It’s professor Dave, let’s discuss conservation of mass.
Let’s say it’s time to cook dinner, and you put a pot of water on the stove to boil.
As it begins to boil, some of the liquid water becomes steam.
Let’s say that rather than cooking up our pasta, we just allowed all of the water to
boil away.
Do you think that if you were to then collect all that steam and measure its mass, it would
be the same as the mass of the water that was in the pot?
For some people, intuition may tell them that the vapor must somehow contain less mass than
the liquid water, because any gas is going to be so much less dense than liquid.
But in actuality, the vapor must contain precisely the same mass as the liquid water, because
this process, like any other chemical reaction, must obey the law of conservation of matter.
This law states that there must be no change in the total amount of matter present when
matter converts from one form to another.
If we wanted to prove this to ourselves, we could collect all of that steam, and we would
find that it contains the same mass as the mass of the liquid water, because when water
molecules go from the liquid phase to the gas phase, nothing happens to each individual
water molecule other than that they speed up and move away from each other.
The volume of the water increases as it becomes a gas, but there is no change occurring to
any individual molecule, so none of the matter, and therefore none of the mass contained by
that matter, can appear or disappear.
This fact pertains not just to physical processes like phase changes, but also chemical reactions.
This is why we balance equations.
In this reaction, represented by this balanced equation, we can see that no atoms are created
or destroyed.
There are the same numbers of each type of atom on each side.
This is a fundamental fact that must be true of every chemical reaction you can imagine.
Let’s imagine some other processes that can demonstrate this law.
Think of a copper pipe.
If we leave this pipe to the outdoor elements, where it will come into contact with water
from the rain, and oxygen from the atmosphere, we know that this pipe will rust.
And as a result, the rusty pipe will have more mass than it did originally.
So we may at first think that matter has appeared out of nowhere to give the pipe more mass.
But in actuality, the rust is due to the oxidation of the pipe, or the reaction with oxygen molecules
in the atmosphere.
This means that oxygen atoms will covalently bond to the copper atoms in the pipe.
This seems strange at first, because it’s easy to think of air as having no mass, since
it’s invisible and is floating all around us, but the atmosphere actually contains many
different tiny, low-mass molecules like oxygen and nitrogen, which do indeed have mass, however
small they may be.
And when the oxygen molecules react with the copper pipe, they are adding their mass to
the pipe.
This is why the mass of the pipe increases, there are literally more atoms on the pipe
as the rust forms.
These are the kinds of observations we have made over the centuries that brought us to
a better understanding of what our atmosphere is made of and how it can interact with different
objects.
The law of conservation of matter applies to many other every-day concepts, even if
it’s not always easy to see it in action.
Say you eat a big burrito for lunch.
Where does all that food go?
Certainly some of it is removed from the body as waste, but by no means all of it, so what
happened to the rest?
Can it have simply vanished?
Amazingly, dozens of enzymes in every cell of your body break down this food into tiny
components that can serve as the raw materials your body needs to build more cells and provide
energy for all the things you do every day, from exercising to simply thinking.
So even though it’s not always easy to directly observe all the changes that matter undergoes
in order to verify the law of conservation of matter, it is this very law that outlines
all of the physical and chemical changes that we will come to understand in our study of chemistry.