra pressure from surface tension.

• The increase in pressure is going to be VERY small compared to atmospheric pressure. Now for a test. Measure the temperature of boiling water at your location. Then put some soap in the water to lower the surface tension and again measure the boiling temperature. Without a VERY precise thermometer, you shouldn't be able to tell a difference. And BTW, you need to do this experiment outdoors, as the boiling soapy water is going to make a big mess. Commented May 21, 2022 at 2:47

The surface tension only affects the pressure inside the liquid if the surface is curved: $$P_{\rm inside}= P_{\rm outside}+ 2\sigma/R,$$ where $$\sigma$$ is the surface tension and $$R$$ the radius of curvature.
It is important in the formation of bubbles during the boiling process as $$R$$ is very small when the bubble forms. This is why boiling tends to be "bumpy" when there are no rough areas on the container walls.