CYCLE 6
Developing Ideas
ACTIVITY 1: Chemical Changes--KEY
Purpose
Physical properties are characteristics that can be observed or measured
without altering the composition of a material. Likewise, when a material
undergoes a physical change, its composition remains the same. At the
particle level, a physical change does not alter the particles themselves, but
rather, affects the attractive forces between the particles that are responsible
for their motion and spatial configuration. At the bulk and particle levels,
physical changes are accompanied by conservation of mass.
While physical changes happen all around us, chemical changes are probably
even more common—digestion of food, leaves changing color and
combustion of fuel in your car’s engine are just a few examples. In this
activity we will perform several experiments in order to answer the
questions:
What is a chemical change? Is mass conserved
during a chemical change?
Collecting and Interpreting Evidence
CAUTION: Wear safety glasses or goggles during this experiment.
Experiment #1: What evidence indicates that a chemical change has
occurred?
Chemical Change #1: Steel wool and air
You will need:
Soap-free steel wool soaked in vinegar for 5-10 minutes, ~ 4 grams
or half of a fine-grade, crafter’s pad
250-mL flasks
Balloon
© 2007 PSET
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Cycle 6
STEP 1. Your teacher will provide you with a steel wool pad that
has been soaked in vinegar for 5-10 minutes. Before removing the
pad from vinegar, stir it around, making sure that the steel wool is
thoroughly soaked. The vinegar will remove the protective coating
of the steel wool, exposing iron to air, but this is not part of the
chemical change we will study.
STEP 2. Remove the pad from the vinegar, squeeze out as much vinegar as
possible, and blot the pad with a paper towel to remove any excess. Do this
quickly.
STEP 3. Place the steel wool pad in a 250-mL flask and seal the
mouth of the flask with a well-stretched balloon. Try to squeeze any
air out of the balloon as you make the seal.
Observe the appearance of the steel wool and the appearance of the
balloon. Record your initial observations in the table below.
Measure and record the initial mass of the steel wool/flask/balloon
system.
Table 1: Observations of Steel Wool/Balloon/Flask System
Initial Observations
steel wool
balloon
Final Observations
Bendable, silver-grey fibers (solid)
Flaky, brittle, reddish-brown (solid)
deflated
Partially inflated, inverted in flask
mass of system
STEP 4. Allow the system to sit for at least five to ten minutes. During this
time, complete Experiment #2. After you have completed Experiment #2
return to Experiment #1 and answer these questions:
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Activity 1: Chemical Changes
Observe the system again and record your final observations in the table
above.
Measure and record the final mass of the steel wool/flask/balloon
system.
Feel the bottom and sides of the flask. Describe the relative
temperature—is the flask cooler than, warmer than, or about the same as
other surroundings at room temperature.
The flask is slightly warmer than the surroundings at room temperature.
From the initial and final appearances of the steel wool, what evidence
do you have that a new material with different properties from the
original materials was formed?
A reddish-brown flaky material has formed; it’s appearance, texture, brittleness, etc. are
clearly different from the original steel wool.
Iron (in the steel wool) is one of the starting materials in this interaction.
From the initial and final appearances of the balloon, what evidence do
you have that a gas in air is the other starting material?
The balloon is inverted in the flask; gas must have been consumed, meaning the
number of gas particles and # of collisions/sec decreased. Since pressure inside the
flask is proportional to # collisions/sec, pressure decreases as the gas is consumed. The
greater pressure outside the flask pushes the balloon inside the flask.
Is mass conserved during this interaction?
Yes, the masses are nearly the same; some variation may occur if same balance is not
used to measure masses before and after reaction, or if the balance is not tared prior to
obtaining measurement.
Chemical Change #2: Alka-SeltzerTM and water
You will need:
1 Alka-SeltzerTM tablet
Water, ~ 100 mL at room temperature
250-mL flask
Balloon
STEP 1. Remove one tablet of Alka-SeltzerTM from the pack.
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Cycle 6
Observe the Alka-SeltzerTM tablet (e.g. appearance, physical state, etc.)
and record your initial observations in the table below.
Table 2: Observations of Alka-SeltzerTM/Balloon/Flask System
Initial Observations
Alka-SeltzerTM
tablet
balloon
Final Observations
White, chalky, brittle (solid)
Bubbled, and solid disappeared
deflated
Inflated upright above the flask’s
mouth
mass of system
STEP 2. Break the tablet into three or four pieces. Insert the pieces in the
mouth of the balloon, and then push them down into the larger part of the
balloon.
STEP 3. Add 100 mL of room temperature water to a 250-mL flask.
STEP 4. Seal the mouth of the flask with the balloon, but do not allow the
Alka-SeltzerTM pieces to fall into the water. Try to squeeze any air out of the
balloon as you make the seal.
Observe the appearance of the balloon. Record your initial observations
in the table above.
Measure
and
record
the
initial
TM
Seltzer /water/flask/balloon system.
mass
of
the
Alka-
STEP 5. Push all of the pieces of Alka-SeltzerTM down to the mouth
of the balloon and into the flask containing water.
