 We know that vapor pressure of a solution decreases when we add a non-volatile solute to a volatile solvent. For example, let's assume that a beaker contains volatile solvent B and now we add a non-volatile solute A into it. So what happens to the total vapor pressure turns out that the vapor pressure of the solution decreases. This is because the non-volatile solute takes up space and reduces the number of solvent particles that are available at the surface which otherwise can escape into the gaseous phase. So because of this, there are only fewer solvent molecules at the surface which can escape into the gas phase and as a result the vapor pressure of the solution decreases. Now this decrease in the vapor pressure of the solution depends entirely on the number of solute particles and not on the nature of the solute particles. For example, let's look at two beakers both of which have 1 kg water as the solvent. This is 1 and this is 2. Now the interesting thing is that when we add 1 mole of sucrose to beaker 1 and 1 mole of urea to beaker 2 in both the cases the decrease in the vapor pressure of water is the same. So when 1 mole of sucrose is added to 1 kg of water, the decrease in the vapor pressure of water is same as when we add 1 mole of urea to 1 kg of water. So from here we can see that the decrease in the vapor pressure of the solution depends entirely on the amount of solute rather than the nature of solute because in this case we have sucrose and in this case we have urea but the decrease in vapor pressure is same in both the cases and these properties that depend on the number of solute particles are known as colligative properties. So in this video let's discuss one such colligative property called elevation of boiling point. Now we know that boiling point of a liquid is the temperature at which its vapor pressure equals the atmospheric vapor pressure or 1 atmospheric pressure. Now for water the boiling point is 100 degree Celsius or 373.15 Kelvin correct? That is it is at this temperature when the vapor pressure of water equals the atmospheric vapor pressure. Now when we add a non-volatile solute to this liquid we know that the vapor pressure of this solution now decreases so that means for this solution to now boil we have to increase the temperature further that is more than the boiling point of the water alone because only in that case will the vapor pressure increase sufficiently so that it becomes equal to the atmospheric vapor pressure. Now this is exactly what is depicted here. As you can see the boiling point of a solution is always higher than the boiling point of the pure solvent. Now if we take the example of water the boiling point of water is 373.15 Kelvin which is that of pure solvent. Now let's say the non-volatile solute that we added is sucrose. Now when we add one mole of sucrose to one kg of water this solution now boils at 373.52 K. So while pure water boils at 100 degree Celsius or 373.15 Kelvin the solution of water and one mole of sucrose boils at 373.52 K. So as you can see the boiling point of the solution is always greater than the boiling point of the pure solvent. Now this difference in temperature which is delta TB which is nothing but the boiling point of solution minus the boiling point of pure solvent is called the elevation of boiling point and for dilute solutions this elevation and boiling point is directly proportional to the molal concentration of the solute. Remember guys we are talking about molality here and not molarity. If we remove the proportionality we get delta TB is equal to KB into molality. Now this KB is called the molal elevation constant or the boiling point elevation constant. Now some of you might be wondering that these are all very textbook concepts right they are very theoretical concepts. Do they even have any application in our everyday life? Let me show you one. How many of you have heard of something called antifreeze? So we add this mixture of antifreeze and water into the radiator of a car engine to help regulate the engine temperature and the most commonly used antifreeze is what we call ethylene glycol which is CH2 CH2 OH that is ethane 1 2 diol. Now when we add ethylene glycol to water it increases the boiling point of water. For example a 50-50 mix of ethylene glycol and water boils at 106 degrees Celsius as compared to the normal boiling point of water which is 100 degrees Celsius. So what is the advantage of this? By raising the boiling point of water this mixture ensures that the water does not boil off and turn into steam as easily when our car engine is running super hot or when we are driving in very hot conditions. This way the antifreeze ensures that the water continues to act as a coolant and help regulate the engine temperature. Now the interesting aspect of ethylene glycol is that it doesn't just increase the boiling point of water and prevent the car engine from getting super hot. It also decreases the freezing point of water. Well turns out that again a 50-50 mix of ethylene glycol and water decreases the freezing point of water to minus 37 degrees Celsius as compared to the normal freezing point of water which is 0 degrees Celsius. Now decreasing the freezing point of the solution means that water in our car's radiator will not freeze even when the temperature outside falls below the normal freezing point of water. Now this becomes super critical when we are driving in very cold regions where you often encounter temperatures which are much below the freezing point of water. So this is how ethylene glycol further help regulate the engine temperature. Now what we have observed here is another colligative property called depression in freezing point. So let's learn about this colligative property in the next video.