When a substance changes phase, that is it goes from either a solid to a liquid or liquid to gas, the energy, it requires energy to do so. The potential energy stored in the interatomics forces between molecules needs to be overcome by the kinetic energy the motion of the particles before the substance can change phase.That is why there is no rise in temprature, as all energy applied is consumed in conversion of states.
When something changes phase from solid to liquid, or from liquid to gas, it takes energy to break the intermolecular interactions. These interactions between the water molecules are what make it solid. When you have ice, these interactions are strongest, which is why ice is hard. Then when you have water, the interactions are not as strong, and although the water still "stays together" it is now a liquid and moves and flows freely. Then when all the interactions are broken, it become a gas, or steam, and now none of the water molecules are attached to any other molecules. Whenever it goes through a phase change like this, the energy goes into breaking up these interactions, and so the temperature stays constant until all the interactions are broken. Once all the ice is melted, or all the water has turned to steam, then any added heat will act to raise them temperature again
The heat released or absorbed by a body or a thermodynamic system during a process that occurs without a change in temperature is Latent Heat.
The latent heat is a hidden energy of the system and does not affect the temperature of a substance
- for example, water remains at 100°C while boiling, this heat added to keep the water boiling is latent heat.
And same for ice.
yes I completely agree with you. But my question is why is it so?? Why temperature does not change during ice to water phase transition?
During the change of phase (from solid to liquid or from liquid to gas), it takes energy to break the intermolecular interactions. In ice, these intermolecular interactions are strongest, thats why ice is hard. In the process of ice melting, heat is being added to ice but that heat is going into turning the water from a solid into a liquid (breaking intermolecular bonds), rather than increasing the temperature. Whenever it goes through a phase change, the energy goes into breaking up these interactions and so the temperature stays constant until all the interactions are broken. Once all the ice is melted then any added heat will act to raise the temperature again.
When a substance changes phase, that is it goes from either a solid to a liquid or liquid to gas, the energy, it requires energy to do so. The potential energy stored in the interatomics forces between molecules needs to be overcome by the kinetic energy the motion of the particles before the substance can change phase.That is why there is no rise in temprature, as all energy applied is consumed in conversion of states.
When something changes phase from solid to liquid, or from liquid to gas, it takes energy to break the intermolecular interactions. These interactions between the water molecules are what make it solid. When you have ice, these interactions are strongest, which is why ice is hard. Then when you have water, the interactions are not as strong, and although the water still "stays together" it is now a liquid and moves and flows freely. Then when all the interactions are broken, it become a gas, or steam, and now none of the water molecules are attached to any other molecules. Whenever it goes through a phase change like this, the energy goes into breaking up these interactions, and so the temperature stays constant until all the interactions are broken. Once all the ice is melted, or all the water has turned to steam, then any added heat will act to raise them temperature again
Yes I agree with both Dr. Jan and Mr. Memon. But the question still remains.
During phase transition, the latent heat is consumed for breaking the intermolecular bonds or overcoming potential energy. But my pinpointed question is why the whole energy is utilized for phase conversion. Why it is not like this that, some amount of energy is used for phase transition and the extra part is used for increasing temperature at the same time. What is prohibiting increase of temperature (even by partial consumption of energy ) during phase transition.
Nature loves symmetry whether in transformation of energy or in energy itself. No natural phenomenon can occur till there is a uniform distribution of energy among the surrounding molecules.. How is it possible to have one molecule at higher energy state while other bonding with at low energy level. All atoms and molecules at temperatures above absolute zero possess thermal energy that keeps them in constant states of motion. A fundamental law of nature mandates that this energy tends to spread out and be shared as widely as possible. Within a single molecular unit, this spreading and sharing can occur by dispersing the energy into the many allowed states of motion (translation, vibration, rotation) of the molecules of the substance itself. There are a huge number of such states, and they are quantized, meaning that they all require different amounts of thermal energy to come into action. Temperature is a measure of the intensity of thermal energy, so the higher the temperature, the greater will be the number of states that can be active, and the more extensively will the energy be dispersed among these allowed states.
