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My question is why a pressure cooker would affect the boiling point at all.

To my understanding, the boiling point of a substance is defined by 'The temperature at which the vapor pressure of the substance equals the surrounding pressure' and according to the Wikipedia article regarding pressure cooking, 'In a sealed pressure cooker, the boiling point of water increases with increasing pressure'.

Now say we fill a pressure cooker with water and turn on the heat for a while. Eventually, it'll start vaporizing and all the pressure accumulated inside the pressure cooker due to vaporization of water can be considered 'vapor pressure', but wouldn't that only mean that the vapor pressure has merely risen? Why should that affect the boiling point at all, when the 'surrounding pressure' itself is still the same? Does a change in vapor pressure also affect the boiling point in any way?

After a bit of thinking, I also started wondering if the expansion of the air inside the cooker alone was enough to increase the pressure sufficiently, but I am not really too sure about it and I couldn't find much clarity on why this pressure increase occurs otherwise.

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    $\begingroup$ So when the pressure cooker hits 100C all the water should turn to vapor. Now what? Given the volume difference of liquid:vapor is roughly 1:1000, unless you put very little water in the pot it simply can't fit all the vapor. $\endgroup$
    – Jon Custer
    Commented Oct 5, 2021 at 13:23
  • $\begingroup$ Partly related: Minimum amount of water in pressure cooker $\endgroup$
    – Loong
    Commented Oct 5, 2021 at 16:51

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The surrounding pressure in the boiling point context is the pressure acting on the liquid, what includes also partial pressures of eventual vapors.

There is always some air in a (fully) sealed cooker and the pressure is always higher than vapor partial pressure, so boiling point is always higher than water temperature.

$$T_\mathrm{boil}=f(p_\mathrm{air}+p_\mathrm{water}(T))$$

That is obviously not true for a pressure cooker with a pressure relieve vent. With the vent, air gradually escapes when the vent relieve pressure is reached. The boiling point raises only to the temperature, at which the saturated vapor pressure equals to the vent relieve pressure.

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  • $\begingroup$ Okay so we can say the now vaporized water exerts a certain pressure on the liquid, and that in itself causes the boiling point to go up. $\endgroup$
    – Kritin Thakur
    Commented Oct 9, 2021 at 16:28
  • $\begingroup$ Sure. Pressure of vapor is not different to pressure of other gases. $\endgroup$
    – Poutnik
    Commented Oct 9, 2021 at 16:40

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