all right welcome back in this video we're going to be looking at chemical bonds and the properties of water in this section of chapter 2 we're going to go over chemical bonds I have the bonds listed here from strongest at the top to the weakest at the bottom where nonpolar covalent bonds are the strongest and Van Der waals interactions are the weakest this might be different from what you learned in chemistry where your professors may have told you that ionic bonds are the strongest in biology we consider covalent bonds the strongest this is because in chemistry it's generally assumed that all the molecules you're looking at the reactions you're looking at those are happening within a vacuum in biology however because we're looking at living organisms we make the assumption that these molecules these reactions they're happening in water in which case covalent bonds and nonpolar covalent bonds in particular will be the strongest so covalent bonds are formed when electrons are shared between atoms in chemistry you may have seen the formation of single bonds double bonds where more than one set of electrons are shared like this and even triple bonds so addends share electrons and covalent bonds however this sharing can be equal or unequal nonpolar covalent bonds are stronger than polar covalent bonds and this is because in nonpolar covalent bonds the atoms have equal or similar electronegativities electronegativities are an atom's affinity for electrons I like to think about this as like how greedy an atom is for electrons and in this first one this first nonpolar covalent bond example we're looking at carbon and hydrogen these have very similar electrone negativities carbon actually has an electronegativity of about 2.5 hydrogen 2.1 so they're both pulling electrons they both like electrons fairly equally and there's no charge difference when we're looking at this Bond foreign looking at this lower example it's kind of interesting because although oxygen Oxygen's electronegativity is 3.0 and it's greater than carbon's electronegativity which is 2.5 making the single interaction between carbon and oxygen polar where oxygen because it's more electronegative is partially negative and carbon because it's not as electronegative it's partially positive the molecule CO2 or carbon dioxide is actually a nonpolar overall because of the linear structure of the molecule there's an equal pull of electrons to the left and right or however it's facing the orientation of the molecule by the two oxygens on either side of the carbon atom so both the bond type and the molecular shape are important to determine if a molecule is polar or nonpolar in polar covalent bonds what's going to happen is one atom like the one shown here oxygen is significantly more electronegative than the other atom here we see this at the top left oxygen has an electronegativity of 3.5 hydrogen is 2.1 so oxygen is more greedy for electrons and it becomes partially negatively charged because it's hogging that electron that negative cloud of the electrons whereas hydrogen is partially positively charged and these symbols these are the lowercase Greek Delta symbols that are scattered throughout the images here are these mean or represent partial charge so this I can't really do that well partially negative partially positive so those are partial charges compared to like full positive or negative charges in ionic bonds the electronegativity difference between the two atoms is so significantly different that the more electronegative atom actually steals an electron completely away from the less electronegative atom so here chlorine is much more electronegative than sodium so it steals an electron away leaving sodium positively charged and now chlorine well it's now it's chloride is negatively charged and that attraction between the opposite charges is what forms the ionic bond so that's different from what we saw earlier the polar covalent bond because oxygen even though it's more electronegative is not actually stealing the electron away it's just that the electrons spend more time spinning around the Oxygen's nucleus than it does spinning around the hydrogen atom's nucleus and here's another one of my wonderful memes that I found on the internet so this one loss an electron so it's positive okay so we had nonpolar covalent bonds polar covalent bonds ionic bonds our fourth type are hydrogen bonds a much weaker type of bond that forms between molecules containing polar covalent bonds and our last type are Van Der waals interactions note that while our stronger bonds require high energy to break these do not and they break and reform pretty readily here's an image showing several hydrogen bonds so we saw that the bond between oxygen and hydrogen is polar polar covalent where oxygen is partially negatively charged and hydrogen is partially positively charged if you happen to have here's one water molecule another water molecule close by hydrogen bonds form between them due to the partial charge that you see on one water molecule being in close proximity to a partial charge of the opposite charge on the other water molecule so hydrogen bonds form between water molecules and if you've ever played with magnets to kind of sense how hydrogen bonds would feel like if you could feel them it feels like when you hold two magnets that are attracted to each other closely together but don't let them touch you can feel that pull between the two magnets which would represent the two water molecules and one hydrogen bond is very weak that many molecules contain hundreds of hydrogen bonds which are collectively strong so note that there are no hydrogen bonds in a single water molecule that's only polar covalent bonds and two of them but there are hydrogen bonds between water molecules and the weakest interactions are Van Der waals interactions which generally form due to Temporary dipoles that are created when two or more molecules come close to one another so a simplified version let's say I have an atom or a molecule over here and it has electrons surrounding it so that's the electron cloud I have another atom over here with its electron cloud I'm going to change it to a different color if they come close to each other what happens is this induces a temporary dipole the electrons don't like each other because they're both the same charge they're both negatively charged so if they come closer together this is going to induce or cause the electrons to spend more time on the left side of this molecular atom that will leave the right side of this molecular atom slightly positive and that induces the shape of the electron cloud in this molecule or atom to also change so the electrons will spend more time on the left side of that atom or molecule so this is what it looks like they can get stronger when you have big lipids that we'll see later on and you have a lot of atoms forming these Van Der waals interactions so water is our next topic and it's the most critical molecule for life on Earth and this is because it's required for most biochemical reactions to occur it makes up most of the cells in our bodies and in terms of chemical reactions sometimes when people are tired instead of reaching for coffee sometimes all you