let's continue our look at chemistry for this lecture we'll explore why water is important for life remember the structure of water what kind of bonds hold the molecule together yes polar covalent bonds well this gives water some really neat properties in fact if it wasn't for these properties life would not exist as we know it remember that the electrons shared between the hydrogen's and oxygens spend most of their time orbiting the highly electronegative oxygen atom so the molecule has a slightly positive end and a slightly negative end well we can represent those partial charges with a special symbol that little squiggle is a Greek letter the lower case Delta and chemistry it means partial so when we consider a water molecule the area around the oxygen atom has a partial negative charge while the areas around the hydrogen atoms have slightly positive charges now we'll find out why this polarity is important if we were to take a glass pipette which contains slightly charged molecules and place it into a beaker of water the polar water molecules rise above the water level of the beaker isn't magic no it's due to a property called adhesion adhesion is when one substance is attracted to another substance in this case the polar water molecules are attracted to the slightly charged glass and stick to the glass inside the tube another property helps to explain this phenomenon cohesion which is the affinity of a substance for itself the water is further attracted up the pipette because each water molecule wants to stick to other water molecules again waters polar nature is responsible for this property the attraction the slightly positive end of one molecule of water has for the slightly negative end of another water molecule forms a hydrogen bond a water molecule can form up to four hydrogen bonds and since this property is just based on polarity hydrogen bonds can form between any polar molecules but the cohesive property of water also helps to explain why water beats all the water molecules are attracted to each other and want to be as close to each other as possible so a sphere of water forms and cohesion can also explain surface tension the water molecules at the surface of this pond can form hydrogen bonds only with the water molecules next to them are underneath them so the water molecules are attracted to the other water molecules around them on the surface and are also being pulled downwards by the attraction to the water molecules below them this forms a skin of water on the surface that is hard to break through this water Strider is so light that he can rest on the surface of the water without breaking the hydrogen bonds holding the molecules together now on to the next interesting property heat of vaporisation what does that mean we'll think of a pot of boiling water as the liquid water reaches its boiling point the water molecules become so energetic that they vaporize into the air becoming a gas this vaporized water is what we call steam and the heat of vaporization is the amount of heat any liquid must absorb in order to change into its gaseous state water has a very high heat of vaporization compared to most other liquids why do you think this would be the case yes those hydrogen bonds between the molecules the water molecules themselves must not only be energized but the hydrogen bonds must also be broken in order for water to vaporize and this is a very good thing for living organisms so what happens to a liquid water when the temperature drops to zero degrees Celsius it becomes ice the solid state of water but something interesting is going on at the molecular level as water becomes ice as the temperature drops from the boiling point to the freezing point of water the molecules are doing a very strange dance at 100 degrees Celsius the water molecules are crazy with energy they are so energetic they can only exist in the gaseous state and they are bouncing around so much the water molecules are not very close to one another as they smack into one another they rapidly form and break hydrogen bonds now remember they must be in the correct orientation slightly positive end of one molecule to the slightly negative end of the other molecule to form hydrogen bonds but if the temperature drops to 50 degrees the water molecules are not so energetic and can be much closer to one another they like being in the liquid state forming and breaking hydrogen bonds at a slower pace a water molecule at this temperature might form hydrogen bonds with several of its neighbors and slide closely past other water molecules without making any bonds soon it bumps into another water molecule breaking the old bonds and reforming new ones with different water molecules this is why water flows but at zero degrees something unexpected occurs as the temperature drops the water molecules slow down they form hydrogen bonds with their neighbors but are less likely to bump into another molecule with enough force to break any of the already formed bonds at zero degrees the water molecules are basically motionless but as they slow down each molecule form the maximum number of hydrogen bonds with its neighbors so each water molecule has formed four hydrogen bonds with other water molecules in the liquid state water molecules can move and get close to one another but at freezing they are spaced away from each other as you can see from just a handful of water molecules at zero degrees the hydrogen bonds occur at a certain distance between molecules and a structure called a crystal lattice is formed this increased spacing results in solid water ice being less dense than liquid water that is why ice floats on liquid water this is very important if ice was more dense than water it would sink this would mean that almost all the life in ponds and lakes would die every winter at least where it gets cold enough to freeze we have learned that water has many unique properties mainly due to the hydrogen bonds that can form but organisms contain more than just water they contain solutions a solution is a mixture of for our biological purposes water and something solid that dissolves in water if we were to take water a solvent because other substances can dissolve in it and salt a solute because it can dissolve in a solvent and mix them together we would get a solution of salt water now if we look very closely at what is going on at the molecular level we would see that the polar water molecules are attracted to the charged ions in the salt the partially negative ends of the water molecules surround the positive sodium ions and the partially positive end surround the chlorine ions breaking the ions up this is why polar substances those with some degree of charge will dissolve in water and we give substances that dissolve in water a special name hydrophilic which literally means water loving now on the flip side some substances won't dissolve in water if we put a non-polar substance which has no charge such as oil in water it tends to clump together and repel water molecules the special term we give to these substances is hydrophobic water fearing we really can't have a solution of oil and water because oil alone does not dissolve in water solutions can have different concentrations based on how much solute is dissolved in the solvent we can measure this by the solute percentage but it is more accurate to use molarity yes the moles have come back molarity is the number of moles of a solute per liter of solution say this lady is holding a 1 molar solution of potassium chloride it is not really this color but let's just pretend one mole would be the mass number of the potassium and chlorine which is 39 and 35.5 add them and we find that one molar solution of potassium chloride is seventy four point five grams per liter as a mole of potassium chloride would weigh seventy four point five grams and on to acids and bases which are also very important when we look at living organisms I'm sure you know some examples of each but what exactly determines an acid or a base if we look at the definition acids add hydrogen ions to a solution while bases reduce hydrogen ions in a solution but what does that mean well let's try to figure it out by looking at an example let's put an acid into water okay we have a hydrochloric acid on the left it is composed of hydrogen and chlorine so it is represented by the chemical formula and on the right we have water if you look closely you'll notice that not all of the water exists as h2o we see some water molecules that have dissociated into hydrogen ions and hydroxide ions we can represent this dissociation of water with an equation the double arrows show that this reaction can go forward with the molecule dissociating or go backward with the ions reforming into water but keep in mind only a very small amount of water actually dissociates most water molecules do not dissociate okay back to the acid so if we put hydrochloric acid into water it will dissociate into hydrogen and chlorine ions as you can see this results in an increase of hydrogen ions now which one increased the hydrogen ion concentration right an acid remember a base reduces the number of hydrogen ions in a solution and we can determine how acidic or basic a solution is by calculating its pH you can do this by first figuring out what the hydrogen ion concentration is of the solution those brackets represent concentration and it is written in exponential form then we take the negative log of that concentration log is short for logarithm if the base number of the concentration is 10 then we just need to take the exponent and multiply that number by negative 1 let's look at an example if we determine that our hydrogen ion concentration is 0.001 or 10 to the negative 3 in exponential form then we take the exponent and multiply it by negative 1 so negative 3 times negative 1 is three our pH of the solution is three what do you think that would be an acid or a base an acid if a solution has a pH below seven it is an acid try this one on your own what put the pH B ten and what do you think that makes it a base solutions with a pH above seven are basic that means that seven is neutral pure water has a pH of seven as it is neither acidic nor basic this diagram shows a pH scale with different substances there hydrogen ion concentrations and the corresponding PHS