STEP 6. Allow the system to sit for a short period, or until the Alka-SeltzerTM
tablet is no longer visible.
6-4
Activity 1: Chemical Changes
Observe the system again and record final observations in the table
above.
Measure
and
record
the
final
mass
of
the
AlkaSeltzerTM/water/flask/balloon system. Report uncertainties in the mass
measurement.
Feel the bottom and sides of the flask. Describe the relative
temperature—is the flask cooler than, warmer than, or about the same as
other surroundings at room temperature.
The flask is slightly cooler than the surroundings at room temperature (Note: this may be
difficult to detect if the tap water used is slightly cooler than 23 degrees Celsius; you may
wish to use bottled water that has been stored at room temperature, or tap water that
has been sitting in a container in the room for some time before the experiment.)
From the initial and final appearances of the Alka-SeltzerTM tablet and
the balloon, what evidence do you have that a new material with
different properties from the original materials was formed?
The solid tablet disappears and forms a colorless gas.
Is mass conserved during this interaction?
Yes, the masses are nearly the same; some variation may occur if same balance is not
used to measure masses before and after reaction, or if the balanced is not tared prior to
obtaining measurement.
Now return to Chemical Change #1, record your final observations and
complete the questions.
You may have noticed that the flask containing the steel wool became
warmer and the flask containing Alka-SeltzerTM became cooler as a result of
the chemical change.
Your instructor will now demonstrate or show you time-lapsed
video clips of these same experiments being performed with a
temperature sensor inserted in the flask. Keep in mind that the
reading given by the temperature sensor, just like with a
thermometer, is a measure of the average kinetic energy of the
particles composing the sensor. As heat energy is transferred to
or from the temperature sensor due to differences in temperature
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Cycle 6
between the sensor and the material in contact with it, the average kinetic
energy of the particles composing the sensor increases or decreases.
What evidence do you have that heat energy is being transferred from
the steel wool to the temperature sensor as the steel wool forms rust?
The temperature increases as rust forms.
What evidence do you have that heat energy is being transferred to the
Alka-SeltzerTM from the temperature sensor as the Alka-SeltzerTM forms
carbon dioxide?
The temperature decreases as carbon dioxide forms.
Chemical Change #3: Potassium iodide and lead nitrate
You will need:
Potassium iodide solution, ~ 2 drops from community bottle
Lead nitrate solution, ~ 2 drops mL from community bottle
Well or spot plate
Plastic pipette
CAUTION: Handle potassium iodide and lead nitrate solutions with care. Wear
safety gloves to ensure that your skin is not contaminated by these solutions or the
material formed when they are mixed.
CAUTION: Do not dispose of lead-containing waste down the sink or in the trash
can. Your instructor will collect this waste for appropriate disposal.
STEP 1. Add ~10 drops of potassium iodide and 10 drops of lead nitrate to
separate wells on a well plate. Keep track of which solution is contained in
each well.
Observe the potassium iodide and lead nitrate solutions (e.g.
appearance, physical state(s), etc.) Record your initial observations in the
table below.
Measure and record the initial mass of the system (the well plate, its
contents, and a plastic pipette).
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Activity 1: Chemical Changes
Table 3: Observations of Lead Nitrate/Potassium Iodide System
Initial observations
Two clear liquids; one with a slight
yellowish tint
Initial mass of system
Final observations
A thick, yellow milky suspension
(solid in liquid) forms
Final mass of system
STEP 2. Use the plastic pipette to transfer the potassium iodide to the well
containing lead nitrate. Stir the contents of the well with the pipette.
Observe the contents of the well (e.g. appearance, physical state(s), etc.)
and record your final observations in the table above.
Measure and record the final mass of the system (the well plate, its
contents and the plastic pipette).
From the initial and final appearance of the solutions, what evidence do
you have that a new material with different properties from the original
materials was formed?
Two clear liquids form a thick, yellow milky suspension when mixed.
Is mass conserved during this interaction?
Yes, the masses are nearly the same; some variation may occur if same balance is not
used to measure masses before and after reaction, or if the balanced is not tared prior to
obtaining measurement.
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Cycle 6
Experiment #2: What is the key difference between physical and
chemical changes?
In Cycle 5 we established that physical changes are changes in shape, volume
(size), or physical state due to some mechanical, heat conduction, or IR
interaction with another object or the surroundings. For example, imagine
that we flatten a piece of lead shot into a thin sheet of lead with a hammer.
We have changed the shape and dimensions of the lead, but the other
properties of the lead (melting point, boiling point, density, etc.) are the same.
Physical changes do not produce new materials. The original form of the
material can usually be recovered through some other physical change such
as mixing, dissolving, expanding or contracting, or changes in state (melting,
boiling, condensing, or freezing). In contrast, chemical changes produce new
materials with properties that are different from the original material. This
means that the original material cannot be recovered using the “physical”
methods listed above but only through additional chemical changes.
However, some of the evidence you recorded for chemical changes can also
be the same as the evidence for a physical change, so you have to be careful
when deciding whether a chemical or physical change has occurred!
