water and hydrogen bonding topic 1.1 of this AP biology review this is Mr W from learn biology.com where we believe that the kind of learning you need to succeed in AP Bio requires interaction and lots of feedback that's what we provide on learn biology.com your subscription comes with a moneyback guarantee first thing to note is that water is a polar molecule there's unequal electron sharing between oxygen and hydrogen and so there's a partial negative region over here there's partial positive regions over here Delta negative Delta positive note that hydrogen bonds are intermolecular bonds they're between molecules not within molecules like coent bonds or ionic bonds or anything like that so what's happening is that the oxygen is partially negative the hydrogen is partially positive and the hydrogen bond is a weak bond that forms between those two areas hydrogen bonds are much weaker much much weaker than calent bonds ionic bonds any of those inra molecular bonds using the diagram below as an example describe how hydrogen bonds can form between molecules besides water the key idea is that hydrogen bonds are everywhere they're not just within water they're essential in biology in general so in this example there are two nen bonds and here's one oxygen to hydrogen over here here's another one between nitrogen and hydrogen this is between the nitrogenous based adenine and thymine and these are the hydrogen bonds that hold together DNA hydrogen bonds again are essential they're um key to the structure of DNA RNA proteins you'll meet them again and again throughout your biology course and they're important to know for the AP Bio exam as well so now let's look at some some of the consequences so describe cohesion adhesion and surface tension and explain how these Key properties of water result from hydrogen bonding cohesion that's hydrogen bonds between water molecules so this is cohesion right here and it's responsible for many of water's very peculiar properties it's a very small molecule but it has a very high heat of vaporization takes a lot of energy to get water to evaporate a high specific heat it can hold a lot of heat and it has high surface tension which we'll talk about below adhesion is water sticking to other stuff so like for example here you see hydrogen bonds between water molecule and the um cellulose walls that make up the conductive tubes of plants which are also called xylm and it's also responsible for a phenomenon called capillary action where water can actually go up for a short distance the sides of a straw as the water molecules bond with the uh plastic or the cellulose that makes up that straw and um a phenomenal thing is called transpiration which is uh how water gets pulled up to the top of trees and that's all based on water's ability to cohere to other water molecules and to adhere to the sides of the conductive tubes of plants as water evaporates from the top and finally surface tension here we see a paper clip that's resting on a net of water molecules and of course that's ridicul ully out of scale you can see how that's keeping this paper clip from sinking down and here it's the force exerted by water molecules on the surface of a body of water creates a kind of web or net upon the surface at learn biology.com we understand why students struggle with AP Bio it's a hard course but we have a plan for your success go to learn biology.com sign up for a free trial and complete our interactive tutorials and interactive AP Bio exam reviews we guarantee you a four or five on the AP Bio exam see you on learn biology.com in terms of hydrogen ions hydroxide ions and pH describe the difference between an acidic and a basic solution first thing to note is that um acidic Solutions are solutions that have more hydrogen ions here's a hydrogen ion over here then hydroxide ion that's represented by o over here so if you dissolve this in water you wind up having lots of hydrogen ions and that pushes the PH down all right um the pH is below seven in bases there are more hydroxide ions than hydrogen ions so if you dissolve sodium hydroxide in water you wind up with a lot more hydroxide ions and that pushes the pH in this direction pH is above seven and one thing that I want to emphasize is that you don't really need to know about pH for the AP exam directly there won't be a question about it but it might be part of an frq or a multiple choice question so it's an essential underlying concept want to learn more sign up for a free trial of the website that guarantees your AP biology success learn biology.com and watch this next video