 We say that temperature is associated with the average kinetic energy of a molecule So if we have a substance that is hot Let's draw a hot substance With its many molecules That's it. This is a hot gas Then we say that these gas molecules are moving around with very high speeds. This is a hot gas If we had a cold gas or if we cool this gas itself Then these molecules would slow down. They would still move around, but they would now be slower Similarly, if we have a hot Liquid, this is a cold gas if we have a hot liquid Then the molecules in a hot liquid do move around, but they have a fixed volume And if we have a hot solid the molecules are all cramped together They cannot move around randomly, but they vibrate and a hotter solid and in a hotter solid The molecules will vibrate More they will push against each other more as compared to a colder solid. This is our understanding of temperature and If we use this understanding Then it is logical to believe that if we go to the lowest of temperatures The absolute zero of temperature zero Kelvin then all Molecular motion should stop. There should be no motion anymore because When there was high temperature, there was a lot of motion and there is low temperature There is very little motion. So if there is no temperature, there would be no motion But this is not true I'll prove this to you in two ways today The first is going to be a theoretical way and the second is going to be an experimental way First of all, I would like to tell you that this definition of temperature fails or breaks down when we are at These very low temperatures at these temperatures There are quantum mechanical effects that come in and you know that when we have started talking about quantum mechanics Anything can happen. Things are not going to be ordinary anymore So the theoretical way to prove that this does not happen Deals with a quantum mechanical principle that you might have heard of it's called the Heisenberg's Uncertainty principle. What does it say? It says that you cannot determine both the position and The momentum of a molecule completely think about this and Pause the video here and think how this would disprove this notion that zero Kelvin at zero Kelvin There should be no molecular motion If you have paused and thought about it then it is a condition like this Let's assume that at zero Kelvin Molecular motion does stop Let's assume that this happens. What is the problem? The problem is that if this happens then we know that the momentum of all of these molecules is zero There is no uncertainty there and we also can observe where these molecules are So there will be no uncertainty in the position as well and Heisenberg's uncertainty does not allow for that so theoretically This cannot happen. There cannot be no molecular motion There has to be some some jiggling some motion even at zero Kelvin Now let's come to the experimental proof Think about gases liquids and solids See gases the molecules are free to move around in liquids They are free to move around but within a specific volume and in solids the volume is completely Fixed so if you are thinking of no molecular motion Are you thinking of a gas or a liquid or a solid? Definitely a solid right because if it is a gas or a liquid there has to be some motion There is still motion that needs to be reduced and then it will become a solid and even after becoming a solid There has to be those vibrate. There will be those vibrations that can be reduced and Then we can say that there is no molecular motion So when we're talking about a gas or a liquid, there is no doubt that there will be some molecular motion so if there was a Substance if there was a material there was an element which was a liquid at zero Kelvin and that would most definitively prove That there is molecular motion at zero Kelvin and There is such a material that material is the second element in our periodic table. It's helium Helium is a liquid even at zero Kelvin under normal pressure conditions So this is definite of proof although we have not been to zero Kelvin We have been very close to zero Kelvin and we know that helium is a liquid there and we can theoretically see That it will remain Liquid even at zero Kelvin and we'll have to apply a lot of pressure before it becomes a solid So experimentally also it is proven that there is some molecular motion at zero Kelvin So to sum up in this video we saw That even though we talk about temperature as being related to the average kinetic energy of the molecules this notion of temperature breaks down when we are at very low temperatures at low temperatures quantum mechanical effects come into play and These effects ensure that there is some energy even at zero Kelvin This is substantiated by Heisenberg's uncertainty principle and the fact that helium is a liquid at zero Kelvin Since it is a liquid it has to have some molecular motion. There is still molecular motion that can be reduced and then it'll become a solid Now you know what happens at zero Kelvin How about you go and ask your friends if they know