Hi. It's Mr. Andersen and this is chemistry essentials video 11. It's on
symbolic representations. When you look at a candle burning, what you're seeing are processes
at work. You're seeing both physical changes. In other words the wax is melting to become
a liquid. And then it's becoming a gas. And then you're also seeing chemical changes as
that wax combines with oxygen in the air. You're seeing combustion. And so what we can
do in chemistry is use symbolic representations of that to show conservation in mass. And
so processes can be physical or chemical. But in chemistry we can use representations
to look at those changes. They could be symbolic representations or they could be particulate
drawings. But in both of those cases we're going to see a conservation of matter. In
other words matter can neither be created nor destroyed. And so we can use that to calculate
reactants based on products. Or products based on reactants. And also we can look at macroscopic
changes. We can look at movement of particles around. An example of that would be the carbon
cycle. And so if we're looking at the physical versus chemical changes, an example of a physical
change could just be boiling of water. In other words we're changing water into a different
phase. From a liquid into a gas phase. But a chemical change is going to require the
breaking and forming of new bonds. And so an example of that could be the combustion
of hydrogen found in the Hindenburg explosion. In other words the hydrogen and the oxygen
are combining to make water. And so we don't have hydrogen gas anymore. And so let's look
at water to start with. And so what would be a symbolic representation of this phase
change in water? We could look at water that's a liquid. And so the l means liquid. We're
going to add a little bit of energy to that. And then we're going to create water that
is a gas. And so what are these bubbles in boiling water? It's simply water that's changed
phase from a liquid to a gas. What would a particulate representation or particulate
drawing look like of that same process? We'd have water molecules here. They're now a liquid.
We're adding a little bit of heat to it. And now they're a gas. And so both of those are
essentially saying the same things. It's just a model so we can wrap our head around it.
Let's look at hydrogen combustion. What would be a symbolic representation of that? We'd
have hydrogen gas with oxygen gas. You can see that these are both gases. And then we're
forming water that's a liquid. And we're releasing a huge amount of energy from that. Now a few
things you should remember. The 2 here, the subscript means that's the number of atoms
of hydrogen that we have in one hydrogen molecule. And then the 2 out front, that coefficient
is going to tell us how many molecules that we have of that. If we were to have a particulate
drawing, here would be our hydrogen. So we have 1, 2 in this drawing we have 4 hydrogen
molecules. So the H2 means we have two of the hydrogen. And then we'd have four of these
molecules combing with two molecules of oxygen. And in this case we'd have four molecules
of water. And so the conservation of matter is incredibly important. It was first developed
by Antoine Lavoisier. Here he is with his wife Marie-Anne. And she worked in his lab
and was really important in helping him make all of these amazing discoveries. And if we
were to say what is he most famous for. He's most famous for moving chemistry from a qualitative,
in other words let's look at the color that's created or smoke is created to more of a quantitative
science. In other words he was always measuring the masses. And he would use these big spheres,
glass spheres so he could collect all the material that was created. And so if we look
at all of these, both symbolic representations and particulate drawings, the mass before
the physical or chemical change is going to equal that after. And so we can actually us
this to solve some simple problems. Let's say the Hindenburg carried 1.77 times 10 to
the 4th kilograms of hydrogen. And this combined with 1.42 times 10 to the 5th kilograms of
oxygen. How much water would be formed in that? Well this is a really simple kind of
a problem. We'd first draw out a symbolic representation. We've got the amount of hydrogen
and the amount of oxygen. And so since mass can neither be created nor destroyed, the
mass that we have before the reaction is going to equal the mass we have after. So if I were
to solve this I'd write out the two masses we have before. You can see that they're going
to be written in scientific notation. But these don't quite match up. So I may change
this so that they match up. And then it makes it a little easier to add those. So we'd make
1.60 times 10 to the 5th kilograms of water. Now most of that water wasn't liquid. It was
probably liquid that was just spread over a large area. If we were to quantify that,
in that Hindenburg explosion, 176 tons of water were made during that explosion. And
so another thing that you should understand is as matter cycles, as it becomes one molecule
and moves into another molecule, it's also going to be conserved. And so an example of
this could be a biogeochemical cycle. So an example could be carbon. And so the carbon
that's found in the atmosphere eventually becomes carbon in plants. Becomes carbon in
food. And then becomes the carbon inside you. And as you die we release that carbon into
the atmosphere. But it's not like we are creating or destroying matter. It's the same mass that's
moving throughout that cycle. And so again in review, we can use representations both
symbolic and particulate drawings to show this conservation of mass. The problems are
very simple, but we can also use it to measure large macroscopic movement. And so did you
learn this? To express the conservation of mass qualitatively and quantitatively? Qualitatively
we could do that through one of these particulate drawings or in some kind of a chemical equation.
But if we want to do a quantitatively we actually have to measure the mass before and relate
that to the mass after or vice versa. And I hope that was helpful.