Prescriptive Activities: Facet Cluster 1.5: PF#80-87
Student Copy
Prescriptive #80-87:
80 The student believes that a compound has the same properties as the elements it
is made of.
81 The student believes that different compounds made of the same elements are the same
substance.
82 The student believes that different compounds made of the same elements have the same
color.
83 The student believes that different compounds made of the same elements have the same
density.
84 The student believes that different compounds made of the same elements have the same
melting and boiling points.
85 The student believes that different compounds made of the same elements have the same
hardness.
86 The student believes that different compounds made of the same elements have the same
chemical properties.
87 The student believes that different compounds made of the same elements have the same
state of matter.
Background
Many different substances can be made out of the same elements and these different
substances have very different properties from the elements
of which they are made. For example, carbon dioxide consists
of carbon and oxygen. Carbon dioxide exists as a colorless gas
under normal conditions, whereas the element carbon exists
as a solid in the forms of diamond, graphite and charcoal. The
image to the right shows pure carbon in the forms of diamond
and graphite.
The melting point of carbon dioxide (CO2) is -78°C whereas the melting point of diamond,
which is one of the hardest materials known, is 3550°C and the melting point of oxygen (O2)
is -218.4°C. Clearly the physical properties of a substance are different than the properties
of the elements from which they are made.
Just as different compounds made of the same elements have different properties, different
forms of a single element can have different
properties. The pure element phosphorous (P) exists
in four different forms (white, red, violet and black;
shown at left) with each one having different physical
and chemical properties. White (or
yellow) phosphorous exists as a P4
molecule (shown at right) and ignites if exposed to air at 30°C. It is insoluble in
water, but does dissolve in the solvents benzene and carbon disulfide.
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Prescriptive Activities: Facet Cluster 1.5: PF#80-87
Student Copy
Red phosphorus, the substance on match sticks, exists as an amorphous network of atoms
(shown to the right) and ignites in air at 240°C. Red phosphorus
can be converted into white phosphorus with pressure, which is
how matches work: the pressure of striking the match changes
the red phosphorus to white phosphorus which spontaneously combusts at lower
temperatures. Violet phosphorous has the more organized
crystalline structure shown at left, does not ignite in air until
heated to 300°C and is insoluble in all solvents. Black
phosphorus is the most stable (least reactive) form of phosphorous, with a
molecular structure, shown at right, similar to that of graphite. It is made of
interlinked six-member rings. As you can see from these example different
compounds made of the same elements, as well as different forms of the
same element, have unique properties.
Materials:
Student handout
Two different sugars (sucrose and fructose)
Vinegar
Vanilla
Directions of Activity:
Part I – Taste Test
Fructose (C6H12O6) and sucrose (C12H22O11) are both sugars made out of carbon, hydrogen
and oxygen. Obtain samples of fructose and sucrose and taste each one.
Part II – Smell Test
Vinegar contains 5% acetic acid (HC2H3O2) in water. The compound vanillin (C8H8O3) is the
main ingredient in vanilla and is the chemical that gives vanilla its distinctive aroma and
taste. Find vinegar and vanilla extract in your kitchen and smell each of them.
Discussion questions
1. Describe the similarities and differences in the physical properties (taste, color,
texture, etc…) of the two sugars you tasted.
2. What three elements combine to form sugar?
3. In what state of matter (solid, liquid or gas) do the pure elements that make up sugar
exist?
4. What state of matter does sugar exist in?
5. How would you describe the smell of vinegar? Vanilla?
6. What three elements make up vinegar? Vanilla?
7. If someone told you that different compounds made of the same elements had the
same properties, what evidence could you provide to this person to show them their
error?
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Prescriptive Activities: Facet Cluster 1.5: PF#80-87
Student Copy
Notes to teacher:
This activity is designed to be done at home. However, you may want to have samples of
sucrose and fructose in the classroom for students to taste in case they do not have access to
these at home.
References:
Diamond, graphite image:
http://en.wikipedia.org/wiki/File:Diamond-and-graphite-with-scale.jpg
Phosphorous allotropes image:
http://chemistry.about.com/od/periodictableelements/ig/Element-Photo-Gallery.-98/Phosphorus-Allotropes.htm
Phosphorus crystal structure images:
http://en.wikipedia.org/wiki/Allotropes_of_phosphorus
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