The Law of
Conservation
of Matter
We’ve talked about changes
in matter…
• The evaporation of a puddle of
water
• Rust forming on a metal fence
HOWEVER…
• No matter is
created or
destroyed by
such changes!
• The total
amount of
matter remains
constant.
The Law of
Conservation of Matter
• During a chemical reaction, matter
cannot be created or destroyed.
• Even though the matter may change
from one form to another, the same
number of atoms exists before and
after the change takes place!
Law of Conservation of Mass
Mass is neither
created nor destroyed
during chemical or
physical reactions.
Antoine Lavoisier
Total mass of reactants
=
Total mass of products
Since the overall mass of the
matter does not change…
• The mass of the reactants – the
substances there before the reaction
occurs – must equal the mass of the
product – the substances there after
the reaction takes place.
Mass of
Reactants
Mass of
Products
An Example of the L.C.M.
• Test tube containing
lead nitrate is placed
upright in a flask
that contains
potassium iodide.
• What are these?
– REACTANTS!
• They don’t mix!
• A stopper is put in place to create a
closed system – an environment where
matter cannot enter or escape.
• The mass of the system is found.
• After the mass is found, the system is
turned upside down so the lead nitrate
can mix with the potassium iodide.
• They create lead iodide and potassium
nitrate.
• Although the matter changed in
form, the total mass of the
system remains unchanged.
The Formation of Rust
• Iron
– Hard, gray-colored
metal
• Rust
– Flaky, orange-red
solid
When iron reacts with oxygen
in the air, they form rust or
IRON OXIDE.
• This is written as:
Iron + Oxygen
Iron Oxide
TAKE A
LOOK AT
THE
NUMBERS
Fe
Fe
Fe
Fe
How many Iron
molecules?
O2
Fe2O3
O2
Fe2O3
O2
How many
Oxygen
molecules?
What about
the
PRODUCT?
All Chemical Reactions follow
the Law of Conservation of
Matter
• With Iron Oxide, the MASS STAYS THE
SAME!
– The total number of iron atoms and
oxygen atoms in the reactants is the same
as that in the product.
– The atoms were just rearranged!
• No new atoms were created and none were
destroyed!
Determining the Mass of Reactants and
Products
• REMEMBER: The mass of the reactants is always equal to the
mass of the products.
Mass of
Reactants
Mass of
Products
Example: Tin Fluoride
• Tin + hydrogen fluoride -> tin fluoride + hydrogen
40.02 g
118.7 g
• What’s the total mass of the reactants?
– 158.72 g
• So what should the mass of the products be?
– 158.72 g
• Tin + hydrogen fluoride -> tin fluoride + hydrogen
Total Mass of
Reactants: 158.72 g
• If we know the mass of the tin fluoride, can
we figure out how much hydrogen was
produced?
– Subtract the mass of one product from the total
mass.
How did scientists come up
with this law?
• Just like we did with the
vinegar and baking soda in a
bag!
• They measured the mass of
the reactants (vinegar and
baking soda) and then, after
the reaction took place, they
measured the mass of the
products (reactants + carbon
dioxide)
ANY QUESTIONS?