Experiment 7
Properties of an Element - Oxygen
Introduction
One of the goals of chemistry as a science is to explain the properties and reactions of
materials through theories about their structure and interactions. Of course, to explain
properties, one must know what they are. Thus the historical development of chemistry
has relied on the collecting, over centuries, of a large amount of data regarding how
materials appear and react. In this experiment, you will experience a part of that process
by collecting a sample of a pure substance – the element oxygen – and observing some of
its physical and chemical properties.
The physical properties of a substance are those properties that apply to the substance on
its own. These include physical state, color, density, melting point, boiling point, and
many others. Chemical properties are the characteristics of how a substance reacts (or
fails to react) when combined with other materials. Since the elements are the basic units
of chemical substances, some of the most fundamental data of chemistry is how each
element reacts with the others.
Oxygen is the most abundant element in the Earth’s crust and is critical to living systems.
Thus we have selected oxygen as the subject for this study. We will prepare samples of
oxygen from a reaction sometimes called an oxygen generator. Moderate heating of the
compound potassium chlorate causes it to decompose into potassium chloride and
oxygen:
2 KClO3 (g) ------ > 2 KCl (s) + 3 O2 (g)
Follow the procedures for the production of an oxygen supply and answer the questions
asked in your lab notebook. The procedure is relatively long so come to lab prepared and
work efficiently on your lab day.
Materials and Safety
Exercise caution during this lab. Most students find it one of the more entertaining labs
of the course, but it can also present hazards. Most of the reactions observed will take the
form of fires. Be aware of your surroundings and keep hair and clothing out of the reach
of flames. Pay particular attention to the setup of the oxygen generator in part A. The
reaction can turn explosive if too much heat or pressure is used.
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Part A – Preparation of Oxygen
Work in groups of 4 or 5. Use the wide mouth bottles from your drawers, and get more
from lab supplies. Assemble the apparatus as shown below (you may want to sketch this
apparatus in your lab notebook).
Caution – Lubricate the glass tubing with water only, not glycerine. Glycerine may
explode on contact with potassium chlorate (KClO3).
Position the apparatus so that a piece of rubber tubing attached to the pneumatic trough
reaches to the sink. Fill the trough with tap water until it begins to overflow into the sink.
Into your large test tube, place about 2-3 inches of potassium chlorate and a pea-sized
pinch of MnO2. Blend well by shaking. Clamp the top of the test tube to a ring stand.
Fill a wide mouth bottle with water, cover it with a glass square, and invert it in the
pneumatic trough, being careful that no water enters the bottle. Repeat with the other
bottles. Once the bottles are in the water, the glass squares can be removed and the water
will not run out.
Position one of the bottles on the shelf in the trough so that its mouth is over the end of
the delivery tube.
Have instructor check the setup before heating.
Heat the mixture in the tube directly with a Bunsen burner. You will find that the rate of
decomposition of KClO3 to give oxygen gas can be easily controlled by the intensity of
heat. Take special care that the molten salt mixture does not froth up so as to plug the
exit tube. When the gas receiver bottle is full, immediately replace it with a second bottle
and discard the first sample as it is mostly air. Record any observations regarding the
appearance of oxygen.
When the second bottle is full, slide a glass square under the mouth as a lid, invert the
bottle, remove it from the water, and immediately replace it with another bottle.
Continue collecting until 8 bottles are filled with oxygen gas.
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Part B – Test for Oxygen
Use the first bottle of oxygen. Light your Bunsen burner and, holding the mouth of the
bottle near the flame, remove the cover. If nothing happens, immediately replace the
cover. Write any observations in your lab notebook. Then, light a wooden splint and
blow out the flame so that a glowing ember remains on the tip. Uncover the bottle and
thrust the glowing splint into the oxygen. Write any observations in your lab notebook.
