 So in this video we are going to talk about ideal and non-ideal solutions. Now basically liquid solutions or liquid-liquid solutions can be categorized into ideal and non-ideal solutions depending on whether they follow or obey Raoult's law or not. Now those solutions that obey Raoult's law over an entire range of concentration are known as ideal solutions and those that do not obey Raoult's law are known as non-ideal solutions. Okay that's all right for definition but what do we actually mean by that? You see if a binary liquid solution is made of two liquids let's say one is A and the other one is B then we know that in their pure components we have AA type of interactions in the liquid A and BB type of interactions in liquid B right and when we prepare a binary solution by mixing these two the resulting solution is ideal if all the type of interactions in that mixture are similar in nature that means the intermolecular attractive forces between AA type of molecules and BB type of molecules are almost equal to those between A and B that is all of these interactions are nearly similar and when that happens we have an ideal solution and if you think this is too idealistic you're not really wrong because a perfectly ideal solution is very rare indeed but that does not mean that we cannot achieve near similar behavior for example a solution of n-hexane and n-hyptane form an almost ideal solution now if you look at a structure of n-hexane it is and n-hyptane would be both are non-polar hydrocarbons and interact with each other through London dispersion forces and when we mix these two together the energy required to separate the n-hexane molecules from each other or the n-hyptane molecules from each other is almost similar to the energy released when n-hexane and n-hyptane interact with each other so what does this mean this means that when we mix these two liquids together the energy of the system does not change drastically and this is why for an ideal solution the enthalpy of mixing would be zero so this means that no heat is absorbed or released when these two liquid components are mixed together and similarly the volume of mixing is also zero the volume of the solution is exactly equal to the sum of the volumes of the two components before they were mixed now this is because you can see from this structure that they're very similar so it is almost like you don't see a different molecule or a different liquid there they are so similar in this structure that one liquid molecule can substitute for the other or fill in the space between the molecules of the other and as a result there's no increase in volume when the two liquids are mixed so these are again the characteristics of ideal solutions another example is benzene and toluene so here again when we mix these two together we get an ideal solution such that the interaction between benzene-benzene molecules and toluene-toluene molecules would be similar to those between benzene and toluene molecules now non-ideal solutions are those that do not obey Raoult's law over the entire range of concentration in such a non-ideal solution the enthalpy of mixing is not zero and similarly the volume of mixing is also not zero now in the case of non-ideal solution the vapor pressure of the solution is either greater than or less than that predicted by the Raoult's law now when the observed vapor pressure is greater than that predicted by the Raoult's law we get positive deviation and when the observed vapor pressure is less than that predicted by the Raoult's law then we get negative deviation so let's understand what we mean by these deviations positive and negative deviations at a molecular level suppose you have a binary liquid solution and let's say it's a close container and A is one type of liquid and B is the other type of liquid let's assume A is a solute and B is the solvent now in the case of a non-ideal solution or specifically the solution that exhibits positive deviation vapor pressure of the solution here is higher than that predicted by the Raoult's law now what type of interaction should happen in the liquid solution so that the vapor pressure of the solution is high so this means that the interactions between these two liquids A and B should be weaker than those between each other that is AA type interaction and BB type interaction are stronger than AB type interaction in other words the intermolecular attractive forces between the solute and the solvent molecules are weaker than those between the solute solute and the solvent solvent molecules for example let's assume that a solvent B is ethanol and our solute A is acetone now if we had only ethanol then the ethanol molecules would associate with each other through hydrogen bonding for example ethanol CH3 CH2 OH right the oxygen atom here can interact with the hydrogen of another ethanol molecule and similarly the hydrogen can interact with the oxygen of another ethanol molecule and so on so you can see that in pure state the ethanol molecules are strongly bonded to each other through hydrogen bonding so that means these molecules do not want to leave each other and go into the vapor state now wait a minute what do you think happens when you add a small amount of acetone to this ethanol some of these acetone molecules will break the hydrogen bonding between the ethanol molecules such that these molecules are now free to escape into the vapor state so because it is easier for the molecules of acetone or ethanol to now escape into the vapor state than when they are in their pure state the vapor pressure of the solution increases and as a result we observe positive deviation now another example of an ideal solution that exhibits positive deviation is a mixture of carbon disulfide added to acetone now what do you think happens in negative deviation exactly the opposite in case of negative deviations from Raoult's law the intermolecular attractive forces between A, A and B, B would be weaker than those between A, B so in this case the liquid molecules are happy to stay connected and bonded with each other and are not in hurry to move or escape into the gaseous phase they're pretty contented and happy in their solution phase now in this case because the individual molecules of A and B are not willing or not interested to move to gaseous phase the vapor pressure of the solution consequently decreases now when does this happen this happens when there is strong interaction between the solute and solvent molecules so let's look at an example of such a solution let's say A is chloroform and B is acetone now when these two liquids are mixed together they result in negative deviation so this is because the chloroform molecules can form hydrogen bonds with the acetone molecules so that looks something like this CH3 CO CH3 CHCl3 so here the electronegative oxygen atom forms hydrogen bonds with the acidic hydrogen of chloroform now the hydrogen here is acidic because the alpha carbon that it is attached to is further attached to three highly electronegative chlorine atoms so these chlorine atoms draw electron density towards itself and that makes this hydrogen atom highly acidic in nature so this strong hydrogen bonding decreases the escaping tendency of these molecules and as a result of that the vapor pressure of the solution decreases and it gives a negative deviation from Raoult's law another example of a binary solution that gives negative deviation is phenol which is C6H5 OH and aniline which is C6H5 NH2 in this case the intermolecular hydrogen bonding occurs between nitrogen and hydrogen of the phenolic OH group so this interaction between the nitrogen and the hydrogen of the OH group is much stronger than their individual AA or BB type of interaction and as a result the vapor pressure of the solution decreases and it exhibits a negative deviation so from here we can get a qualitative sense of what type of molecules can give which type of deviation when they form a non-ideal solution