might need is just water we'll see that water is a reactant and many many biochemical reactions what makes it so special so it makes up a huge amount of our body as well as most cells the reason water is unique is because that it it's polar like we saw in the previous slides and it can form hydrogen bonds these two properties gives it its essential characteristics an important role to life so this slide is a review of what I went over earlier we remember that hydrogen as a partially positive charge oxygen is partially negatively charged and hydrogen bonds are formed between adjacent water molecules molecules that can mix with or dissolve in water are known as hydrophilic molecules Hydro means water and phyllic means loving so these molecules are usually comprised of polar covalent bonds hydrogen bonds ionic bonds in contrast hydrophobic or water fearing molecules do not mix with water they're usually made up of molecules with many many nonpolar covalent bonds Van Der waals interactions are actually found in both types if you ever try to mix oil which is hydrophobic and water which is hydrophilic you'll see that they don't mix and after a few seconds and it might look like this at first but a few seconds later all the oil will kind of gather together to get as far away from the water as possible in chemistry they often say like dissolves like so hydrophilic molecules mix well with other hydrophilic molecules we often say polar instead of hydrophilic or short imagine you're making toast and you're getting the butter out you're spreading it over your bread and you get some on your hands how do you get butter off of your hands can I just rinse with water now that doesn't usually work I still feel greasy my fingers will still feel greasy so that doesn't work but soap soap works pretty well and the reason it works well we'll see later on that it's because that soap has both hydrophilic and hydrophobic Properties or components the hydrophobic portion will bind to the butter and form these like cool spherical structures where the oil the butter is in the middle and the hydrophilic portion will be facing the outside and allows water to pull and rinse the butter off of your hands oh my gosh another one okay why did the white bear dissolve in water because it was polar so cute hydrogen bonds are also why water has unique characteristics in its various States usually you think of the solid state of a molecule as denser than the liquid state but for water it's like the opposite right ice or solid water is less dense than liquid water and that's very important one of the reasons is because it allows ice to float above water and this happens because the average distance of hydrogen bonds between water molecules in the solid form the water molecules are actually further apart in the solid form in ice than in the liquid state in lakes and ponds ice will form on the water surface creating a nice insulation or insulating barrier that protects all the animals and plants inside the pond or inside the lake from freezing without ice being less dense than water then the whole thing would freeze and all the animals and plants and other microbes in the pond or lake would freeze and not survive in the winter time something else that can be both detrimental or beneficial is when Isis formed it expands relative to liquid water that can be detrimental because it could freeze when it freezes it could kill living organisms when the cells burst it can also be beneficial to us and we actually use this in some cases such as flash freezing certain foods that we want to eat raw like raw fish sushi grade fish is flash frozen because we know it's not going to be cooked to kill any microbes so in order to kill anything that could be harmful it's frozen and those cells are ruptured and it kills any microbes or anything that could be harmful and cause food poisoning another important property of water it has a very high heat capacity so it takes a lot of energy to heat one gram of water up and raise it by one degree Celsius it takes a lot of energy and it takes a lot of time to heat water up but also to cool it down which means water can absorb a lot of energy before it changes temperature and this is important in maintaining Earth's temperature and also for example on a smaller scale our own bodies allowing us to sweat and cool down similarly water also has a really high heat of vaporization going from liquid to gas so very similar to a heat capacity so water changes temperature as it changes temperature it does so very slowly and again these properties are due to the polar covalent bonds found within each water molecule and that giving the ability of water to form hydrogen bonds with one another so it's hard to break these bonds and form gas for example from liquid water it takes a lot of work water is also great because most things can dissolve in water right most Polar Polar molecules ionic molecules they can dissolve in water and this would make water the solvent and whatever you're trying to dissolve into the water the solute our book gives us a nice example we all have salt probably at home if you put a little bit of salt into a cup and add some water and stir you'll see that we form these little hydration spheres where water surrounds the ions that are formed and if you look closely at that hydration spheres you can see since water is polar the bonds within water are polar I know oxygen is partially negative hydrogen is partially positive I can see that the partially positive hydrogens are facing the negatively charged chloride ion whereas in the right side it's opposite they negatively or partially negative oxygen atoms are facing the positively charged sodium ion cohesion is another property what is cohesion so that means that water likes to stick to water they bind together through those hydrogen bonds and one example is for example whenever I'm trying to boil water it takes forever because the water molecules that are forming the hydrogen bonds in the liquid state don't want to let go it takes a lot of work for me to get them to release one of the hard the water molecules and release it into a gas state and that cohesion those water molecules that are bonded through those hydrogen bonds allows the formation of surface tension so certain things can be placed on top of a surface of water without falling down without falling through another property is adhesion so cohesion was when we looked at Water sticking to other water molecules adhesion is when water sticks to something else like glass we will often see when we put water into some kind of glass container like a capillary tube or maybe a graduated cylinder that's made of glass we'll see a meniscus form due to capillary action and this is because water is actually more attracted to the glass walls of the container than to each other it's like adhesion winds over cohesion and you see water moves up the sides of the glass container because of its attraction to The Container to that glass and this takes us to the end of part two of chapter two there's one more part to the set of videos and in that third part we're going to be looking at pH and buffers carbon a little bit more closely as well as a few functional groups all right thank you