You will need:
1 Alka-SeltzerTM tablet
Water, ~ 250 mL
Table salt
2 250-mL flasks
Consider what happens when you add table salt and Alka-SeltzerTM to water:
STEP 1. Add 125 mL of water to each of two 250-mL flasks.
STEP 2. Measure and add 3 grams of table salt to the water in the one flask.
Add an Alka-Seltzer tablet (which is ~ 3 grams) to the water in the second
flask.
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Activity 1: Chemical Changes
What is the result of adding salt to water?
The salt dissolves.
Based on your experience in Cycle 5 Activity 7 HW, what could you do
to recover the original salt?
Boil off the water and salt reforms.
How is adding the Alka-SeltzerTM tablet to water different from adding
salt to water?
As the tablet disappears, a solution forms and carbon dioxide gas is formed. This is
clearly not simply a physical change like dissolving.
You cannot recover the original materials in an Alka-SeltzerTM tablet
using physical changes. If Alka-SeltzerTM and water produce carbon
dioxide gas and a solution, what do you think you could do to recover
the Alka-Seltzer?
In theory, we should be able to perform other chemical changes to regain the AlkaSeltzer. Practically, this would be difficult to accomplish in the classroom.
Energy and Chemical Changes
When making mass measurements, we considered the various containers
(flasks, balloons, beakers) as well as their contents as our “system.” When we
discuss energy transfers and changes, we will define the system as the
materials undergoing chemical change inside a container, but the container
itself will be considered part of the surroundings.
In Experiment #1 you observed that changes in the temperature of the
surroundings accompany chemical changes of the system (even for Chemical
Change #3, but in this case the temperature change was not perceivable by
touch alone.) The changes in temperature that we detected in the
surroundings (the flasks or the temperature sensor) are due to heat transfer
between the system and the surroundings during chemical changes of the
system (the contents of the flasks). When heat energy is transferred from the
surroundings to the system during a chemical change, the temperature of the
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Cycle 6
surroundings decreases (therefore, the flask feels colder to the touch or the
temperature sensor displays a lower temperature). When heat energy is
transferred from the system to the surroundings during a chemical change,
the temperature of the surroundings increases (therefore, the flask feels
warmer to the touch or the temperature sensor displays a higher
temperature).
A chemical change in which heat energy is transferred to the system is called
endothermic (endo- is an English prefix derived from Latin prepositions
meaning into or within).
HC/IR Interactions
System of
materials
undergoing
chemical change
Heat
energy
Increase in
?
Surroundings
Decrease in
thermal
energy
A chemical change in which heat energy is transferred from the system is
called exothermic (exo- is an English prefix derived from Latin prepositions
meaning out or away).
HC/IR Interactions
System of
materials
undergoing
chemical change
Decrease in
?
6-10
Heat
energy
Surroundings
Increase in
thermal
energy
Activity 1: Chemical Changes
You will study exothermic and endothermic chemical changes in further
detail, including developing a more complete definition for endothermic and
exothermic, and identifying the “?” (the type of system energy that changes)
in Cycle 6 Activity 7.
Summarizing Questions
Discuss these questions with your group and note your ideas. Leave
space to add any different ideas that may emerge when the whole class
discusses their thinking.
S1. List all the evidences of chemical change that you are aware of at this
point:
Consumption or production of a gas
Consumption or production of a solid
Temperature change/heat energy transfer (without a heat source)
Change in physical appearance (color, texture, brittleness, etc.)
S2. Some evidences that you listed for chemical changes may be the same as
the evidence for a physical change. A few examples and comparable
physical phenomena are: disappearance of a solid (melting or
dissolving), generation of a solid (freezing), or generation of a gas
(evaporation or boiling). So, what distinguishes physical changes from
chemical changes?
The production of a new material with characteristic properties that are distinct (different)
from the original starting materials.
S3. Does the evidence gathered in this activity support, or not support, the
claim that mass is conserved during chemical changes? Explain.
The evidence gathered supports the claim that mass is conserved. The masses of each
system were the same before and after the chemical change occurred.
S4. Which chemical change from Experiment 1 would you consider
endothermic? Exothermic? Explain your reasoning.
The formation of rust is exothermic because heat is transferred to surroundings (we feel
the flask is warmer, the temperature probe is warmed); the formation of carbon dioxide is
endothermic because heat is transferred to the system (we feel the flask is cooler, the
temperature probe is cooled).
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Cycle 6
S5.
Deduce whether each scenario below is a physical or chemical
change. Give your reasoning for each.
a. CyanalumeTM sticks (‘glowsticks’) glow after they are “cracked”
Chemical change; the new material glows (light energy is transferred from the
system)
b. paper is torn into smaller pieces
physical change; the paper is still paper, all the same properties (except for size)
c. a penny turns from copper color to green (patina) over time
Chemical change; the new material undergoes a color change
d. a wood log burns
Chemical change; the new materials include ash and gas (light and heat energy
are transferred from the system)
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