CYCLE 6
Developing Ideas
ACTIVITY 2: Chemical Changes and the Small
Particle Model--KEY
Purpose
Chemical changes have two things in common: they produce new materials
with different properties than the original materials and the original materials
are not recoverable by physical changes alone.
We have investigated how the Small Particle Model describes physical
properties of gases, liquids, and solids, and changes in those physical
properties. The Small Particle Model suggests that the motion and spacing of
particles are affected during physical changes, but the identities of the
particles themselves are not affected. In this activity we will begin to explore
the composition of materials and how the Small Particle Model describes
chemical changes of materials.
How does the Small Particle Model describe
chemical changes?
Initial Ideas
Recall Chemical Change #3 in the previous activity. You mixed two solutions,
potassium iodide dissolved in water and lead nitrate dissolved in water. We
know a chemical change occurred because a yellow solid formed (when a
solid is “suspended” in liquid we call this a suspension). The yellow solid
clearly has different properties than the solutions that mixed to make it.
What is the yellow solid? What occurred at the level of small particles?
Where did the yellow solid come from? Is it made up of particles that were
originally present in the solutions? Are the particles making up the yellow
solid the same particles or different particles from the ones making up the
original solutions?
Two students are trying to use the Small Particle Model to account for their
observation of the formation of the yellow solid:
© 2007 PSET
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Cycle 6
I think that the dissolved particles are moving around, collide with each other,
and new particles are made--almost as if the two original particles disappear
and a completely new particle appears. When enough of the new particles are
made, they clump together and eventually we can observe the solid forming.
They might look something like this:
Samantha
I also think that the particles collide with one another. But instead of
“morphing” to make a new particle, we think that the original particles are just
rearranged into some new combination. When enough of the new
combination of particles has formed, they clump together and eventually we
see the solid forming in the beaker.
Luisa
Discuss the following questions with your group.
In which explanation do the original particles lose their “identity”?
Samantha’s—they morph into something that is different from the original particles.
In which explanation do the original particles keep their “identity”?
Luisia’s—they remain A and B, but just clump together.
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Activity 2: Chemical Changes and the SPM
Which explanation do you think is more correct? Explain your
reasoning.
Samantha’s seems correct—new particles with new identity.
Luisa’s seems correct—reorganization accounts for conservation of particles.
Perhaps we don’t know enough about particles???
B Participate in a whole class discussion. Be prepared to share your ideas
with the rest of the class.
Collecting and Interpreting Evidence
You may have found it difficult to choose one explanation over the other. To
make a better decision, you really need to know more about the materials and
their small particles. Specifically, what are materials made of and what gives
materials (and their small particles) their “identity”?
Experiment #1: What happens when the yellow solid is heated?
You are now going to conduct two experiments on the yellow solid that
formed when potassium iodide and sodium nitrate solutions were mixed in
Activity 1 of this cycle.
You will need:
Pea sized sample of the yellow solid, in a test tube (provided by your
instructor)
Matches
Charcoal stick
Test tube and stopper
Test tube holder
Alcohol burner or candle
Tweezers
Beaker
CAUTION: Wear safety glasses or goggles.
CAUTION: Handle the yellow solid with care. Wear safety gloves to ensure that your
skin is not contaminated by it.
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Cycle 6
STEP 1. Observe the yellow solid (e.g. appearance, texture, etc.)
Record the properties of the yellow solid in the first column of the data
table below.
Table 1: Heating Yellow Solid
Observations
before heating
Yellow brittle solid
(noncrystalline, may be
powdery or chalky)
Observations after heating
With charcoal stick:
A shiny, silvery metallic coating forms on the
stick
By itself:
A purple solid forms in the bottom and a purple
gas is released
Heating the yellow solid with a charcoal stick:
STEP 2. Smear a thin layer of the yellow solid onto one end
of the charcoal stick.1 Do not use so much yellow solid that
clumps can fall off the stick or cling to the walls of the test
tube. If the yellow solid is too dry to smear onto the stick,
moisten it with a little distilled water. Drop the charcoal stick
with the yellow solid into a clean test tube--the smeared end
of the stick should be at the bottom of the test tube.
STEP 3. Make sure that your instructor has adjusted your alcohol burner to
give a small, cooler flame. Light the alcohol burner.
STEP 4. Hold the test tube with the test tube holder and
carefully heat the end of the test tube until the yellow solid
is no longer visible. Remove the test tube from the flame
and allow it to cool in a large beaker.
STEP 5. When the test tube is cool, dump the charcoal
stick onto a piece of paper and examine the end of the
charcoal stick.
1
Alternatively, your instructor may provide you with a charcoal stick that already has a thin
coating of the yellow solid suspension produced in Activity 1.
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Activity 2: Chemical Changes and the SPM
In the data table, describe the appearance of the charcoal stick.
