Chemical and Physical Change
How can we tell whether a change is chemical or physical?
Why?
Did you know that virtually all modern materials are made using chemical
reactions? Everything from prescription drugs that support your body while it
fights off diseases to the cushy foam in the seat of your car has been made
possible by a deep understanding of how chemical and physical changes affect
materials. Gaining a deeper understanding of chemical and physical changes
will allow you to better appreciate the all customized materials that help you
live a safe, happy, and productive life. Who knows, one day you may even be
part of the process of designing and making these materials for future
generations!
Activating Prior Knowledge
In the space below, write down everything you know about chemical and
physical changes.
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Model 1 — Physical Changes
A
Physical Change
Before
After
10 CSq
B
Physical Change
Before
After
8 TC
C
10 CSq
3 TC
Physical Change
Change
Before
After
6 TCSq2 + 4 C3Sq
6 TCSq2 + 4 C3Sq
2
1. In Box A, the particle CSq is ( an element / a compound ). I can tell this
from the drawing because…
2. For Box A, indicate how many of the following are present.
Before Side
After Side
# of molecules of CSq = ____
# of molecules of CSq = ____
# of total atoms = ____
# of total atoms = ____
# of different elements = ____
# of different elements = ____
Read This!
Chemical bonds are forces that hold atoms together in a compound or
molecule.
1. How are chemical bonds represented in Model 1?
2. For each box, write a brief description of what’s you see happening to the
particles.
Box A
Box B
Box C
3. At the bottom of each box, there is an equation that summarizes the
change. What does the arrow represent in the equation?
a. Look at Box C, how are different compounds separated in the equation?
4. For each item, indicate which drawing (A, B, or C) is the best representation.
a.
b.
c.
d.
e.
f.
Evaporating a mixture of salt and water ______
Ice melting in to water ______
Letting air out of a bicycle tire to reduce the pressure _____
Heating up a solid and seeing it expand _____
Cutting a piece of wood into a smaller chunk _____
Filtering muddy water so that you can drink it _____
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Model 2 — Chemical Changes
D
Chemical Change
Before
After
6C+6T
E
6 TC
Chemical Change
Before
After
6 C2 + 3 Sq2
F
6 C2Sq
Chemical Change
Before
After
4 TCSq2
?
4
5. For Box F, indicate how many of the following are present.
Before Side
After Side
# of molecules of TCSq2 = ____
# of molecules of TCSq2 = ____
# of molecules of CSq = ____
# of molecules of CSq = ____
# of molecules of TSq = ____
# of molecules of TSq = ____
Total # of C atoms = ____
Total # of C atoms = ____
Total # of Sq atoms = ____
Total # of Sq atoms = ____
Total # of T atoms = ____
Total # of T atoms = ____
a. Describe what you think happened to the TCSq2 molecules in Box F.
b. Complete the equation in Box F.
c. During a chemical reaction the total number of atoms of each element
( increases / decreases / stays the same ). Which boxes from Model 2
support your answer?
d. During a chemical reaction the total number of atoms ( increases /
decreases / stays the same ). Which boxes in Model 2 support your
answer?
e. In Box F, we started with 4 total particles and ended with 8 total
particles. Explain how the number of particles can increase without
changing the number or type of atoms.
6. In Box F, the equation for the before says 4 TCSq2.
a. What does the 4 tell you?
b. How can you use this equation to figure out how many total atoms
there are of each element?
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7. Consider Box D, where do you think the molecules of TC came from.
a. In Box D, we started with ____ total particles and ended with ____
particles. Explain how the number of particles can decrease without
changing the number or type of atoms.
8. Consider Box E, describe what happened to the atoms. In your answer be
sure to describe any changes in the chemical bonds.
9. Use the equation in Box G to draw the before and after pictures.
a. In the title above the
G
____________ Change
box, write whether the
Before
After
change is Physical or
Chemical. Explain your
reasoning below.
2 Sq2 + 2 TC
2 TCSq2
b. Which should be heavier, the 2 Sq2 + 2 TC in the before box or 2 TCSq2
in the after box? Explain your reasoning.
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10. Compare Model 1 and Model 2. How does the equation for a physical
change differ from the equation of a chemical change?
a. How can you tell whether a material has undergone a chemical change
rather than a physical change?
11. Complete the statements below.
a. During a physical change…
b. During a chemical change…
Read This!
During a chemical reaction (also called a chemical change), the chemical bonds
between atoms are either broken, formed, or both. The result is that new a
new compound or molecule is formed using the atoms from the starting
material. The starting materials are called the reactants. The compounds or
molecules that are formed are called the products. It is important to note that
the products are made by recombining the atoms in the reactants.
12. In Box F, the reactants are _______ and the products are _____ and _____.
13. Read each statement below and indicate whether it describes a physical or
chemical reaction.
a. Jestin filters sand out of a solution of salt water.
b. The log Bethany added to the campfire burns.
c. Henry uses a magnet to separate iron from some soggy cereal.
d. The water Olivia spilled on the ground slowly evaporates.
e. The metal on Paul’s car starts to rust and flake off.
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Model 3 — Chemical Formulas
Use the formulas below to draw the reactants and products for reaction below.
Possible formulas to be used in Boxes H - K
H
H
C
O O
H H
O2- oxygen
H2 - hydrogen
O
H
H
H2O - water
O C O
CO2- carbon
dioxide
H
H
CH4 - methane
C
C - carbon
H
Chemical Change
Reactants
Products
2 H 2 + O2
I
2 H2O
Chemical Change
Reactants
Products
C + O2
CO2
Notice that reaction J and K are both missing one of the numbers. Use your
drawing to figure out how many molecules you need of H2 you need for reaction
J and how many molecules of O2 you need for reaction K.
J
Chemical Change
Reactants
Products
C + __H2
CH4
K
Chemical Change
Reactants
Products
CH4 + __O2
CO2 + 2 H2O
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