You may have come across the idea of conservation in science or in everyday life. Conservation of
nature or historical artefacts is the idea that you protect or preserve them.
Conservation of energy is the idea that energy cannot be created or destroyed, only transferred
from one form to another. The Law of Conservation of Mass is very similar. Matter, in this case atoms,
cannot be created or destroyed during a chemical reaction. All the atoms that you start with in your
reactants will still exist during and after the chemical reaction. However, the atoms may have been
rearranged to form new products. Therefore, the mass of the products at the end of the reaction
will be equal to the mass of the reactants at the beginning of the reaction.
Mass is always conserved. When an ice cube melts it changes state from solid to liquid.
The mass of the solid ice cube is exactly the same as the mass of the liquid water molecules.
This is because we have exactly the same number of molecules throughout. Conservation of Mass
applies even when there is a change in state. In this video, we are going to investigate the
Conservation of Mass with reactions where there is an increase or decrease in mass.
There are two parts to this experiment: the first is the reaction of magnesium with oxygen,
the second is the reaction of calcium carbonate with hydrochloric acid.
Hopefully, you're familiar with the reaction of magnesium with oxygen, and remember that it burns
with a bright white flame to produce a white powder. Let's look at this, but before we do I
must make sure that I have eye protection on and that I do not look directly at the
flame.
In this experiment, we're going to contain the reaction within a crucible to ensure that we
don't lose any of the product. The crucible has a loose-fitting lid to ensure the oxygen can get in
and the reaction can complete. I'm going to curl my magnesium into a loose curl so that it will fit in
the base of the crucible, but not so tight that the oxygen cannot react with the surface of the metal.
Before we begin the experiment, we need to record the mass of the crucible and the magnesium strip.
We need to record this in the table because at the end of the experiment
we cannot ensure that we get all of the magnesium oxide product out of the crucible.
Therefore, we have to measure the crucible and its contents together at the end.
Now, I'm going to heat the magnesium. Remember the reactivity series.
Magnesium is more reactive than aluminium so it will oxidize fairly easily. However,
it's not as reactive as lithium or sodium so it won't happen very quickly at room temperature.
The Bunsen flame ensures there is enough energy to get the reaction started.
The crucible, with the magnesium in, will be heated on a pipe clay triangle on a tripod.
First, I need to put the Bunsen burner on a roaring blue flame.
I'm going to use tongs because the crucible will get very hot.
We need to lift the lid every minute or so, to check whether the reaction has fully completed,
taking care that no products are allowed to escape.
The reaction will not fully complete with the lid on because not enough oxygen is in the crucible.
Once the magnesium has completely reacted, you'll be left with a white powder,
maybe glowing orange until you take the Bunsen flame away.
You need to leave the crucible for a few minutes to cool before measuring the mass of the products.
Remember not to remove the lid or you could lose some of the fine white powder
and this will affect the final mass. Once the crucible has cooled we can transfer the crucible
to the mass balance, complete with lid and contents, then record the final mass: 48.36g. We will calculate
the change in mass after we've completed the second experiment, so that we can compare the two.
For the second part of this experiment, we're going to react calcium carbonate, in the form
of small marble chips, with hydrochloric acid. This experiment also requires eye protection.
I'm going to place 20 centimetres cubed of hydrochloric acid into my conical flask,
which has already been measured out using a measuring cylinder.
Again, we need to record the starting mass of our reactants, so I will place this conical flask
onto the mass balance with the hydrochloric acid and the marble chips, which are already
on the mass balance. 80.60g
Now, we can add the calcium carbonate to the hydrochloric acid.
Straight away we can see that a reaction is happening because there are bubbles forming.
This tells us that a gas is being released. I'm going to
leave this reaction for a few minutes to ensure that the reaction completes.
We can give the flask a little swirl to ensure no more bubbles are being produced.
All of the calcium carbonate should now have reacted with the hydrochloric acid,
so we can measure the mass again using the mass balance.
80.13g
It's now time to complete the results table by calculating the change in mass.
To do this, you need to take the total mass before the experiment away from the total mass after
the experiment. Your answer could be positive or negative. Show this in the results table by putting
a positive or negative sign in the final column. A positive change in mass means we have observed an
increase in mass whereas, a negative change in mass means we have observed a decrease in mass.
If we look at our results table, we can see that the magnesium has increased in mass
whereas, the calcium carbonate and hydrochloric acid has decreased in mass. Taking into account
the law of conservation of mass, we know that no atoms have been created or destroyed
during the reaction. So, how can we account for the change in mass? Let's look at the word equation for
the reaction between oxygen and magnesium. Here we have two reactants combining to make one product.
One of the reactants, magnesium, is a solid so we were able to record it in the crucible at
the start of the reaction. The other reactant, oxygen, came from the surrounding environment
during the reaction. The product of this reaction, magnesium oxide is a white powdery solid.
If we were able to contain all of that solid in the crucible during the reaction,
then the increase in mass represents the amount of oxygen that has combined with the magnesium.
Now, let's look at the reaction between calcium carbonate and hydrochloric acid:
The products had a total mass that was less than the mass of the reactants.
These changes in mass have occurred because we conducted both experiments in an open system.
In an open system, mass or energy can be transferred to or from the surroundings.
Now, let's see what happens if we conduct the second experiment in a closed system.
This time I'm going to repeat the experiment inside a sealed bag. Starting mass: 91.86g
I'm sealing the plastic bag before I let the chips go so that no carbon dioxide can escape.
This time the carbon dioxide gas cannot escape to the surroundings and is included in the total
mass of the sealed bag, so it's a closed system. This is the law of conservation of mass in action.
We have explored some examples of reactions where we're able to observe an increase or decrease
in mass. However, when reactions take place in an open system, they may appear to violate the laws
of conservation of mass. Link to the additional resources in this video to practice applying
the law of conservation of mass. Thank you for watching and have fun with your experiments.
Inspiring your teaching and learning.