Conductivity of Molecular and Ionic Compounds
Introduction
The salt and sugar on your kitchen table both dissolve easily in water, but the solution they form
have an important difference. One of those kinds of white crystals is an ionic compound, and when it
dissolves, it dissociates into ions. The ions are free to move in the solution, and that solution, therefore,
conducts electricity. The other kind of crystal, however, is a molecular compound, and its molecules
remain whole when they dissolve. With no ions, the solution conducts no electricity.
This investigation involves testing several different liquids that are distributed among the work
areas in the laboratory. When you measure the conductivity of each liquid, you will find that some are
good conductors, some are fair or poor conductors, and some are nonconductors. Using the conductivities
you have measured, you will decide which solutions contain ionic compounds, and which contain
molecular compounds.
After you have classified your solutions, you will examine sugar and salt from another point of
view. Bonding theory generally predicts that ionic compounds should form from combinations of elements
that are far apart on the periodic table, while molecular compounds should form from elements that are
close together. You will see whether your findings on conductivity agree with this prediction.
Pre-Lab Questions
1. What are ions, and how do they form?
2. What is an ionic bond?
3. What is a covalent bond?
4. How do aqueous solutions of ionic and molecular compounds differ?
5. When some ionic compounds dissolve, not all of their bonds dissociate. What kind of
conductivity would you expect such a solution to have?
Problem
How can the conductivity of a solution help you classify the bonds in the solute?
Materials
Chemical splash goggles
Laboratory apron
Equipment at each work area:
Conductivity tester
Wash bottle with distilled water
Test solution in 250-mL beaker
Beaker, 250-mL
Safety
Wear your goggles and apron at all times during the investigation. Beware of electrical shocks. Use and
dispose of chemicals as specified by your teacher, and wash your hands thoroughly before leaving the
laboratory.
Procedure
1. Put on your goggles and lab apron.
2. Using the conductivity tester as described by your teacher, test the solution at your lab station.
CAUTION: Some of the test solutions may be corrosive, or may stain or discolor clothing.
Avoid spills and splashes. If spills occur, wash with plenty of water and notify your teacher
immediately. Avoid inhaling fumes from the bleach or ammonia solutions. DO NOT mix
bleach and ammonia solutions. The resulting vapors are toxic and explosive.
3. Note whether the test light is lit, and if so, how brightly. Record your observations in the data
table.
4. Rinse the conductivity probes with distilled water over an empty beaker.
5. Move to the next lab station and check the conductivity of the solution at that station in the same
manner. Record your observations.
6. Repeat steps 2-5 until you have tested and recorded data for all the solutions.
7. Dispose of all solutions as instructed by your teacher.
8. Clean up your work area and wash your hands before leaving the laboratory.
Observations
DATA TABLE
Test Solution
vegetable oil
salt water
sugar water
antacid in water
detergent in water
liquid hand soap in water
borax in water
household bleach in water
carbonated drink
distilled water
tap water
1 M HCl
1 M NaOH
lime water
aspirin in water
baking soda in water
vinegar
fruit juice
coffee
household ammonia
Good
Fair
Conductivity
Poor
None
Analysis and Conclusion Questions
1. What types of bonds do you think the good conductors of electricity have? Explain your
reasoning.
2. What type of bonds do you think the nonconductors of electricity have? Explain your reasoning.
3. How can you account for the fair and poor conductors of electricity?
Applications Questions
1. What kind of ions does sodium chloride (table salt) produce when it dissolves?
2. Where are sodium and chlorine found on the periodic table? Do the relative positions of these
elements in the periodic table agree with the prediction made in the Introduction about their
structure? Explain.
3. Look up the chemical formula for sucrose. Where are the elements that form sucrose found on the
periodic table? Do the relative locations of these elements in the periodic table agree with the
theoretical prediction about the kind of compound these elements should form? Explain.
4. What conclusions can you draw from this investigation?
Extra Credit
1. Using the electronegativity values listed for the elements in the periodic table, compute the
electronegativity differences for the bonds in table salt and sugar (sucrose). Use these differences
to predict what type of bonding will occur between the various atoms in these compounds. Do the
predictions agree with your observations on table salt and sugar?
2. Investigate how Linus Pauling used bond energies to derive electronegativity values.