PROPERTIES OF MATTER
Raymond Zmaczynski Senn Metropolitan Academy
5900 N. Glenwood Avenue
Chicago, IL 60660
1-312-989-3565
The following properties of matter will be shown: density (two approaches);
diffusion in a liquid; the effect of pressure on the boiling point of water;
polarity of liquids; and the comparison of bond types and the physical properties
of solids. For each of these properties, a separate summary will be made.
Objective:
To show the concept of density.
Materials:
Samples of metals of equal sizes (Al, Fe, Cu, Pb, Sn); unknown density
samples (irregularly shaped pieces of sulfur and/or marble chips or assorted
collection of fishing sinkers, available at Sears in a variety of metals and
shapes); graduated cylinders (size depends on size and shape of unknowns);
chemical balances; experiment sheets; graph paper.
Strategy:
The concept of density appears very early in most high school chemistry
courses. In many cases, the student already understands the concept or has been
exposed to the concept. Density is a useful and necessary property of matter and
the student should have a good understanding of the concept. It is assumed that
the student has been taught to use a chemical balance and a graduated cylinder as
tools of measurement. The idea of determining the volume of an irregularly shaped
solid by the water displacement method may be taught previously or may be
introduced as part of the experiment. The student usually has the idea of
identifying properties based on his or her general life experience.
Pass out for general classroom inspection or for group inspection, samples
of some or all of the following metals: Al, Zn, Sn, Pb, Cu, and Fe (galvanized
steel is all right to use). Inquire of the students whether the samples are
alike or different. Ask whether any of the samples might possibly be alike,
especially those that look or feel alike. Suggest that certain pairs might be
alike (Pb and Sn or Zn, Al, and Fe) and ask why the students don't think they are
alike. After some discussion, someone will usually say that they are not alike
because one of the pieces is "heavier" than the other one. From this, the
discussion should lead to ways to determine which is "heavier" when the pieces are
not the same size, have an irregular shape, or appear to have the same properties.
The idea of density can be developed by having the student do a "density"
experiment. Two approaches are suggested and a possible experimental design for
each is shown on the following pages. The first, adapted from the Chemical Guide
and Laboratory Activities by McGill, Bradbury, and Sigler published by Lyons and
Carnahan in 1966, introduces the concept of density and continues from there. The
second, adapted from the Interdisciplinary Approach to Chemistry (IAC), develops
the concept of density by doing mass and volume measurements of different size
samples of the same materials and then developing density by graphing the data
collected from the entire class.
EXPERIMENT -- Determining the Density of a Solid
INTRODUCTION: One of the important physical properties of a substance is
its density, or what is its mass per unit volume. The property of density
may be used to help identify a substance. Gold, for example, is much more
dense (heavier) a substance than copper even though their colors may be
similar.
Density is the mass in grams (g) for each milliliter (ml) or cubic
centimeter (cm3) of a substance. This relationship may be expressed as
follows: Density = Mass / Volume or D = M/V .
PROCEDURE: Obtain an unknown from your instructor and identify it by name
or number in the data table below. Determine the mass of the unknown and
enter it in the data table. determine the volume of the unknown and then
calculate the density of the unknown substance. From your instructor
obtain the accepted density of the unknown and determine the error and
percentage of error in your experiment. An error of only a few percent is
considered good work. Dry off your unknown and return it to the container
your instructor supplies. If you have time, you may wish to try another
unknown.
DATA: Name or number of unknown ____________________________________
(A) Mass of unknown ------------------------------ ________________ g
(B) Volume of water in graduate ------------------ ________________ ml
(C) Volume of unknown and water in graduate ------ ________________ ml
(D) Volume of unknown ---------------------------- ________________ ml
(E) Density of unknown --------------------------- ________________ g/ml
(F) Accepted density of unknown ------------------ ________________ g/ml
(G) Error = difference between (E) and (F) ------- ________________ g/ml
(H) Percentage of error = (G)/(F) x 100 % -------- ________________ %
QUESTIONS.
1. What is the density of a solid whose mass is 19.57 g and whose volume
is 3.6 ml ?
2. An irregular solid has a mass of 12.77 grams. When the solid is
lowered into a graduated cylinder filled with 10.0 ml of water, the water
level rises to 13.1 ml. What is the density of the solid?
3. The method used in this experiment would not be a satisfactory way to
measure the volume of salt or sugar crystals. Why?
EXPERIMENT -- Mass and Volume
Several different kinds of solid objects will be available in the
laboratory. Ask your instructor to identify each type and to tell you how
many grams (g) of each to use. Plan to keep a record of all the
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measurements you make. Consult your instructor about how you should
arrange your data in a table.
Determine the mass of the specified number of grams of one of the solids.
Then use the water-displacement method to determine the volume (in ml) of
this quantity of material. Be sure that you record on your data sheet the
mass and the two measurements that you used to determine the volume of each
material.
