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Let's talk about water. Water, as you may know,
is two hydrogens bonded to an oxygen.
H2O. Oxygen has
has an element number of 8. That means it has six electrons in its outside,
or valence electron, shell. It wants two more electrons.
Hydrogen wants one more, so the two hydrogens get together
with the oxygen and shares with the oxygen.
Now because oxygen is bigger than carbon
it can keep the electrons a little bit longer
instead of stealing them and so what happens is is the electron
zipper on hydrogen once, then twice around oxygen, once around the hydrogen,
twice around oxygen, once around hydrogen twice around oxygen.
So that what happens is the molecule gets a slight positive and negative
charge.
Oxygen gets a slight negative charge because the electrons are spending more
time around the
nucleus of the oxygen and the hydrogens get a slight positive charge.
We call this a polar covalent bond.
Think of it as the polls of the earth. The earth is a big magnet
and it also has a negative pole and a positive pole.
Now, because water has a positive and a negative
side, if you will, a positive and negative pole
it forms bonds with other water molecules.
The negative oxygen is attracted to the positive
hydrogen and so on and so on, and we call these hydrogen bonds.
Don't get confused between the bond,
nonpolar covalent bond
between two hydrogens in gaseous hydrogen
and the hydrogen bond between molecules. Now, hydrogen bonds can form
between any molecules where there is a hydrogen bonded to an oxygen
or hydrogen bonded to a nitrogen.
Those are the ones you need to worry about for this class.
Now, let's talk about what happens when you mix
water with molecules that are not capable of hydrogen bonding.
Here we have an example of a string of carbons
with nonpolar covalent bonds between the carbon
and the hydrogens that are attached to it, and we have a bunch of waters.
Well the waters, because of their polar covalent bonds, they've got a slight
positive and slight
negative charges, they're attracted to each other so this one comes over and
says,"H ey I can form a hydrogen bond with you"
and it's all happy and this one over here is saying, "Oh, well, don't leave me out
off the party," and
the carbon molecules are saying, "Hey, hey, you're stepping on my toes,"
and it keeps getting more more crowded. "Hey! Hey!"
And it can't form hydrogen bonds with the water
because it doesn't have an oxygen
bonded to a hydrogen nor does it have a nitrogen bonded to a hydrogen.
So what happens is, it is excluded by the water.
We call this hydrophobic interactions. Molecules that are
incapable
a forming hydrogen bonds are excluded by the water that wants to hang out by
itself.
Kinda sounds like cliques in high school, doesn't it?
Molecules that can form interactions
hydrogen bonds with water are called hydrophilic.
"Philic" meaning "loving", "hydro" from "water."
So they are water loving molecules whereas molecules that are excluded by water
that can't
hydrogen-bond are called hydrophobic or "water fearing".
If you take salt and put it in water,
what happens? It disappears. It dissolves.
If you taste the water, it's still salty. The salt is still there.
What's happening on a molecular basis as
the water because it has a slight positive and a slight negative end
can interact with the ions of
sodium chloride. So here's what's happening.
The sodium ion says, "Hey, hi, Water,"
and the chlorine ion says, "Hey, hi, Water"
and they separate. We call this dissolving.
Now, water can do this because the slight positive charge and slight negative
charge.
It can also do this with molecules
that are hydrophilic. So, for example,
sugar is not an ionic compound
but it does have, um, oxygens bonded the hydrogens
so we can do the same thing. So when you add sugar
to water it dissolves.
Now, we call this a solution,
either salt solution or sugar solution.
The solid stuff that you put into water we call a solute
and we call water the solvent.
You put a solute and a solvent together
and you get a solution. Water is important because it's the universal
solvent. It can dissolve most things
and your cells exist as a water solution.
so it becomes very important. From here,
we're going to discuss what else happens
in different types the solutions. There are two types of ionic substances:
those, like hydrochloric acid,
that release hydrogen photons,
hydrogen ions into water when they dissolve.
We call this an acid. That's why we call it hydrochloric acid.
There's also those that when they're put into water
they release hydrogen
oxygen bonded ion. We call this a hydroxide ion
and these have a tendency to
combine with any hydrogens that are floating around
in the water and for more water. We call these bases.
So, an acid is anything that releases hydrogens
in solution and a base is anything the removes hydrogens from solution
namely if it has a hydroxide ion or,
will get to this later on in this chapter, an ammonium ion.
Now, if you combine
an acid with the base in solution they will combine
and make a salt and water. These hydrogen
and hydroxide ions get together to make H2O.
The sodium and the chloride get together to make sodium chloride.
We call that a salt. Both acids and bases are important
in Biology as well as salts, since your cells are composed of both.
And your cells are very busy keeping the balance between
acids and bases. To measure whether something is
more acidic or more basic we have a pH scale.
It's based on how many hydrogen ions, how many free protons,
are floating around in the solution. Now, this is a
scale the goes both up and down. So let's start from the middle
or the starting the starting point.
This is where we're considered to be neutral, where we have equal numbers
of hydrogen ions and in this case hydroxide ions
or ions that are taking hydrogen ions out of solution.
This is a pH of 7. Pure water is considered to be a pH
7. Now, as we go
to smaller numbers or down the scale,
unfortunately this scale has it going up, and as the pH lowers
you get increasingly more and more hydrogens.
So, if you look at the substances that are listed
as they get more and more acidic you can recognize that. They get, ah,
when you taste them you get a more a more sour, more corrosive.
So, milk is just a little bit acidic urine is just a little bit
more acidic,
tomato juice its kinda tart unless you've got some nice garden fresh
tomatoes
and wine, grapefruit juice, lemon juice, stomach acid,
is a pH between 2 and 1.
Now going the other direction this is where you have fewer and fewer hydroxide,
excuse me, fewer and fewer hydrogen ions
versus hydroxide ions or anything removing hydrogens.
So human blood is just a little bit basic,
or alkaline is another term for it, sea waters a little bit more alkaline,
milk of magnesia - um, most of you are probably not familiar with
that, they still sell up to use for an upset stomach -
so if you have too much acid you drink something that's a little bit basic to
neutralize the acid and it creates salts.
Ammonia, you wanna drink that, household bleach, wouldn't wanna drink that,
and you get to heaven cleaner. That's our sodium hydroxide that we have a in the last
slide,
and lime water. OK, now,
your body needs to stay at a pH
just a little bit alkaline, so it uses buffers
to maintain that pH. Buffers are salts
or, more usually, weak acids or weak bases
that can adjust to an increase or decrease in hydrogen ions by either,
um, taking up hydrogen ions or releasing them.