Hello and welcome to NinteyEast! When you put a container
of water on a table, the combined weight of the container itself and the
water inside it, exert a downward force on the table. If you were to replace
that container with an identical one with for example, vegetable oil,
the force exerted would be slightly smaller. But why is that?
Well because water weighs less than water for that same volume.
Another way to describe this is to say that oil is less dense than
water. Density is simply a way of describing how much mass
a substance has for a given volume. We calculate it using the following
equation. Density equals mass divided by volume.
Interestingly the density of a substance isn't constant - it changes.
And it changes depending on lots of environmental factors.
We can see that here, where a thousand grams of liquid water at room temperature takes up a volume
of approximately one litre, or approximately 1000 cubic centimetres.
It has a density of 1 gram per cubic centimetre.
If we were to heat the entire mass of this water up to
create steam, it would take up a much larger volume.
What do you think would happen to its density? Well let's have a look at the equation here.
Density equals mass divided by volume. So the mass is the same, because it's the same
number of water molecules. But what's changed is the volume - it's increased.
Because volume is the denominator in this equation,
- that is, it's down the bottom here - as it increases,
density will do the exact opposite. It will decrease.
So, the general trend is that, as a substance moves from a solid to liquid,
and from liquid to gas - that is as it's temperature increases
its molecules will get further apart from one another, so its volume will increase
and the density will decrease. Now of course there are exceptions
to all of the rules. And one of the exceptions in this instance, is water.
As water is cooled from steam to liquid, and from liquid to
approximately 4 degrees Celsius, or 39 degrees Fahrenheit
its molecules will get closer to one another, as we've described. But at temperatures
below that 4 degree mark, or 39 degrees Fahrenheit mark,
the molecules don't actually get any closer to one another. Hmmm. Why is that?
Well, the best way to think about it is like this:
Water molecules are like tiny tiny little magnets. They're not actually magnets
but that kind of act like them with one another.
If you have a magnet in each of your hands and you move them around really, really fast, nothing much will happen.
But then as you slow them down, and you bring them close to one another
all of a sudden, the positive side of one magnet, will attract the
negative side of the other.
You may know already that water molecules are made up of an oxygen atom, and
two hydrogen atoms. Because of the way the electrons are distributed
in a water molecule, the oxygen atoms are actually a little negative;
and the hydrogen atoms a little positive. In
a liquid or gaseous state, the water molecules are moving around relatively fast
and far away from one another, so these positive and negative poles
don't really matter. But as the temperature of the water decreases, the molecules
slow down, they move, and then just like magents, their opposites attract.
So when two molecules get really slow and near each other,
they line up so that the negative oxygen atom of one molecule is near the positive
hydrogen atom of the other. As the temperature of the water
decreases further, more and more molecules line up like this, and it creates a crystalline
structure. And this crystalline structure has extra space in the middle
that wasn't necessarily there before. And suddenly the water will expand as it freezes.
This is actually a really well known phenomena, and is the reason why you should
make sure you don't fill water bottles right up to the top before you put them in the freezer. Because if you do,
the water won't have space to expand into, and the bottle will burst as the
water tries to freeze. Now there are also other factors that affect the density of
a substance, like Pressure, but they're beyond the scope of what we're looking at in this video.
For now, you just need to remember that density is simply a way of describing
how much mass a substance has for a given volume; that it's calculated
by dividing mass by volume; and that as the
temperature of the substance decreases, its density will generally increase.
In our next video, we're going to be looking at another interesting characteristic about frozen water.
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