Heating the yellow solid by itself:
STEP 6. Lightly stopper the test tube that contains the rest
of the yellow solid. Do not press the stopper firmly into
the tube.
STEP 7. Hold the test tube with the test tube holder and
carefully heat the sides and end of the test tube. You will
notice that the yellow solid turns orange and then reddishpink. Continue to heat just until you begin to see a gas forming
inside the tube. Remove the test tube from the flame and turn
off the burner.
STEP 8. Hold the test tube against a white background,
like a piece of paper.
In the data table, describe what you see inside the
test tube. You may wish to observe the tube again
after the material inside has cooled.
What evidence do you have that gently heating the yellow solid (with or
without the charcoal stick) causes a chemical change to occur?
New materials (silvery metal and purple solid/gas) with different properties form.
Simulator Exploration #2: What is lead iodide composed of?
STEP 1. Open Cycle 6 Activity 2 Setup 1.
When the simulator opens you should see the
macro region which contains the Chemical
Analyzer. The Chemical Analyzer can be
used to study the components of a material. A
balance, a thermal container, and a solubility
container are also located near the bottom the
macro region and can be used to determine
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Cycle 6
characteristic physical properties of materials. In this exploration, we will use
the Chemical Analyzer Simulator to help us determine what materials form
when the yellow solid (called lead iodide) undergoes chemical changes due to
heating. We will also use the other instruments to determine the characteristic
physical properties of lead iodide and its components.
The scroll menu and corresponding legend below the Chemical Analyzer
provide a list of possible materials that can be analyzed. The materials in the
scroll menu can also be identified by scrolling over the colored dot.
STEP 2. Scroll down to the third row and click the
mouse button once to select lead iodide from menu of
materials.
STEP 3. Click the mouse button once in an empty space of the
macro region to deposit a lump of lead iodide.
To determine the density (g/cm3) of lead iodide, we will use the balance to
measure the mass of a 1 cm3 scoop of the material.
STEP 4. Click the cm3 scoop on the bottom
toolbar to select it.
Then move the mouse (empty scoop) over the name of the lump you
deposited in Step 3 and click the mouse button once to fill the scoop. If the
scoop is full, it turns the same color as the lump.
Move the mouse (full scoop) over the balance pan.
When a black rectangle appears, click the mouse button
once to deposit the 1 cm3 scoop to the pan.
Record the density (g/cm3) of lead iodide in Table 1. Follow the
instructions given in Steps 5 and 6 for completing the table.
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Activity 2: Chemical Changes and the SPM
Table 1. Characteristic Properties of Lead Iodide
6.16 g/cm3
402ºC
954ºC
0.04 g solute/100 g water
Density
Melting point
Boiling point
Solubility
To determine the melting and boiling points (in ºC) of lead iodide, we will
use the thermal container to heat a sample of it.
STEP 5. Click on the gram scoop on the
bottom toolbar to select it.
Then move the mouse (empty scoop) over the name of the lump you
deposited in Step 3 and click the mouse button once to fill the scoop. If the
scoop is full, it turns the same color as the lump.
Move the mouse (full scoop) over the thermal container.
When a black rectangle appears, click the mouse button once
to deposit the 1 g scoop to the container.
As the lead iodide melts and boils you will see changes in the state:
solid
liquid
melting
gas
boiling
Run the simulator, pausing to record the melting temperature as soon as
you observe a change of state from solid to liquid. Run the simulator again,
pausing to record the boiling temperature as soon as you observe a change
of state from liquid to gas. Record the information in the table.
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Cycle 6
To determine the solubility (g solute/100g water) of lead iodide we will use
the solubility container to dissolve a sample of it.
STEP 6. Click on the gram scoop on the bottom toolbar to select it.
Then move the mouse (empty scoop) over the name of the lump you
deposited in Step 3 and click the mouse button once to fill the scoop. If the
scoop is full, it turns the same color as the lump.
Move the mouse (full scoop) over the solubility container.
When a black rectangle appears, click the mouse button
once to deposit the 1 g scoop to the container.
If a fractional number (less than 1.0) appears in the window, then less
than a 1.0 g scoop is soluble in 100 g of water. Record this value in Table
1 for solubility. If 1.0 g appears in the window, add more scoops until
the value in the window stops changing. Record this value in Table 1 for
solubility.
To determine the components of lead iodide, we will use the Chemical
Analyzer. The chemical analyzer simulates what happens when we heat lead
iodide.
STEP 7. Click on the refresh or reload icon on your browser to reset the
simulator to the default parameters.
STEP 8. Repeat Steps 2 and 3 to deposit a lump of lead iodide to the
workspace.
STEP 9. Click on the substance scoop on the
bottom toolbar to select it.
Then move the mouse (empty scoop) over the name of the lump you
deposited and click the mouse button once to fill the scoop. If the scoop is
full, it turns the same color as the lump.
