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Can't we measure the boiling point of a liquid in a closed vessel

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closed as unclear what you're asking by stafusa, sammy gerbil, Jon Custer, AccidentalFourierTransform, Cosmas Zachos May 20 '18 at 20:23

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  • $\begingroup$ Have you heard of the phase diagram of water? $\endgroup$ – ahemmetter May 6 '18 at 11:33
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    $\begingroup$ Boiling is the phase change associated with adding heat to overcome the local vapor pressure, an enclosed vessel will have a larger vapor pressure which means the boiling point will raise $\endgroup$ – Triatticus May 6 '18 at 15:07
  • $\begingroup$ @Triatticus: That should be an answer. $\endgroup$ – Michael Seifert May 16 '18 at 17:10
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When you boil a liquid, what you are doing is adding heat until the vapor pressure of the liquid is greater than that of the local external pressure (that is say the pressure of the atmosphere). If you were to enclose the liquid, the vapor pressure due to the liquid evaporating and the air molecules will increase the local pressure. This means the vapor pressure of the liquid is no longer in excess of that external pressure, this raises the boiling point of the liquid (the entire principle upon which pressure cookers function and why cooking times are longer at higher altitudes where atmospheric pressure is lower). Hence the boiling point you measure will be higher in an enclosed vessel, and in fact may not even reach a boiling point at all before the pressure causes the vessel to fail. Boiling in an open vessel, the escaping steam doesn't appreciably change the atmospheric pressure at all, so the boiling point remains the same.

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