Part C – Chemical Properties of Oxygen
1. Reactions with Metals
Iron
Use the second bottle of oxygen. The primary ingredient in steel wool is the
element iron. Hold a small wad of steel wool with your crucible tongs and heat it
in the flame until it glows. This only takes a second. Uncover a bottle of oxygen,
insert the steel wool, and recover the bottle as much as possible. DO NOT
DROP THE STEEL WOOL INTO THE BOTTLE – THE GLASS WILL
BREAK. When the reaction is finished, it is safe to drop whatever remains into
the bottle. Record any observations of the reaction in your lab notebook, as well
as a description of the product.
Add 1 mL (if necessary) of water to the bottle, cover it with a glass plate, and
shake it several times. Drop a piece of each kind of litmus paper into the water in
the bottle. After a moment, record the color of the paper strips.
Magnesium
Use the third bottle of oxygen. Repeat the preceding procedure using a 4-cm
piece of magnesium ribbon. CAUTION – DO NOT LOOK DIRECTLY AT
THE BURNING MAGNESIUM – THE INTENSE LIGHT CAN CAUSE
EYE DAMAGE. Be sure to do the litmus test after the reaction also.
Be sure to record ALL observations.
2. Reactions with nonmetals
Sulfur
Use the fourth bottle of oxygen. This experiment should be performed in the
hood since the burning of sulfur produces irritating fumes. Clean a combustion
spoon by holding it in a hot flame until it glows red. ALLOW IT TO COOL
BEFORE PROCEEDING. Place a lump of sulfur the size of a pea in the bowl
of the spoon and hold it in the flame until it ignites. Then lower it carefully into
the bottle of oxygen. Record any observations of the reaction in your lab
notebook, as well as a description of the product. Test with litmus as before,
recording all observations.
Phosphorus
Use the fifth bottle of oxygen. This experiment should also be performed in the
hood since the burning of phosphorus produced poisonous vapors. Clean the
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spoon as before by heating. Allow to cool. Using a spatula, place a small amount
of phosphorus in the bowl and heat gently to ignite. Lower carefully into the
bottle of oxygen, recording any observations. Test with litmus.
Carbon
Use the sixth bottle of oxygen. Repeat the procedure using a small lump of
charcoal.
Be sure to record ALL observations.
Part D – Physical Properties of Oxygen
Use the seventh bottle of oxygen. Remove the cover, sniff the gas to note its smell
(record observations), and quickly replace the cover. Invert a bottle of air over the
oxygen bottle. Slide the cover out of the way so that the mouths of the two bottles are
joined. After 1 minute replace the cover. Remove the upper bottle and cover it also.
Perform the test for oxygen as in part B on both bottles. Record whether there was a
positive test for oxygen in the upper bottle and in the lower bottle.
Use the eighth bottle for errors and/or repeats
Part E – Reactions of Litmus Paper with Acids and Bases
Obtain a few mL of dilute hydrochloric acid (HCl) in a small beaker. Drop a strip of
each type of litmus paper into the acid. Note and record the colors.
Repeat the procedure using dilute sodium hydroxide (NaOH, a base).
Be sure to record ALL observations.
Using this information, go back to part C and determine whether the reactions produced
acidic or basic solutions. Record this information next to where you recorded the results
from the litmus test.
Prelab
In addition to writing up the procedure in your laboratory notebook, answer the following
questions.
1. How is oxygen generated in this exercise?
2. What precaution should be taken when inserting glass tubing into a rubber
stopper?
3. How do you clean a combustion spoon?
4. What safety precaution must be taken when carrying out a reaction between
magnesium and oxygen?
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Postlab
Answer the following questions in your lab notebook:
1. List all the physical properties of oxygen that you observed during this exercise.
You should have at least four properties.
2. What did the result of part D tell you about the density of oxygen (compared to
the density of air)? How did you determine this?
3. List all the chemical properties of oxygen that you observed during this exercise.
4. What did the results of part B tell you about oxygen?
5. Formulate a general statement from the results of the litmus tests in part C.
(Hint: What types of oxides (metal/nonmetal) form acidic solutions? Basic
solution?)
Be sure to include the discussion section in your lab notebook.
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