Move the mouse (full scoop) over the Chemical
Analyzer Input Receptacle. When a black rectangle
appears, click the mouse button once to deposit the
scoop to the receptacle.
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Activity 2: Chemical Changes and the SPM
STEP 10. Run the Chemical Analyzer. Stop the simulator once something
appears in the output receptacle(s).
STEP 11. Repeat Steps 4, 5, and 6 to determine the characteristic properties of
the material found in the left output receptacle. We will call this “Unknown
A”. Instead of scooping from a lump of lead iodide you have placed in the
workspace, scoop from the lump of Unknown A found in the left output
receptacle.
Record the characteristic properties for Unknown A in Table 2.
Table 2. Characteristic Properties of Unknown A
Density
Melting point
Boiling point
Solubility
11.3 g/cm3
327 ºC
1630ºC
0.0 g solute/100 g water
What does it mean if the solubility measurement is 0.0 g? Is this material
soluble in water?
If the solubility is 0.0 then the material is not soluble in water, it will not appreciably
dissolve.
STEP 12. To discard Unknown A samples from the balance, thermal
container, and solubility container, select the delete tool (red X) and click in
the center of the material. If you should accidentally delete any of the
instruments, select the instrument icon from the bottom toolbar and click
once in the workspace to add the instrument. Alternatively, you could refresh
the browser and repeat Steps 7-10.
STEP 13. Repeat Steps 4, 5, and 6 to determine the characteristic properties of
the material found in the middle output receptacle. We will call this
“Unknown B”. Instead of scooping from a lump of lead iodide you have
placed in the workspace, scoop from the lump of Unknown B found in the
left output receptacle.
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Cycle 6
Record the characteristic properties for Unknown A in Table 2.
Table 3. Characteristic Properties of Unknown B
Density
Melting point
Boiling point
Solubility
4.93 g/cm3
113 ºC
184ºC
0.03 g solute/100 g water
The characteristic physical properties of the materials that can be analyzed
with the Chemical Analyzer are listed in the Appendix.
Use the data you collected to identify Unknowns A and B.
Unknown A = lead; Unknown B = iodine
What is the identity of the silvery solid on the charcoal stick? What is the
identity of the purple gas2 produced?
The silvery solid is lead and the purple solid/gas is iodine.
In Activity 1 you learned that chemical changes produce new materials
with properties that are different from the original material. If the
Chemical Analyzer simulator simulates heating lead iodide (Experiment
#1), can you now say with certainty that a chemical change occurs when
lead iodide is heated? Explain your reasoning.
Yes, we have determined that lead and iodine have very different properties from lead
iodide.
2
You may have predicted from melting and boiling points that Unknown B should be a solid
at room temperature, not a gas. In fact, if you observed the test tube after allowing the purple
gas to cool, you probably observed tiny purple crystals collecting on the test tube walls. What
is happening here? Some solids can appreciably vaporize at temperatures below their boiling
points. Unknown B is one such example (carbon dioxide, or dry ice, is another). The process
of a solid evaporating to gas is called sublimation. The process of a solid reforming from gas
is called deposition. Sublimation and deposition are changes of state, and therefore, are
physical changes. In this case, as the purple solid formed in the test tube, and its particles
gained sufficient energy from continued heating to vaporize. Once you stopped heating the
test tube, vapor particles lost sufficient energy to re-establish attractive forces, forming small
crystals on the cooler surfaces of the test tube.
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Activity 2: Chemical Changes and the SPM
Elements, Compounds, and Mixtures
This simulator demonstrates some key ideas about the components of
materials. Some materials are more complex--that is, they are made from
other, simpler materials. We call these complex materials compounds.
Compounds can be “broken down” into simpler materials through chemical
changes, as simulated by the Chemical Analyzer. The compound has
characteristic physical properties that are different and distinct from those of
the simpler materials. The simpler materials that form when compounds are
broken down are called elements. Elements cannot be broken down into
simpler materials through chemical changes.
How would you classify lead iodide—compound or element? Why?
Lead iodide is a compound; it can be broken down into simpler components—in this
case, lead and iodine-- that have different properties from the compound.
How would you classify lead and iodine—compounds or elements?
Why?
Both are elements—they are the simplest materials, do not break down into simpler
materials.
Scientists have found 92 different elements in nature, here on Earth. They
have made more than 25 elements in the laboratory. Most of the natural 92
elements combine with other elements or compounds during chemical
changes to make new compounds. In other words, all materials found on
Earth are made up of some combination of the 92 elements. The elements are
the Earth’s building blocks, just as the letters of an alphabet are the building
blocks of a written language!
Simulator Exploration #3: What are “small particles” of lead iodide
composed of?
STEP 1. Open Cycle 6 Activity 2 Setup 2.