Repeat these measurements with each of the other two solids that have been
made available to you. Record these measurements in your data table.
Prepare a graph of the data that you and the other students have collected.
Plot the mass (in g) of each sample along the vertical axis. Plot the
volume (in ml) of each sample along the horizontal axis. Consult your
teacher for suggestions in preparing your graph and for identifying the
different kinds of solid materials plotted.
Draw the best straight line you can through the points for each material.
Study your graph. What general pattern do you see?
Could the graph be used to identify an unknown sample of one of the solid
materials? Explain.
What is your interpretation of the results?
Objective:
To show diffusion in liquids.
Materials:
Two large beakers or cylinders (usually 1000 ml, although somewhat smaller
size probably will work too); potassium permanganate crystals or liquid food
coloring.
Strategy:
While diffusion occurs in all states of matter, it is easier to observe in
the liquid state. Once observed, the concept can be related to all states of
matter and to the idea of the energy and velocity of molecules. Fill the one of
the containers with cold water and the other container with hot water. Allow the
liquid in the containers to "settle." Add a small crystal of potassium
permanganate (or a drop of food coloring) to each container and have students make
observations. If desired, the containers may sit, undisturbed, until the end of
the class period or until the next day.
The students should observe that the crystal of the permanganate dissolves
faster and the color spreads out faster in the hot water than in the cold water.
Discussion should center on why this is so. Discussion should relate the rate of
diffusion to the energy and velocity of the water molecules. The discussion may be
extended to the kinetic theory and to all the states of matter.
Objective:
To show the effect of pressure on the boiling point of water.
Materials:
Florence flask (250 or 500 ml); rubber stopper (a stopper-thermometer
assembly may also be used); clamp to hold the Florence flask; hot plate; ice or
cold water.
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Strategy:
[CAUTION: Make certain that the flask contains no cracks so that it will not
explode. An implosion hazard also exists due to the reduced pressure in the
flask. Goggles probably should be worn and this demonstration should be performed
at a safe distance from the students.]
To many students, water boils at 100 C regardless of the atmospheric
pressure. This demonstration of the effect of atmospheric pressure on the boiling
point of water may serve as an introduction to or as reinforcement of the standard
definition of boiling point that "the boiling point is the temperature at which
the vapor pressure is equal to the atmospheric pressure."
Fill the flask about one-third with water. Heat the water to boiling in the
unstoppered flask. As the water is heated, ask the class about the temperature
change and its effect on the energy in the water and on the vapor pressure of the
water. As the water reaches its boiling point, relate the vapor pressure of the
water to the atmospheric pressure as a definition of the boiling point. Remove
the flask from the heat and wait until the boiling stops. Stopper the flask
tightly. Using the clamp, invert the flask. Rub the portion of the flask now
containing the air (the bottom) with ice or put the inverted flask under running
cold water. The water will begin to boil again. Ask the students to explain
their observations. After the demonstration turn the flask right side up and
remove the stopper to equalize the pressure. If the stopper that is inserted into
the flask is fitted with a thermometer, the temperature at which the water
boils can be determined. The discussion should be related to the relationship
between temperature and pressure of a gas (air, in this case) and the relationship
among boiling behavior, air pressure, vapor pressure of a liquid, and temperature.
The class should be asked to define boiling and boiling point.
Objective:
To show the polarity of liquids.
Materials:
Burettes (the number depends on how many liquids you wish to show); 100 ml
beakers, as needed; liquids to be tested (suggested liquids: water, carbon
tetrachloride, ethanol, acetone, benzene); source of static electricity
(suggested sources or methods: plastic rod or comb and a piece of flannel cloth;
plastic rod and animal fur; have students comb their hair vigorously with a
plastic comb). If only the polarity of water is to be shown, produce a fine
stream of water from an available water faucet and have a source of static
electricity.
Strategy:
Water is a common substance but one with special properties. Much is usually
said about the polarity or non-polarity of molecules in chemistry. The polarity
of the water molecule and the importance of this polarity in the ability of water
to act as an "almost universal solvent" is usually stressed in the high school
chemistry course. What about other common liquids or solvents? The following
demonstration should help the student to "see" the polarity of some common
liquids. Following the demonstration, the shapes and structures of the molecules
of the liquids can be related to their polarity. From this starting point,
depending on when in the course the demonstration is used, the teacher may develop
further ideas on the structure of molecules and their shapes or deal with the
properties of liquids and solutions.
Fill each burette with a different liquid. Open the stopcock of the first
burette and let the liquid flow out into a receiving beaker with a fairly fine
stream. Charge the plastic rod or comb and bring it near the stream of liquid as
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it flows from the burette. Move the charged rod up and down near the stream. If
the liquid is polar (water, acetone, or ethanol), the stream will be deflected
toward the charged rod. If the liquid is non-polar (benzene or carbon
tetrachloride), the stream flow will be unaffected by the charged rod. When the
burette is emptied, it may be refilled with the liquid from the collecting beaker.
