Explain why there is no rise in temperature when it undergoes a change of state although it still being heated?
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For a long time scientist knew about the invariance of temperature during phase changes, but this phenomenon cannot be explained in terms of the usually observable variables, that is, the macroscopic or thermodynamic variables. To understand the WHY we need to look at the phase change from a microscopic point of view, in other words, we need to explain the observed behavior in terms of the motion of the atoms and molecules that form the substance.
As an example, let's talk about boiling a pure liquid, but the same reasoning can explain freezing/melting. When you start heating a mass of liquid, the heat absorbed by the system increases the kinetic energy of the molecules, they will move faster. This energy increase is what we can observe as an increase in the temperature. More heat in and temperature will go higher and higher and higher, until there comes a moment when the molecules have acquired enough energy to overcome the forces that keep them together in the liquid phase and they start to escape from every part of the liquid into the gas phase, that means the liquid boils. (The key point here is FROM EVERY PART OF THE LIQUID PHASE, because normally during the heating up, molecules will be escaping from the surface into the gas phase all the time). When this happens, the molecules that escape to the gas phase take away some energy from the liquid phase, and since we continue heating they will continue to escape. The effect we had observed before (increase of temperature as we give more heat to the system) is not possible anymore, because the vaporizing molecules are taking away the energy that is being supplied to the system. That means, boiling occurs at constant temperature until all the liquid is gone, in which case, if we continue to heat up, then we will increase the temperature of the gas formed.
All this process has occurred at constant pressure, as when we do our experiments in an open container.
As an example, let's talk about boiling a pure liquid, but the same reasoning can explain freezing/melting. When you start heating a mass of liquid, the heat absorbed by the system increases the kinetic energy of the molecules, they will move faster. This energy increase is what we can observe as an increase in the temperature. More heat in and temperature will go higher and higher and higher, until there comes a moment when the molecules have acquired enough energy to overcome the forces that keep them together in the liquid phase and they start to escape from every part of the liquid into the gas phase, that means the liquid boils. (The key point here is FROM EVERY PART OF THE LIQUID PHASE, because normally during the heating up, molecules will be escaping from the surface into the gas phase all the time). When this happens, the molecules that escape to the gas phase take away some energy from the liquid phase, and since we continue heating they will continue to escape. The effect we had observed before (increase of temperature as we give more heat to the system) is not possible anymore, because the vaporizing molecules are taking away the energy that is being supplied to the system. That means, boiling occurs at constant temperature until all the liquid is gone, in which case, if we continue to heat up, then we will increase the temperature of the gas formed.
All this process has occurred at constant pressure, as when we do our experiments in an open container.
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It is so because the energy that is given to it is utilised by the particles to convert from one state to another. This heat energy brings about a change in it's potential energy. Temperature can only be detected when the energy is converted into kinetic energy.
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