When the simulator opens you should see the
micro region of the Chemical Analyzer.
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Cycle 6
STEP 2. Select lead iodide from the scroll down menu of materials.
STEP 3. Click the mouse button once in the input
receptacle of the micro region to place a particle of
lead iodide in the Chemical Analyzer.
STEP 4. Run the Chemical Analyzer. Stop the
simulator once something appears in the output receptacle(s).
STEP 5. Activate the Select tool. To investigate the particles produced,
double click on the particle in the output receptacle of the micro region of the
Chemical Analyzer. An information window will appear that provides the
identity and characteristic physical properties. NOTE: The characteristic
physical properties listed are those of the bulk material, not of the
individual particle. Characteristic physical properties arise from a
collection of particles that are held together by attractive forces. An
individual particle does not have a density, melting point, boiling point, or
solubility.
What happens when you run the simulator? What are the identities of
the components in the output receptacle?
One lead particle and two iodine particles.
What is a lead iodide particle composed of? Be as specific as possible.
One lead particle and two iodine particles.
Atoms, Molecules, and Formula Units
The 10 million x magnification monitor on the right side of the chemical
analyzer simulator will help us to clarify certain aspects of the Small Particle
Model. Initially, we described all materials as consisting of tiny particles. But
now, the 10 million x magnification monitor indicates that what we have been
referring to as “small particles” may consist of a collection of even smaller
particles bound to each other in some configuration.
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Activity 2: Chemical Changes and the SPM
In the 10 million x magnification monitor, each sphere represents an atom.
Most elements, such as lead, are monatomic, meaning that the atom is the
smallest particle of the element that has the same chemical properties as the
bulk material. For the elements hydrogen, nitrogen, oxygen, fluorine,
chlorine, bromine, and iodine the diatomic molecule (two atoms of the same
element bound to one another) is the smallest particle of the element that has
the same chemical properties and behaviors as the bulk material. Molecules,
such as water, consist of two or more atoms that are bound to one another in
a distinct unit. The smallest part of some compounds is a molecule (e.g. a
molecule of table sugar contains 12 carbon atoms, 22 hydrogen atoms, and 11
oxygen atoms connected in a certain configuration.) Some materials, such as
lead iodide, are not made of molecules, rather of large numbers of repeating
formula units with a certain ratio (e.g. two iodine for every one lead); these
repeating formula units are held together in a rigid three dimensional
network called a crystal lattice. Since the crystal lattice structure is difficult
to represent with two-dimensional models, we will often represent formula
units as though they are molecules. In later activities, we will learn how to
predict whether a compound’s smallest units are molecules or formula units.
These images below summarize pictorially the three types of “Small
Particles” described above, using lead, iodine, and lead iodide as examples:
Monatomic
Diatomic Molecule
Formula Unit
When heated, lead iodide undergoes a chemical change, producing lead
and iodine. According to these 10 million x magnification views, what
happens during a chemical change—the identities of the atoms change,
the spatial configurations of the atoms change, both the identities and
spatial configurations of the atoms change?
The spatial configurations of the atoms change.
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Cycle 6
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. Revisit Luisa’s ideas about chemical change in the Initial Ideas scenario.
How could you change Luisa’s statements to reflect our new
understandings of “small particles” more accurately? What do ‘A’ and
‘B’ represent in her model?
Change “particles” to atoms. A and B must be atoms—they are reorganized but their
identity doesn’t change.
S2. The first column of the data table below lists some compounds that you
used or generated in Cycle 6 Activity 1. Use the micro Chemical
Analyzer to define a particle for each material (i.e. what elements and
how many atoms of each element).
Material
Iron Oxide
(rust)
Chemical Analyzer
2 iron atoms and 3 oxygen atoms
Water
2 hydrogen atoms and 1 oxygen atom
Carbon dioxide
(fizz from AlkaSeltzerTM)
Potassium iodide
(in colorless solution)
Lead nitrate
(in colorless solution)
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1 carbon atom and 2 oxygen atoms
1 potassium atom and 1 iodine atom
1 lead atom, 2 nitrogen atoms, and 6 oxygen atoms
Activity 2: Chemical Changes and the SPM
S3. Imagine that another student in the class tells you: “I’m confused! Water
vapor is made up of hydrogen and oxygen atoms and is a compound.
But air contains oxygen and hydrogen molecules and is a solution
mixture. Why aren’t they either both compounds, or both solutions
mixtures?”
Read the paragraph “Atoms, Molecules, and Formula Units”. Also
examine the data you collected for water in S2. Then draw x 10 million
magnification Small Particle Models like those on the previous page for
water and air that demonstrate the difference between them. Write a
brief description of each model.
Hydrogen and oxygen atoms are “connected” in water. In air, hydrogen and oxygen
gases (diatomic molecules) exist separate from one another.
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