Bonds
Back Ground: Compounds can be classified by the types of bonds that hold
their atoms together. Ions are held together by ionic bonds in ionic compounds; atoms are held together by covalent bonds in molecular compounds.
You cannot tell whether a compound is ionic or molecular simply by looking
at a sample of it because both types of compounds can look similar.
However, simple tests can be done to classify compounds by type because
each type has a set of characteristic properties shared by most members.
Ionic compounds are usually hard, brittle, water-soluble, have high melting
points, and can conduct electricity when dissolved in water. Molecular compounds can be soft, hard, or flexible; are usually less water-soluble; have
lower melting points; and cannot conduct electricity when dissolved in water.
PROCEDURE:
PROBLEM: How can you identify ionic and molecular compounds by their
properties?
OBJECTIVES:
• Examine the properties of several common substances.
• Interpret the property data to classify each substance as ionic or molecular.
Materials: glass microscope slide, grease pencil or crayon, hot plate, spatula, 4
small beakers (50- or 100-mL), stirring rod, balance, conductivity tester, graduated
cylinder, small, stop watch, 1- to 2-g samples of any 4 of the following: salt
substitute (KC1), fructose, aspirin, paraffin, urea, table salt, table sugar, Epsom salt,
Use care when handling hot objects.
Part I
1. Use a grease pencil or crayon to draw lines dividing a glass slide into four parts.
Label the parts A, B, C, and D. Label a second slide E, F, G, and H.
Part II
10. Using a well plate put a small sample of A, B, C, D, E, F, G, and H into each
separate labeled well.
2. Record all data in the table under Data and Observations.
11. Add 10 mL of distilled water to each beaker.
3. Use a spatula to place about one-tenth (about 0.1 to 0.2 g) of the first of your four
substances on section A of the slide.
12. Stir each substance, using a clean toothpick for each sample. Note on your table
whether or not the sample dissolved completely.
4. Repeat step 3 with your other substances on sections B, C, D, E, F, G, and H. Be
sure to use the spatula that is specific for each sample. Record in your data table each
compound with its corresponding letter.
13. Test each substance for the presence of electrolytes by using a conductivity
tester. Record whether or not each acts as a conductor. Make sure to rinse the
conductivity tester with distilled water between each sample test
5. Place the slide on a hot plate. Turn the heat setting to a medium temperature and
begin to heat the slide.
Part III
14. Cut a piece of 12 to 15 cm nichrome wire.
6. If you have a TI thermometer, gently hold a TI thermometer so that the bulb just
rests on the slide. Be careful not to disturb your compounds. Be careful not to burn
the wire!
15. With needle-nose pliers, from a closed, oblong loop on the
end of the wire approximately 7 mm long and 3 mm wide.
16. Heat loop end of wire with available torch until it begins
to turn red in color. Note: If the wire gets too hot it can melt.
7. Continue heating until the temperature reaches 135°C. Observe each section on
the slide and record at what temperature the substances melted. Turn off the hot
plate.
8. On a glass slide put a small piece of moth ball, heat on hot plate until melted.
Remove and look at under a specimen scope.
17. Dip heated end into borax, then carefully heat with torch until glass bead in
formed. If bead is too small or incomplete, then dip it in borax again and heat until
desired bead is formed. Continue melting the bead until it forms a droplet that is
glassy and transparent. To keep the bead from dripping, gently rock or rotate the
wire. Note Overheating will evaporate the borax , do the melting in the cool, outer
portion of the flame as demonstrated above.
9. Clean slides and dry them.
18. Record your observations.
Data and OBSERVATIONS:
Substance
Did it
Name and Formula if melt?
Time
available
or
temp
Did it
disso
lve in
water
?
Did the
solution
conduct
electricity?
Classification:
Ionic
Polar Covalent
Covalent
Network (Giant)
Covalent
Metallic
Analyze:
1. Interpreting Observations: What happened to the bonds between the
molecules when a substance melted?
2. Comparing and Contrasting: Did all compounds melt at the same
temperature?
A
3. Classifying: Complete your data table by classifying each of the
substances you tested as ionic or molecular compounds based on your
observations.
B
Analyze and Apply:
1. What are the differences in properties between ionic and molecular
compounds?
C
D
E
F
2. How did the melting points of the ionic compounds and the molecular
compounds compare? What factors affect melting point? (Discuss
intermolecular forces (bonding) too – hydrogen, dipole/dipole, and London
dispersion)
3. The solutions of some molecular compounds are good conductors of
electricity. Explain how this can be true when ions are required to conduct
electricity.
4. Consider a mixture of sand, salt, and water. How can you make use of the
differences in properties of these materials to separate them? Detail step by
step…1, 2, 3, …
G
5. Give the definitions for the following in terms of what would be seen in
lab by testing them. Ionic, Polar Covalent, Covalent , Network (Giant) Covalent,
Metallic
H
Conclusion: (Wrap it up!)
1. What where the possible sources of error in your lab?
2. What was learned from this lab?