Interested students should be invited to test the liquids to see if individual
differences in combs, rubbing techniques, and movements of the rod make a
difference in the polarity of the liquids. After all the students have satisfied
themselves as to which liquids are polar and non-polar, class discussion should
focus on which liquids are affected by the charged rod and why this is so.
NOTES:
Static electrical charges are more easily produced in dry weather (winter
time), although this demonstration will work on a hot, humid day in the summer
with the right materials and vigorous rubbing of the rod or comb. Benzene and
carbon tetrachloride are substances that are known carcinogens, but they are also
the most common non-polar substances available. The amounts used and the length
of this demonstration, about 10 to 15 minutes, should not preclude their use.
Nevertheless, other non-polar liquids that are apparently non-carcinogens , such
as hexane or cyclohexane, may be used and should probably work just as well as
benzene and carbon tetrachloride. If one wishes to show only the polarity of
water and does not wish to use a burette (or does not have a burette), an
alternate procedure is available. Adjust the water stream from a classroom faucet
so that a fine stream of water is flowing. This flow is approximately the same as
that from a burette. The same effect can now be demonstrated using the water flow
from the faucet when the charged rod is brought near the water flow.
Objectives:
To examine some of the physical properties of typical ionic and covalent
molecular solids and to relate these properties to the type of bond found in each
solid.
Materials:
sodium chloride, paradichlorobenzene, test tubes, bunsen burner, water,
benzene ( or some other non-polar solvent). The amounts of materials and
equipment used will depend on whether the experiment is performed individually or
in groups by the class or whether the instructor demonstrates the experiment for
the class.
Strategy:
In a chemistry (or another science) course, it is helpful for the students to
"see" the differences in properties between an ionic and molecular covalent solid.
Sodium chloride and paradichlorobenzene are chosen as the typical solids because
they are usually readily available and inexpensive for high school use.
Additional typical ionic and covalent solids may be added to or substituted for
these two substances depending on the purposes of the instructor. This experiment
may be used to introduce chemical bonding or the types of chemical bonds. Since
ionic and covalent molecular bonds are generally synonymous with inorganic and
organic chemistry respectively, this experiment could serve as an introduction to
the topic of organic chemistry. In a biology, physical or general science course,
the experiment could be demonstrated to summarize the general differences between
ionic and covalent bonding compounds. The experimental format that follows is
based on an experiment in Laboratory Keys to Chemistry by Ledbetter and Young,
published by Addison-Wesley in 1973.
NOTES:
Benzene is the best non-polar solvent for this experiment although carbon
tetrachloride may also be used satisfactorily. Unfortunately, both have been
identified as known or probable carcinogens to humans. Other non-polar solvents
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that are not probable carcinogens could probably be used instead of benzene or
carbon tetrachloride but such an inexpensive, readily available substitute does
not immediately come to mind. This is a dilemma for the instructor, but not a
readily soluble one.
EXPERIMENT -- BOND TYPES AND PHYSICAL PROPERTIES
The bonds in ionic solids are formed by the transfer of electrons between
atoms. As a result, such solids consist of positive and negative
ions arranged symmetrically. The bonds in molecular solids are formed by
sharing electrons; ions do not exist in such solids. The forces between
molecules in molecular solids are weak van der Waals forces. In this
experiment, you will investigate the relationship between bond types and
the physical properties of solids.
Procedure: Obtain samples of sodium chloride (an ionic solid) and
paradichlorobenzene (a molecular solid).
1. Note the odor of each solid. An odor indicates that some of the solid's
molecules can evaporate at room temperature. Record your observations.
2. Rub a sample of each solid between your fingers. Record the hardness
of the solids by noting whether the "feel" is soft or hard and granular.
3. Place a few crystals of sodium chloride in one test tube and a similar
amount of paradichlorobenzene (PDCB) in a second test tube. Heat each
test tube separately. Heat cautiously at first and then more intensely if
the solid does not melt readily. (CAUTION. If the solid does melt with
gentle heating, do not heat it further. It may also vaporize, producing
fumes. Be careful not to inhale such fumes since they may be poisonous.)
Record your observations.
4. Place a few crystals of each solid in separate test tubes. Add about 5
ml of water to each test tube. Shake or stir to mix. Compare the
solubilities of the two solids in water. Record your observations.
5. Place a few crystals of each solid in separate test tubes. Add about 5
ml of benzene to each, shake or stir to mix. Compare the solubilities of
the two solids in benzene. Record your observations.
Conclusions. 1. Prepare a chart in which you summarize the observations made
on the ionic and covalent molecular solids.
2. Compare the general properties of an ionic solid and of a covalent
molecular solid.
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