[MUSIC] Have you ever wondered why ice floats on top of water? Ice being a solid, we would expect it to be denser than the liquid. However, ice is not denser than water and floats. Rock on the other hand is denser than water and sinks to the bottom. This property is very unique to water and water is anomalous in many ways from other liquids. In this module, we will learn about the many unique things about water. By the end of this module, you should be able to develop a thermodynamic model for the two phase water mixture, and using this two phase water mixture thermodynamic model, reconcile the anomalous properties of water. And, in the end, you should be able to explain the properties that give water its unique functionality in a wide variety of applications. Water at 4 degrees Celsius is denser than ice. Now, the liquid phase being denser than its solid phase is extremely unusual. Why does this happen? Now, let's start with the picture that water really is not microscopically a single substance but has two components, two distinct phases of water, a low-density liquid and a high-density liquid. Now, what I'm presenting to you is very recent research, and the picture that I'm presenting is one that's still in development. From the available spectroscopic studies, the spectrum of water is extremely unusual and interesting. It seems from the spectre that water itself consist of two phases. A high density liquid phase and a lower density liquid face. The high density liquid face prefers to arrange itself in a near-tetrahedral network with about 4.4 neighbors. Now, remember ice on the other hand, arranges itself in a perfect tetrahedral network, and has only 4 neighbors. Now, this subtle change is what makes water at low temperature denser than ice. Now, what happens when you increase the temperature? As you increase the temperature, a more dominant phase emerges. This more dominant phase is the low density liquid phase, which prefers to arrange itself in a less than 4 network, at room temperature, the density of the 2 phases of water start as the average value of 1,000 kilograms per meter cubed. The high density liquid phase is slightly higher than 1,000, while the low density liquid phase is less than 1,000. Now, you might ask the question, well, why are there two distinct phases? Now, water forms very strong hydrogen bonding with other water molecules. Hence, having more water in its nearest neighbor shell is a favorable configuration for water. So one can easily rationalize the high-density liquid phase. Now, however, the molecules that are present in the nearest neighboring shell of water is rigid, that is, the neighbors are blocking the movement of water. And this results in sort of an entropic jam. In the low-density liquid phase, on the other hand, there's an entropic gain because the water molecules can wiggle around, move around, and as a result, they gain entropy. However, this comes at the loss of enthalpy. As it has lesser water neighbors of favorable interaction. Hence, the loss in entropy is made up in part by its ability to take different configurations in thermal dynamic partons. The configurational entropy. One can immediately now ask the question, well, this is all very interesting theoretically, is there any practical relevance to this? Now, this phenomenon has a really important consequence for biology and life. In lakes, during the winter, there's a thin but distinct layer of ice that forms. This blanket separates the upper layer from the calm, deep sea water. The solid layer, being lighter than water at 4 degrees Celsius, rises to the top and absorbs heat during the day and cools during the night. While the temperature below remains solidly fixed around 4 degrees Celsius, this lasts central spring and in the spring, the water heats up. When the water heats up, the ice melts at the top, and this is followed by spring turnover. Where the oxygen rich upper water mixes with the nutrient rich lower water. As a result you have blooming and a wide variety of plant life emerges during this period. This whole cycle is called a thermocline. And this is simply because water at low temperature is denser than ice. Now, this emerging picture has important industrial implications which is not yet been realized. I think the design of devices that use water require a detailed model such as the thermodynamic model that we have discussed. And this is essential to make reliable devices that use water as its working substance. To summarize, in this video, we learned that water really is not a pure substance but consist of two phases. The low density liquid and the high density liquid. The high density liquid arranges itself in a tetrahedral network with nearly 4.4 neighbors on an average. Making it denser than ice, which is perfectly tetrahedral and has 4 neighbors. The high density liquid phase is enthalpically stabilized, while the low density liquid is entropically stabilized.