There is no partial consumption of energy for any natural phenomenon. This whole universe works on Chemical and physical equillibrioum of energy states. Observe it in every aspect of daily life, you will difinietly get an elaborate reply.
Thanks Dr. Jan. I got the philosophy. Is it possible to prove it mathematically. Is it possible to show that partial consumption of energy violates laws of thermodynamics.
What I said is not my personal philosophy it is established sceintific fact. Well for mathematical derivation follow up with de Broglie, Planck's law that describes the amount of electromagnetic energy with a certain wavelength radiated by a black body in thermal equilibrium. For violation of laws of thermodynamics, there is research article entitled " Quantum Power Source by ALEXEY NIKULOV, read it
Barid,
Dr Jan has described the situation well.
You have molecules with a very definite binding energy between them. You are asking that a bound cluster of water molecules somehow become more energetic (ie, 'hotter') *without* breaking the bonds that ties them into a cluster. This is like asking that one can toss a cup and saucer into a running washing machine and somehow have them preserve their orientation with respect to each other. The available modes of excitation are not picked and chosen deliberately.
Try not to think about 'latent heat' - imagine the molecules. There are certain forces at work between them that in turn manifest as binding energies.
May I suggest a few days with a good undergraduate text on statistical thermodynamics? Flowers and Mendoza was my companion 25 years ago, and I'd suggest wikipedia, but it's not for the faint of heart - you're asking, in a sense, why the partition function is even-handed and does not promote certain microstates over others. The reason for that is anchored in the blind nature of probability theory - and as such that is grounded in logic and experimental evidence.
The temperature remains constant because the given heat is used to increase only the potential energy of the molecules of the given substance (which results in the change of its state) and not the kinetic energy as temperature of a body is actually the measure of the average kinetic energy of its molecules.
Thanks to all of you for your kind explanations.
@ James: I will surely look for the book you mentioned. I understand about the blindness of probability and hence one molecule is not preferentially heated up while the rest are not. Agreed.
But as far I understand, the situation is like this. As we input heat the thermal energy of the molecules increases, And as we travel up the vibration levels after a certain point we reach the dissociation level and the bonds break, But this is valid for gaseous atoms. Whether the same stands for solid to liq. transitions. Moreover under this picture I do not know how to explain critical pressure, critical temperature of first order phase transitions.
About the evenness of the partition function, completely agreed. But situation does not remain same when we move out of mean field approximation. How do we defend the situation at that point. Let us take for example a 1-D ising chain. Spontaneous phase transformation is not possible. But field induced phase transformation is possible. How do we defend the concept of "latent heat" at those cases. It can generally be defined as a inward flow of energy required to overcome the potential barrier required for transformation from one phase to another. How do we generally define that there must always be such energy associated with phase transformation? I dont know the answer. Is it really true??
@ Dr. Jan. Thanks for the article. It is really a good one and it was pleasure reading it. Unfortunately it does not speak about phase transitions. But anyway thanks.
I know Barid it is about energy relations... That is what we were discussing ..
Yes. I agree. But in heat engines we a continuous change of energy. As the loop has to be completed for sink to source transfer. But in phase transition we are concerned about the divergence. And as far I understand this divergence point is the source of all troubles. Be it first order or second order or pseudo first order. The correlation length diverges over all significant length scales. And the scaling of the correlation function gives the physical laws governing the transitions. Thats what I meant.
Barid,
Your description of the events taking place in a warming material are exactly right.
"...as we travel up the vibration levels after a certain point we reach the dissociation level and the bonds break,"
It does not matter whether the material is a solid or a liquid. Granted, the binding forces arise from different phenomena, and are of different magnitudes, but the result is the same. At some point the average 'thermal' photon has sufficient energy to break whatever bond joins one object (molecule, atom, etc.) to another. The photon does not heat that object further.
