Chemical and Physical Changes Lab
In this activity (third in a series),
students develop observation- and peer argumentation skills.
They mix chemicals without any preconceptions of what may
occur. This approximates the discovery process that Mendeleev
and his precursors experienced discovering properties of
the sixty-some elements that provided enough data from
which to generalize a periodic law. From standard textbook
definitions given by the instructor, they must create their
own criteria to distinguish a physical change from a chemical
change, but they must draw on observable evidence.
The procedures chosen include some obvious physical or chemical
changes, and some which can be interpreted with less certainty,
creating another opportunity for students to defend their
conclusions in a peer review. It is important that student
lab reports are evaluated not on whether or not
a physical or chemical change is correctly identified, but
by how successfully they defend their conclusions based on
the observations they made in lab. A large group criteria
is then created, hopefully, leading to some consensus. Finally,
a lab performance assessment is used via laserdisc, in which
students observe a new experiment, make observations, write
and defend their conclusions.
Purpose: To identify chemical and physical changes
in matter.
Background: Most science research is done by
people working as part of a team. In most fields, individual
success is dependent on the group. Your skills as a team
member are important to any future job or relationship
with other people. Research also shows that students of
all ability levels learn better in cooperative groups.
This lab will be a group effort, and each member will
be responsible for the learning of the other members. Each
member of the group will write a section of the lab, and
these sections will be put together to determine the group's
grade for the lab report.
Each member of the group will be "in charge" of
a section of the lab. This means that she/he will:
- prepare a data/observation table in advance to be filled
out during the lab,
- execute the lab techniques properly, teaching the other
members of the group how to do them,
- and write the conclusion for that section.
Theory: A physical change is a chenge in the appearance
of a substance, but not its identity. A chemical change produces
one or more new substances.
Data/Observations: The quantities of chemicals
used in this lab are not absolute--this is a qualitative
procedure. Do not waste time measuring the exact amount--do
not double or triple the amounts, either--a close estimate
is good enough.
Have a data chart or observation section prepared before
you come into lab, so that it can be filled out as you
do the lab, possibly by another member of your group. It
will be especially important to notice and describe the
appearance of the reagents (another word for chemicals)
BEFORE, DURING, and AFTER the experiment. Be sure to write
both qualitative and quantitatvie observations.
Conclusions: As a group, you must create a crtiteris
which you can use to distinguish between a physical and
a chemical change. Use observations from the lab procedures
to create this criteria. Then, for each procedure, write
a conclusion:
- Was there a chemical or physical change, did both occur,
or did neither occur? Discuss what evidence you observed
to support this.
- Was there an endothermic or exothermic change or both?
Describe your reason for deciding which one of the two
occurred. If you cannot decide, then discuss why you
cannot decide.
Part 1: Silver nitrate (AgNO3) and Sodium chloride (NaCl)
Caution: Solid silver nitrate is corrosive
and is used to cauterize wounds. Even dilute silver nitrate
may be corrosive. Silver nitrate produces photochemical stains
which darken to a brown, purple, or black color. These stains
are permanent, but will wear off as skin is shed naturally
through showers, hand washing, etc. You may use gloves when
pouring this solution. Spray N' Wash has been somewhat successful
in removing stains from clothing if treated quickly. Technique:
Pouring Corrosive Liquids, Filtering Procedure:
- Obtain 2 small test tubes and transfer them to the
reagent table by carrying them in a test tube rack. The
test tube you use for silver nitrate must be clean and
dry. Measure silver nitrate solution to a depth of 2
cm in one tube using the correct technique for a corrosive
liquid. Add sodium chloride solution to a depth of 4-5
cm into the other tube. Return to your lab table.
- After observing the properties of the two liquids,
add the sodium chloride to the silver nitrate.
- Next, transfer the contents of the test tube to a filter
setup, using tapwater to rinse out the test tube. After
filtering, remove the filter paper with tongs or tweezers,
and save it on a paper towel labeled with your names.
There will be a tray in which to save the papers overnight,
during which they will be exposed to sunlight or fluorescent
light. Dispose of the filtrate as indicated below.
- The next day, observe any change in the product left
on the filter paper.
Disposal: Pour the filtrate into the labeled waste
container. Silver solutions should not be disposed of in
the sewer untreated. Then clean all glassware and the funnel
with soapy water. The next day, the filter paper which contains
silver chloride should be placed in the labeled waste bag
for storage in a secure chemical landfill.
Part 2: Sodium Polyacrylate and Water
Technique: Measuring a Solid
Procedure:
- Measure one small scoop of sodium polyacrylate into
a beaker (100, 150, or 250 mL). Return to the lab table.
- Add 10 mL of water. Record observations. Invert the
beaker over the sink, record observations. Repeat process
until beaker is full, or you are bored with the results.
You may touch the product with your hands.
Disposal: Dispose of the mixture in a plastic-lined
wastebasket. Clean beaker with soap and tapwater.
Part 3: Stearic Acid ( CH3(CH2)16COOH )
Technique: Measuring a Solid, Heating a Test Tube--waterbath
Procedure:
- Scoop enough stearic acid into a micro test tube to
fill it to a depth of 1-2 cm.
- Half-fill a 100 or 150 mL beaker with tapwater and
place it on a hotplate as a water bath. Place the test
tube in the water bath--making sure it does not tip over.
Note any changes that occur during heating--heat until
water boils or there are no more changes in the appearance
of the stearic acid.
- Also prepare a 150mL cold water bath. Use a crucible
tongs to remove the tube from the hot water and place
it into the cold water bath. Record any changes that
occur during cooling.
