A starting point for
the scientific investigation of anything is to observe and find similarities and
differences among things. Scientific work must be able to be understood
and repeated by others studying the same thing in order to be useful. One
way that scientists accomplish these goals is to classify things according to
their traits. To help others to use their method of classification,
scientists
often construct a dichotomous key.
A dichotomy is a division
into two parts. In a dichotomous key there are a series of paired
statements. If one is true about the thing you wish to classify, you go on
to another pair of statements. The alternative answer leads you to a
different pair of statements. In this way, by choosing one or the other of
paired statements, you are led to a final category for the thing you are
observing.
This lab covers both the concept and use of a dichotomous key
and the classification of matter. Using informational questions and vials
containing various types of matter, you will classify the vials into mixtures;
compounds; elements; etc. This
flowchart
provides a visual reference on how to classify matter and this
flowchart provides
a visual reference on all the properties of matter. Both of these charts must be
included in the data section of your lab report (for a better printing, set your
page setup to "landscape" and then print.)
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Procedure:
This lab exercise uses a dichotomous key to help you classify the contents of
small vials into the proper type of matter. The classification of matter
is a fundamental step in physical science. As you cannot directly observe
all of the traits you will be looking for, you must rely on previous
knowledge; charts within the science lab room and any information that others
may share with you during the classification procedure. As you make each
choice for the contents of a vial, write the number and letter of each statement
you choose in the column labeled "justification" on the line for that vial.
Once you reach your final category, the statements you chose will be
justification for your choice. The chart of your findings should look like
this.
Materials:
| pen/pencil |
Dichotomous Key
Chart |
matter flowchart |
| student worksheet |
vial of sand |
vial of glue |
| vial of gravel |
vial of sugar (C12H22O11) |
vial of salt in
water |
| vial of sulfur (S) |
vial of lead (Pb) |
vial of beads |
| vial of milk of
magnesia |
vial of bromothymol
blue in water |
vial of copper (Cu) |
| vial of water (H2O) |
|
|
A Dichotomous Key to the Classification of Matter
|
1.
|
a) |
the contents of the vial have mass and take up
space....................................................go to Q2 |
| |
b) |
the contents of the vial does not have mass or does not
take up space....................it
is ENERGY |
|
2. |
a) |
the contents of the vial have a variable composition, a
sample of this material is not uniform or different samples could differ
in the proportions of the things that make it
up.............................. ....................................................................................................it
is a MIXTURE; go to Q3 |
| |
b) |
the contents of the vial have a definite composition;
every sample of this material will have the same
composition............................................................it is a PURE
SUBSTANCE; go to Q6 |
|
3. |
a) |
the contents of the vial are uniform throughout,
although their proportions could differ from one sample to
another..............................................it is a HOMOGENEOUS MIXTURE; go to
Q5 |
| |
b) |
the contents of the vial vary in composition or traits
from one part of the sample to another; they do not appear uniform or are
known to vary from one part of a sample to another or to settle out over
time....................................................it is a HETEROGENEOUS MIXTURE; go to Q4 |
|
4. |
a) |
the contents of the vial may appear uniformly
distributed, but solid particles will settle out of the liquid portion of
the mixture.....................................................................it is a
SUSPENSION |
| |
b) |
the contents of the vial visibly vary in
composition........it is a HETEROGENEOUS MIXTURE |
|
5. |
a) |
the particles of the homogeneous mixture are atoms,
ions, or molecules; in a transparent mixture they will not scatter a beam
of light.............................................................it is a SOLUTION |
| |
b) |
the particles of the homogeneous mixture are larger
than molecules, they make the mixture appear cloudy or milky or they
scatter light in a transparent mixture...................it is a COLLOID |
|
6. |
a) |
the substance in the vial is made up of a single type
of atom that has a symbol and appears on the Periodic Table of the
Elements....................................................................it is an ELEMENT |
| |
b) |
the substance in the vial is made up of two or more
atoms chemically combined in a fixed
ratio........................................................................................................it
is a COMPOUND |
Student Work Sheet
- for a printer friendly version, click
here.
|
Vial Number
& Substance |
What is it? |
Justification |
| 1. gravel |
|
|
| 2. sugar (C12H22O11) |
|
|
| 3. salt in water |
|
|
| 4. sulfur (S) |
|
|
| 5. lead (Pb) |
|
|
| 6. beads |
|
|
| 7. milk of magnesia |
|
|
| 8. sand |
|
|
| 9. copper (Cu) |
|
|
| 10. water (H2O) |
|
|
| 11. bromothymol blue in water |
|
|
| 12. glue |
|
|
When doing the lab report write-up, be sure to follow
the guidelines.
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