Back to the cup and saucer analogy. Agitating a whole room full of tea cups and saucers does not result in one extremely agitated cup/saucer pair. All crockery becomes shaken uniformly as we increase the agitation. When the agitation reaches a certain point, the average cup is shaken from its saucer. A vanishingly small number of cup/saucer pairs exist beyond this point till essentially all cup/saucer pairs are fragmented.
Critical materials exist at that rather narrow region of phase space where *some* cups and saucers are separated, some are intact, and some cups have been 'kicked' back onto their saucers. It is a point of dynamic equilibrium.
As for the 1D Ising model, I see no difficulty in calculating the latent 'heat' (actually the specific enthalpy change) of a field-induced transition, after all work is being done by the external field against the interactions that are presumed between the elements in the chain. That work manifests as a phase change, and the change in entropy (after all, this is isothermal) is the latent 'heat'.
Forgive me, I'm reaching the outer limits of my recollections of thermodynamics.
Latent heat is the stored heat energy in a substance used for changing the state of a substance. Consider ice at -10 ˚C. As the heat energy is supplied the internal energy increases and thereby the temperature of the ice increases like -9,-8,-7,.....0 ˚C. After reaching 0 ˚C, the heat energy supplied to the ice is not used to increase the internal energy of the ice any further but stored in the ice itself which is used for increasing the intermolecular distance and thereby the ice is converted into water at 0 ˚C. Hence, whenever there is no change in temperature with respect to the heat energy that has been supplied,definutely there will be the change of state.Therefore, latent heat is used to change the state of a substance and not for increasing the temperature of a substance.
Actually temperature implies the physical motion of the molecules in liquid/gas OR the electrons in lattice, which interchanges with the infrared photons during collision in lattice or within the molecules. Two systems are called to be at the same temperature when the speed of generation and consumption of the IR photons is same.
Now in the case of latent heat energy, the generation of IR photons from the lattice and the next phase, let us say liquid remains the same. This happens because in the state transition the consumption and production of IR photons is not the same because a part of the electrons kinetic energy is lost in breaking the lattice bonds. That energy is never recovered. In fact under the right conditions a phase change can be a cooling thing, like evaporation pots.
When ice is heated from -5 degree C to 0 degree C the kinetic energy (movements) of the molecules gradually increase, then from 0 degree C (ice) to 0 degree C (water) the intermolecular bonds are broken by the heat. When the first bond breaks, simultaneously why kinetic energy (movements) of the molecules does not increase (i.e. temperature does not change)? Why after all the molecular bonds are broken, the kinetic energy (movement) of the molecules again increases raising the temperature (i.e. after absorbing latent heat)?
Thanks parag for still following the question stream. This was the original question. Kindly share your views.
Barid,
You’re asking why adding Latent Heat does not increase a substances temperature! In order to answer that question let’s use water as an example. When water is at 0 C it is a solid because its intermolecular bonds are strong enough to hold its molecules strongly together resulting in it being a solid. Also these bonds are the result of all the molecules in the substance being at a lower energy level. All these bonds are strong and each will require sufficient energy to be broken and as they are being broken the solid slowly begins to melt. When energy is being applied to this substance all these bonds behave as pseudo heatsinks, this behavior absorbs away all the applied heat energy until all bonds are broken, then and only then will any additional applied energy result in an increase in temperature because there are no more bonds to absorb away the applied energy. At this point the substance has had a phase change because it has melted. Such is the nature of Latent Heat.
I know that molecular degrees of freedom are translational, rotational, vibrational and electronic and they are increasingly ordered with respect to the threshold of the energy required for excitation. Since the latent heat is consumed in bond breaking, which is electronic responsibility, why can't we say that latent heat is dissipated on electronic excitation ?
the latent heat involves during the phase change when the Q heat provided is used to break the molecule bonds from each other. During solid to liquid energy is required only to lose the bond while during liquid to gas energy is required to completely separate the molecules.
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