I'm Finding it hard to visualize how higher pressure makes the boiling point higher. I understand how pressure increases then temperature increases such that say air molecules compressed gives it energy and then the molecules move fast and average kinetic energy increases hence temperature increases.

But When we can say pressure increasing the bonds between molecules can easily be broken with the external force , but it somehow takes more temperature to disassociate bonds such that it transfers into a gas?


1 Answer 1


Remember that it is not the case that nothing happens below the boiling point and then everything changes at it. The dissociation of molecules from the liquid is happening all the time.

Liquid water has a temperature-dependent vapor pressure. And this pressure is still there below the boiling point. The water in your glass creates a fairly small pressure as molecules dissociate temporarily. But this pressure cannot overcome the external pressure of the atmosphere, so the water just "sits there".

As you raise the temperature of the liquid, the dissociation rate increases and so does the vapor pressure. In an open vessel at sea level, this increases until the vapor pressure equals atmospheric pressure and it can begin forming bubbles in the liquid.

When you do the same thing in a pressure vessel, nothing different is happening to the water or to the bonds. But the increased pressure prevents the vapor pressure from forming bubbles at that same temperature. Instead the temperature has to rise until the vapor pressure equals the actual pressure inside the vessel.

  • $\begingroup$ I get it , its just that external pressure doesn't allow liquid's vapor pressure to change states. But pressure increases temperature and temperature is a factor for vapor pressure so is there any relation such that this temperature induced vapor pressure and external pressure such that this induced vapor pressure becomes large or it always stays behind ? $\endgroup$
    – Naveen V
    Jun 8, 2023 at 6:05
  • $\begingroup$ Vapor pressure depends on temperature. Large temp, large vapor pressure. "pressure increases temperature", not true. There are times when compression increases temperature, but you can have high pressure without high temperature. $\endgroup$
    – BowlOfRed
    Jun 8, 2023 at 7:17
  • $\begingroup$ how does that work since pressure is force per unit area then there must be some kind of temperature induced ? By gay lussac's law of pressure and temperature we can say both are proportional right ? $\endgroup$
    – Naveen V
    Jun 8, 2023 at 10:17
  • $\begingroup$ Are you assuming no heat flow? That's required for that law, but that's a poor assumption for a pressure cooker. $\endgroup$
    – BowlOfRed
    Jun 8, 2023 at 16:00
  • $\begingroup$ ooh I got it thank you for answering all my questions ! $\endgroup$
    – Naveen V
    Jun 8, 2023 at 17:13

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