Water [2/2]: Edexcel Unit 1 (A Level Biology)


Uploaded by freeeschool on 13.10.2012

Transcript:
A Level Biology: Edexcel Unit 1 Ð Water (Part 2)
Hi! Welcome to our second video on Water and its properties. We are going to cover on from
where we left off.
Another one of waterÕs special properties which is linked to its ability to form hydrogen
bonds is this ability of cohesion. So cohesion is basically the attraction between particles
of the same substance. Water is basically attracted to itself and this was known as
something known as surface tension which is the measure of the strength of waterÕs surface.
It produces a surface film on the water and that allows insects and other small organisms
to actually walk on the surface of the water.
Cohesion also forms water droplets on the surface of the leaves. If cohesion is the
attraction between two different substances, water will actually make those hydrogen bonds
with other surfaces such as glass, soil, plant tissue and cotton. It allows a capillary action
as well. It also leads to transpiration where water is taken in by the roots and through
this process of transpiration, water is actually moves the xylem against gravity.
Water also has some very important thermal properties as well. One of those being that
it has a high specific heat capacity. What this basically means is that it takes a lot
of energy for the temperature of water to change. This means that the temperature remains
relatively stable, which is useful for the organisms because a lot of organisms are adapted
for only a narrow range of results. This slow heating and slow cooling of water is ideal
for many organisms because there is less risk of their being massive fluctuations in temperature.
Because it takes a lot of energy to make water evaporate and actually break all those hydrogen
bonds before it can evaporate, it is a good coolant. Evaporating water actually removes
lots of heat energy from an organism and that is the other property that water has. It has
a high heat or vaporization or sometimes known as a high heat of evaporation.
The other thermal property of water is this high heat of vaporization. Once again, high
temperatures can obviously damage tissues and the denature proteins which obviously
causes enzymes to stop working. Now, it takes a lot of energy to actually change the temperature
of water. ThatÕs because the amount of heat needed to raise one gram of water is about
1?C.
Now waterÕs heat for vaporization is about 540 calories per gram which means that in
order for water to evaporate, each gram of water needs to gain 540 calories, otherwise,
the temperature doesnÕt actually change.
As water evaporates, it actually removes a lot of heat with it, which gives it a cooling
effect. Animals which live in warm countries, what they can do is use water or mud to cool
off during the hot day. We, humans, when we sweat, it gives us a cooling sensation because
the excess heat energy is removed by the body through this latent heat of evaporation.
Now water also has amphoteric properties. In fact, it can act as both as an acid and
as a base. Before that, we need to look at their dissociation of a water molecule. Here
you got two water molecules and you can see that one hydrogen atom has actually left its
electron behind and travelled as a single proton or a hydrogen ion and joins up with
this water molecule, forming hydronium or a hydronium ion. What you left behind with
is a hydroxide ion.
What actually happens is impure water, only one water molecule in 554 million is dissociated
and this is simplified into this reaction here. So you can obviously see this arrow
means that itÕs reversible. Because only one water molecule in every 555 million is
dissociated, if we add an acid into water, an acid is just a substance that increases
the hydrogen ion contents, so it just increases the number of hydrogen ions that can become
dissociated. Any substance that reduces the hydrogen ion concentration in the solution
is known as a base.
Strong acids and base is completely dissociates in water while weak acids and bases dissociated
only partially or reversibly. The fact about water accident buffer means that it eliminates
any large or sudden changes in pH. So buffers actually help organisms maintain the pH of
body fluids within a small narrow range necessary for life. Buffers are usually a combination
of hydrogen ion acceptors and the word by accepting hydrogen ions from solutions, whether
in excess and also by donating those hydrogen ions when they have been depleted.
Here, we got an overview of some of the key properties of water. Now, you might have noticed
that we have actually looked to the fact that itÕs immiscible with hydrophobic molecules
because that is actually covered within Unit 2.
That concludes the short presentation. I hoped you enjoyed it. DonÕt forget to keep a lookout
for some more videos coming out soon.
[end of audio Ð 07:24] A Level Biology: Edexcel Unit 1 Water Part
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