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CONDUCTIVITY EXPERIMENT

VIEWS: 168 PAGES: 3

									                                CONDUCTIVITY LAB
Introduction:
       In general, only those substances which have free-moving charged particles will be found to
conduct electricity. In this experiment, you will use a micro—conductivity prob., a device that enables
you to test with a high degree of sensitivity the electrical conductivity of very small samples in a very
safe and time efficient way. In solids, only loosely held electrons are free to move. An electric current in
a solid ( usually an active metal) consists of a flow of electrons. In liquids, the presence of free ions
(positive and negative charges) is necessary for conductivity. When two electrodes, one positively
charged and the other negatively charged, are introduced into a liquid containing free ions, the positive
and negative ions will move in opposite directions (towards oppositely charged electrodes). This
movement of charged particles constitutes an electric current through the liquid.
       Free ions are produced as a result of dissociation or ionization. Dissociation describes the
behavior of ionic compounds. In the solid phase, ions are held rigidly in the crystal structure by ionic
bonds. Dissociation occurs when ionic solids are dissolved in water or melted. In either case, the bonds
holding the ions in the crystal lattice are broken and the ions are free to move. Dissociation by solution
on water can be illustrated as follows:
                                                      +               -
                                     NaCl(s)  Na         (aq)   + Cl (aq)

       Ionization describes the behavior of polar-molecular substances. These substances do not consist
of ions in their undissolved state. Many acids fall into this category. When dissolved in water ( also a
polar-molecular substance), these polar molecules ionize, as illustrated in the following equation:

                                                                             -
                             HCl + H2O ————> H3O+ (aq) + Cl (aq)
        Most organic compounds (the compounds of carbon) do not conduct an electric current, either
when pure or when dissolved in water. This is because all organic substances have molecular structures.
Except for certain compounds that have polar molecules, organic compounds do not dissolve in water.
Of those that do dissolve in water, only same are ionized in solution. Organic acids do ionize to varying
degrees in solution and thus, conduct an electric current.
        In general gases are said to be nonconductors; though, given low enough pressure and high
enough temperatures and voltages, gases can be made to conduct electricity. The free-moving charged
particles in such cases are the electrons that have been stripped from the gas atoms or molecules. These
conducting, ionized gases constitute a fourth state, of matter known as plasma, and plasma tends to emit
light as it conducts due to excitation of same of the electrons and subsequent falling from higher energy
levels to lower ones.

Purpose:

      To test the electrical conductivity of various liquids, solids and mixtures, and in so doing, to gain
an appreciation for network covalent, ionic, metallic, and molecular crystals and for the dissolving and
melting processes.
Equipment:

safety goggles              .1M ammonium hydroxide          tap water              zinc
conductivity probe          .1M sodium hydroxide            distilled water        copper
spot plate                  .1M acetic acid                 ethanol                charcoal
                            .1M hydrochloric acid           mineral oil            graphite(pencil)
iodine crystals             magnesium strips                calcium chloride       sodium chloride


Procedure:
1.         Obtain a spot plate place several drops of each liquids to be tested into the wells on the plate.

2.         Using the probe, test the conductivity of each of the liquids. Remember to clean the probe
           between trials using water and a paper towel. Wash the liquids down the sink with tap water and
           dry the spot plate.

3.         Obtain the solids to be tested in the wells of the spot plate. Using the probe, test the conductivity
           on each of the solids: zinc, copper, charcoal, graphite (pencil), magnesium. When finished return
           the solids to their containers. DO NOT DISCARD.

4.         Using the spot plate obtain the compounds to be tested: calcium chloride, sugar, salt and iodine.
           Remember to clean the probe between trials.

5.         Add a few drops of distilled water to the solid compounds in #4 and again test their conductivity.
           Remember to clean the probe between trials.

6.         Place a few drops of water in the palm of your hand. Rub the water with a finger and test your
           palm with the probe for conductivity.

7.        Wash the spot plate off with tap water and dry.

8.         From your knowledge of the subject matter, from the textbook, or from in—class discussion, fill
           out the first four columns of the data table.


     Conclusions and Questions:
     1.      What classes of solids conduct electricity.
     2.      What type of charged particles do ionic crystals, such as NaCl and KI, possess? Though these
             charged particles are present, they are not free—moving, and so ionic crystals, in general, do not
             conduct electricity. List and describe two separate processes that can act to free—up these charge
             particles and allow ionic substances to conduct.
     3.      Based on your experimental evidence, how do organic solids and liquids rate as electrical
             conductors?
     4.      If graphite and charcoal are both chemically the same ( composed of carbon, and only carbon),
             explain the results of your test.
     5.       Why would tap water conduct?
     6.       What enables you to conduct?
           DATA TABLE
                             PHASE    TYPE OF           DESCRIPTION       CONDUCITIVITY
NAME OF            FORMULA   (SOLID   SUBSTANCE         (ELEMENT,
                                      (METAL,NETWORK,   MOLECULE,         GOOD
SUBSTANCE                    LIQUID                     IONIC COMPOUND)   POOR
                                      IONIC,COVALENT)
                             GAS)                                         NON-CONDUCTIVE

Set 1-liquids
Tap Water

Distilled

Ethanol

 .1M ammonium
hydroxide
.1M sodium
hydroxide

.1M acetic acid
.1M hydrochloric
acid
mineral oil        none                 COVALENT         MOLECULE



Set 2-solids
Zinc

Copper

Charcoal

Graphite
(pencil)
Magnesium


                                                                           Solid   Aqueous
Set 3-compounds                                                            state    state
Calcium chloride

Sugar

Sodium Chloride

Iodine

yourself           none

								
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