Disposal: When the tube is at room temperature or
colder, discard it in the glass disposal box. The chemical
does not dissolve in water which makes the glassware clenaup
difficult . This chemical is not hazardous and can be disposed
of in ordinary garbage, but the glass tube is separated from
burnable waste, such as paper towels.
Part 4: Heating Metals
Technique: Heating a Solid, Lighting a Burner
Procedure:
- Platinum: Examine the platinum wire (attached to glass
rod). Observe the color and luster of the metal. Hold
the wire in the high temperature flame of your burner
for 10 seconds. Allow the wire to cool and re-examine
it. Disposal: Platinum is expensive and reusable, return
to labeled beaker.
- Copper: Examine the copper strip. Heat the strip in
the cool outer portion of the burner flame for 15 seconds
or until a coating forms. Be careful not to melt the
strip. Place the strip on a glass plate or watch glass
and use tongs to hold it and another pair of tongs or
tweezers to scrape off some of the coating produced by
heating. Record your observations. Disposal: Place copper
scrap in the Cu waste container. Copper may be disposed
of in a landfill; large amounts of the coating are toxic.
- Magnesium: Examine the magnesium ribbon. Obtain a Mg
waste can and light a burner.
Caution: DO NOT LOOK DIRECTLY
AT THE MAGNESIUM WHILE IT IS BURNING. EYE DAMAGE
TO THE RETINA RESULTS FROM UV LIGHT.
UV goggles may be worn to reduce exposure to ultraviolet
light--see instructor.
With tongs, hold the ribbon in the hottest part of
the flame (tip of inner blue cone) to ignite it. Then
hold it inside the waste can to complete the burning.
Disposal: Magnesium ribbon and ash are left in the
waste can. The ash can be safely disposed of in a regular
landfill. The ribbon is too reactive for an ordinary
landfill and the ribbon scraps will either be heated
to ash in a crucible or reacted with acid.
Part 5: Liver and Hydrogen Peroxide (H2O2)
Caution: Concentrated hydrogen peroxide is very
hazardous. It will bleach skin and clothes and is very corrosive
to eyes.
Technique: Pouring Corrosive Liquids, Lighting
a Burner, Splint Test for Gases
Procedure:
- Obtain a small piece of liver with crucible tongs,
and place it into a 100 or 150 mL beaker.
- Pour 10-15 mL of 6% hydrogen peroxide (Clairoxide)
into a graduated cylinder. Add to the beaker.
- Test any gas produced with a glowing splint. To prepare
the splint, hold it in the tongs in a burner flame, then
blow it out, leaving a glowing ember at the tip of the
splint.
Disposal: Remove the piece of liver with the tongs
and return it to the waste beaker--DO NOT put it in the wastebasket
or sink. Then add tapwater to the hydrogen peroxide carefully,
so as not to splash. Empty the beaker into the sink. Wash
and brush beaker with soapy water. Rinse with tapwater.
Part 6: Citric Acid (H3C6H5O7 ) and Baking Soda (NaHCO3)
Technique: Measuring a Solid, Lighting a Burner,
Splint Test for Gases
Procedure:
- Measure one scoop of citric acid and one scoop of baking
soda into a Ziploc bag, and observe any reactions. Return
to your lab table.
- Pour about 25 mL of tapwater into the bag and quickly
seal it. Hold the bag to observe any temperature change.
- Test any gas produced with a burning splint. Open the
bag and quickly test the gas before it all escapes and
without igniting the plastic bag.
Disposal: Empty contents of bag into the sink, flush
with water. Discard plastic bag in wastebasket.
Lab Report Check
Give your observation/conclusion sheet to one of your lab
partners for them to evaluate and sign--then staple this
sheet to it and turn it in before the end of the class period.
Do not sign the lab sheet until all are completed:
- Name of person responsible for lab conclusion and number
of experiment (1-6)
- Observations labeled before/during/after and quantitative/qualitative
- At least one quantitative observation
- Observations include all these items: color, odor,
temperature, clear or cloudy (for liquids only)
- Observations tell if heat, light, or electricity were
added during the experiment
- Conclusion uses the term physical or chemical change
(may be both). Observations are used to back up an opinion.
- Conclusion uses the term endothermic or exothermic
(may be both). Observations are used to back up an opinion.
- Check observations to see if more than one change occurred.
There is a conclusion written for each change.
Signature of evaluator___________________________________________________
CONCLUSIONS
These may not be the only right answers--REMEMBER, we grade
on how well your conclusions fit your observations and the
evidence you use to back up your conclusion.
- SILVER NITRATE AND SODIUM CHLORIDE
- chemical change--precipitate forms
no evidence of energy change
chemical change--color change from white to grey-violet
endothermic--light-induced
- SODIUM POLYACRYLATE AND WATER
- physical change--gel forms when water added
no evidence of energy change
- STEARIC ACID
- physical change--solid melts
endothermic--requires input of heat to melt
physical change--liquid freezes
(exothermic)-- but no evidence of energy change
- HEATING METALS:
- PLATINUM
- no change or physical change--color change only
in flame, returns to original state
either endothermic--only glows in burner flame
or exothermic--gives off light when in flame
- COPPER
- chemical change--tarnish forms, new compound
endothermic--requires addition of heat
- MAGNESIUM
- chemical change--new compound (ash) forms
exothermic--emits its own heat and light after removed
from burner
- LIVER AND HYDROGEN PEROXIDE
- chemical change--oxygen gas produced
may be exothermic--temperature increase often observed
- CITRIC ACID AND BAKING SODA
- chemical change--carbon dioxide gas produced
endothermic--temperature decreases
Link to overview of "Mendeleev
Puzzle" curriculum module.
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