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I have somehow gathered that when you heat water as you record its temperature, it's temperature begins to rise until it boils. After that it's temperature will not rise by adding anymore heat.

So the question is why? And the answer is: It is due to water's "Phase transition".

So have I learned correctly to have concluded the following and be able to pass the exam should such a question arise?

The reason why the water will not get hotter is due to "phase transition".

What is phase-transition? it is when water begins to boil after that no matter how much heat you put into it, it's temperature will not increase.

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What is phase-transition? it is when water begins to boil after that no matter how much heat you put into it, it's temperature will not increase.

Phase transition is the breaking of the hydrogen bonds that hold the water molecules together making it a liquid, converting the water to vapor. The temperature doesn't change because when the heat (latent heat of vaporization) breaks the bonds it increases the internal potential energy of the water, instead of increasing the internal kinetic energy (random motion) of the molecules, the latter of which is responsible for temperature change. The properties of saturated steam (mixture of water and water vapor) for a given temperature can be found in the Saturated Water-Temperature Table.

Once the conversion is complete so that it is all vapor, it is called saturated vapor. Then if the saturated vapor is further heated at the same constant pressure it becomes superheated vapor (superheated steam). The properties of the superheated steam for various temperatures at the same pressure as the saturated steam can be found in the Superheated Water Tables.

Hope this helps.

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It must be required that the pressure is kept constant. At any temperature there is a vapour pressure, and that is why water evaporates, even well below the boiling point.

At the boiling point, the vapour pressure equals the surrounding pressure. For 1 atm it is $100^\circ$C for water. In a pressure cooker it is about 2 atm what increases the BP to $120^\circ$C.

In a pool with 10m of depth of water being heated, even after reaching $100^\circ$C and starts boiling, its temperature can keep increasing in the depth, where the pressure is 2 atm.

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The phase transition is the change of the liquid water to gaseous water vapour. These two different states are called phases of water.

The reason that the temperature no longer increases is, that the water begins to boil and the phase transition from the liquid to the gaseous state requires heat (the enthalpy of vaporization or latent heat), so the heat drives the phase change instead of heating the liquid further. The amount of water in your pot gets less and less as it evaporates, so you can't add infinite amounts of heat without increasing the temperature. Once all the liquid is evaporated, the heat input will heat the vapour to higher temperatures.

You can't heat the liquid above its boiling point while keeping the pressure constant, because the liquid is no longer stable if the temperature gets higher (so if you heat an infinitesimal parcel to a higher temperature, it will just be vapour afterwards), at the same time the phase transition can't happen instantaneously, because you need to put energy in for the phase change to happen. (We'll ignore the effect of super.heating here, where under special conditions you can heat the liquid above the boiling point.)

Outlook

This kind of phase transition, that shows latent heat, is called discontinuous (or first order) phase transition. There are phase transitions that do not involve latent heat, e.g. the change from a ferromagnet to a paramagnet at the Curie temperature. Those phase transitions are called continuous (or second order) phase transitions.

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  • $\begingroup$ So what I said is wrong and that I didn't understand phase transition? $\endgroup$
    – user232620
    May 2, 2021 at 1:46
  • $\begingroup$ You were not entirely wrong: It is correct that the temperature of boiling water will not rise when you add heat. But since the liquid water gets less and less, at some point no liquid will be left and then there is nothing left that prevents the system from getting hotter. (And I should emphasize again, as the other answer does, that the constant boiling temperature requires an ensemble, where the pressure remains constant, the behaviour will be different for water enclosed in a pressure cylinder of constant volume.) $\endgroup$ May 2, 2021 at 11:22

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