hi everybody my name is Andre welcome back to med school EU in today's lecture we are going to talk about Solutions and more specifically we're actually going to discuss solvent properties of water and solubility first let's talk about what a solution is and in order to know we first have to get the definitions of solvent and solute so just to put a plainly Solutions involve two types of molecules or two types of compounds one of them are going to be considered solvents because they are the ones that will have the capability to dissolve things dissolve other things and solute is the one that's being broken down that's the one that's being dissolved so the most classic case is you could have water and salt and ACL so the water will be the solvent and NaCl will be the solute and as you know typically the solvent is going to be in a much greater concentration much greater amount will have the solvent compared to the solute and so what would happen is the solvent has the capability to break down the sodium chlorine bond which is why it would be called the solute because it's being dissolved whereas the solvent does the dissolving now let's talk a little bit about properties of water that make it a solvent what really makes a solvent so water is considered a universal solvent and this just simply means that it has these quote unquote magical powers to be able to dissolve a variety of substances and that is primarily due to two factors one of them being polarity and the other being hydrogen bonds or the ability to form hydrogen bonds so if we take a look at the oxygen molecule here it is connected by covalent bonds so the hydrogens are connected to the oxygen with the covalent bonds now these covalent bonds are covalent bonds by definition do sharing of electrons however this sharing is unequal because oxygen has a greater electronegativity so then oxygen is going to form a slightly negative charge because it's electrons that it's sharing are going to be pulled closer to the oxygen so we'll have a slight negative and both hydrogens will have a slight positive charge because their own electrons are being pulled away closer to the oxygen they're still being shared however they're not shared equally so there's an imbalance whenever we have an imbalance it creates polarity to find out more about polarity and electronegativity watch a video that I have linked here which would be lecture 4.2 on chemistry where I talk more about polarity and electronegativity in much greater detail and now if we have multiple water molecules together as we would in a regular cup of water as you can see we would form plenty of hydrogen bonds between them and so these hydrogen bonds are the ones that will be responsible for the majority of breaking down and dissolving and this is why water is such a good universal solvent all of these bonds that I have highlighted with red dots these are all formed due to hydrogen bonds and remember hydrogen bonds are between Fawn atoms fluorine oxygen and nitrogen so here we got the oxygen bonded to hydrogen and therefore they would be forming plenty of hydrogen bonds now alone they're weak hydrogen bonds are very weak however when there's a lot of them and water would have a whole lot of hydrogen bonds they become extremely strong and so because of these two properties they're able to break down something like sodium chloride very easily and I'm going to show you an example of that later on in this lecture now let's talk about how solubility Works generally speaking you're going to be asked questions about does this dissolve this or is this a solute going to be dissolved in this solvent and essentially what it's asking is for you to determine the polarity of both substances the solute and the solvent and then you will be able to tell whether dissolving and and a solution will occur so let's talk about how this works well what dissolves what um typically the general saying is that like dissolves like this is a classic saying and what it really means is that polar dissolves polar and nonpolar substances dissolve non-polar substances so if you have a Polar cell vent it will dissolve a polar solute if you have a nonpolar solvent it will only dissolve in a nonpolar solute now water is polar as we have seen and sodium chloride is a ionic bond and ionic bonds by Nature are going to be polar because they have real charges and when that happens it forms ions so it easily dissolves however something that has uh for example chlorine gas where there's no polarity it will not dissolve in something like water or even carbohydrates something like methane methane is a nonpolar substance and it's not going to dissolve in water however there are non-polar substances that are able to dissolve something like methane so let's first discuss how polar dissolves polar so I use the classic example of water being the solvent the polar solvent dissolving sodium chloride which is a polar solute and in this case the sodium and the chlorine are going to break their bond inside the water and what's going to happen is they're going to form ions and that is due to the hydrogen bonds and polarity of the water and as you can see the oxygens partially negatively charged oxygens are going to Clump around the positive sodium whereas The partially positively charged hydrogens are going to Clump around negatively charged chlorine and this is exactly how they would separate and break those bonds between sodium and chlorine and they will simply form their ions which means this has been dissolved now in terms of nonpolar dissolving nonpolar there's going to be no breakage of bonds so essentially if we have a longer chain of carbohydrates for example right so this is a pentane and this this pentane these molecules of pentane are never going to break their their Bonds in between the carbon and hydrogen or the bonds between these the carbons together so essentially what is going to happen is these molecules are simply going to disperse so if this is our solute I could add some sort of polar solvent that will simply go around the molecules and they're just going to mix very nicely so it's kind of like mixing vegetable oil and mixing olive oil they're just going to mix together and they will nicely disperse between each other whereas if you were to mix it with water the oil is going to make hydrophobic interactions and it's then going to Clump up however here as you can see the white being the solvent is going to separate the individual molecules of the solute however it's never going to break its bonds that's really the only difference with the nonpolar to nonpolar dissolution compared to the polar and polar where the bonds are actually going to be broken so this is a bit of a summary all you really need to know is like dissolves like and you will be able to solve many many problems by being able to identify if they're polar or nonpolar so that's the key here determine if the compound is polar and see if your solvent and solute are polar they will dissolve if your solvent and solute are nonpolar they will dissolve but if they do not match they will not dissolve and that's how you solve those questions so it's a very simple concept so this concludes our lecture for today in the next video we are going to discuss the concentration of solutions and how to express those as well as the concept of equilibrium foreign [Music]