CONDUCTIVITY AND

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					      DAY 6


CONDUCTIVITY AND                                                Name_______________________________
CHEMICAL BONDING
            In this experiment, you will measure the conductivity of several different compounds and solutions in
       order to predict the bond type (ionic or covalent) in the substances tested. To understand conductivity, we
       must first define electric current. An electric current is produced by the motion of positive or negative
       particles. An electric current is measured in terms of the total number of positive or negative charges
       passing a given point per second. In a solid, such as wire, only the negative charges (free electrons) are
       able to move. Conductivity is a property that describes the ability of a substance to transmit an electric
       current.
            Experimentation has shown that pure water is a very poor conductor of electricity. The poor
       conductivity of water can be attributed to its molecular bonding. When solid sodium chloride, an ionic
       solid, is added to the water, a conducting solution is formed. The salt water solution is a good conductor
       because the NaCI forms Na+ and CI- ions in solution. These ions migrate toward oppositely charged
       electrodes. The moving ions complete the electric circuit. You can predict bond type in some solids by
       dissolving some of the solid in water and measuring the conductivity of the resulting solution.


       Objectives
                 In this experiment, you will
                 • test a number of different substances and solutions for their ability to conduct an electric
                    current, and
                 • predict the bonding nature of each substance, on the basis of your results.




 EQUIPMENT
goggles and apron
conductivity apparatus                                                    Solid-partially fill beaker with crystals. Test for
beakers (100 or 150 mL)                                                   conductivity (see Step 4). Retain a small
                                                                          amount for further testing; return the remainder
PROCEDURE                                                                 to the appropriate container.
1. Safety goggles and lab apron must be worn for this
   experiment.                                                           Dilute Water Solution-add a few crystals of
2. Support the conductivity apparatus as shown in                        solid to 30 mL of distilled water.
   Figure 20-1. (As there are many types of conductivity                  Water Solution-add about 1 g of solid to 30
   set-ups available, your teacher may provide one that                   mL of distilled water.
   is not the same as the one pictured here.) CAUTION:                    Alcohol Solution-add about 1 g of solid to
    There is danger of electrical shock; apparatus must be                30 mL of alcohol.
    turned off when placing it in or removing it from sample.            Acetone Solution- about 1 g of the solid to 30
3.Obtain one of the compounds and prepare (one at a time)                mL of acetone.
the solutions listed in the table. Listed to the right are               CAUTION: Alcohols and acetone can be poisonous
general directions for each test condition. Use 100 or 150               through skin absorption or inhalation. They are also
mL beakers.                                                              highly flammable. Keep away from flames; avoid
                                                                         skin, eye, lung contact.

                                                                     To test for conductivity lower the electrodes into the 4
                                                                 .   material in the beaker. Rate the conductivity of each
                                                                     test material as good, poor, or none. Record your
                                                                     rating and any other observations.
                                                                     The pure alcohol and all solids should be returned to
                                                                     the reagent table. All water solutions may be rinsed
                                                                     down the drain using plenty of water.
Test Material                         Conductivity                           Other Observations
Distilled water

Tap water

Alcohol – See CAUTION above

Acetone – See CAUTION above

NaCl solid

NaCl dilute water solution

NaCl water solution

NaCl alcohol solution

NaCl acetone solution

Sucrose solid

Sucrose water solution

Sucrose alcohol solution

Sucrose acetone solution

CaCl2 solid

CaCl2 water solution

CaCl2 alcohol solution

CaCl2 acetone solution



 CONCLUSIONS                 Answer the following using complete sentences on a separate piece of paper
1. Explain the similarities and differences in your conductivity results for NaCl and CaCl 2.
2. What was the purpose of using both water and alcohol as solvents?
3. Explain any differences in your results for distilled and tap water. Why is it important that laboratory solutions are
made with distilled water instead of tap water?
4. On the basis of this experiment, what conclusions can you draw concerning the bonding in the four solids tested?

5. What effect does concentration of a solution have on its conductivity?