Physical
versus Chemical Change
This is an archival
version of the web assignment. It is after the due date,
so the assignment can no longer be submitted.
Now that you
have revisited the textbook and have viewed the movies,
you are ready to proceed to the assignment. Don't worry
if you struggle to answer the questions–that is the
plan! One goal of the web assignments is to make you stretch
beyond what you currently know or have already
figured out. In doing so, you will get prepared for future
discussion of the topic in class, and you will likely take
yourself to the next level of understanding.
Here is an old exam question
on physical and chemical changes of matter.
How might you approach answering this question? The original
question and multiple choice answers are in black and the
thought processes used to answer the question are in blue.
Which of the following is a chemical
property (or change)?
| |
a. |
oil floating on water |
| |
|
When mixing oil and water,
neither substance changes and no new substance is formed.
The oil is still oil and the water is still water. They
retain their physical properties like melting point, boiling
point, slipperiness, color, odor, and density. Since a
chemical change did not occur, choice A is not
the correct answer. |
| |
b. |
wax melting |
| |
|
Melting is changing from
the solid to the liquid state. Changes of state are physical
changes. The composition of the wax did not change and
no new substance was formed. Simply cooling the hot wax
returns it to its original state. Choice B is not
the correct answer. |
| |
c. |
a candle burning |
| |
|
When a candle burns,
the wax is consumed and becomes soot, heat, and other
stuff. When new materials (soot) are produced that have
different physical properties than the original substance
(wax), a chemical change has taken place. C is the
correct answer. |
| |
d. |
sea water drying up to leave a residue
of salt |
| |
|
When two substances can
be separated by a physical means (in this case evaporation),
no chemical change has occurred. Notice, if you added
water to the residue, you would get back to the original
stuff. D is not the correct answer. |
| |
e. |
mixing together rust (iron oxide) and
metallic iron |
| |
|
In everyday language
we talk about mixing. Sometimes mixing produces mixtures.
Other times mixing results in a chemical reaction. How
can we tell? If a chemical reaction happened, new substances
form and and we may see a change in color, bubbles indicating
the formation of a gas, or other tell-tale signs. Here,
simpling passing a magnet near the sample would separate
the iron and iron oxide, so the iron and the iron oxide
were chemically unchanged. E is not the correct
answer. |
Now it is your turn. Give careful
thought to each question and justify your answers
by telling me what you were thinking and how you approached
each decision. Edit your answers so they make sense and convey
your understanding of the concepts. When you are satisfied
with your responses, click the SUBMIT button at the bottom
of the page.
Question 1. Aspirin
tablets are made by pressing together starch and aspirin.
Explain your
choice.
Question 2. Burning
an aspirin tablet produces carbon dioxide and water vapor.
Explain your
choice.
Question 3. When
a person takes an aspirin tablet, the drug is metabolized
in the liver and the metabolites are excreted in the urine.
Explain your
choice.
Question 4. Milk
curdles (forms curds) when it is mixed with vinegar or lemon
juice.
Explain your
choice.
Question 5. A
sidewalk is formed when concrete sets (hardens).
Explain your
choice.
Question 6. Identify
the physical change in the following list.
Explain your
reasoning.
