Distinguishing Ionic and Covalent Compounds
When ionically bonded substances dissolve in water, the ions separate
from each other enabling electric current to flow through the solution.
An ionic compound can usually be identified by its high melting point.
As a result of this high melting point, ionic compounds are crystalline
solids at room temperature.
A covalent compound is made of molecules. Weak intermolecular forces
hold molecules of covalent materials together. The weak forces are
responsible for their relatively low melting points. Covalent compounds
are usually liquids or gases at room temperature. With a few important
exceptions, covalent compounds do not conduct electricity when dissolved
in water, when they dissolve in water at all.
In this laboratory activity you will:
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build a simple conductivity tester to determine which substances
produce ions in aqueous solution;
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examine various compounds to determine their state;
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research the literature to determine the melting points of these
compounds;
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predict whether the compounds are more likely to be ionic or covalent.
OBJECTIVES
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Construct a conductivity tester.
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Measure conductivity of solutions.
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Observe the states of compounds.
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Research the melting temperatures of compounds.
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Relate properties to bond type.
MATERIALS
general purpose LED
9 V battery
battery clip
1k-ohm resistor
connecting wire
alligator clips (3)
96-well microplate
96-well template
soda straw
thin-stem pipet
wire stripper
apron
goggles
Handbook of Chemistry and Physics
PROCEDURE
Part 1: Building a Conductivity Tester
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Refer to Figure A to assemble the LED, resistor, battery, and wires
together.
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Connect the red lead of the battery to one end of the resistor with an
alligator clip.
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Using an alligator clip, connect the other end of the resistor to the
positive lead of the LED (usually the longer lead).
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Strip the insulation from both ends of two pieces of connecting wire,
each 3–4 cm long.
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Connect one piece of wire to the other lead of the LED with an
alligator clip. Connect the second piece of wire to the lead of the
battery clip.
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Push the end of one of the connecting wires crosswise through a soda
straw.
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Push the end of the other wire through the straw about 0.5 cm from the
first wire. (See Figure B.) The ends of the connecting wires will be
the electrodes of the conductivity tester.
Part 2: Testing Solutions for Conductivity
CAUTION: Many of the chemicals you will use are toxic. Both
hydrochloric acid (HCl) and sulfuric acid (H2SO4)are
very corrosive. Avoid contact with skin and eyes. Follow proper chemical
hygiene procedures. Wash your hands thoroughly after completing this
laboratory activity.
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Place 10 drops of solution 1 in well A1 of your microplate. Record the
name and formula of the solute on your template and Microplate Data
Form. Place 10 drops of solution 2 in well A2 of your microplate.
Record the name and formula of this solute on your template and
Microplate Data Form. Continue in like manner until each solution
available to you has been dispensed into your microplate. (Rinse the
pipet both inside and outside after each use).
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Insert the electrodes of the tester into a well that contains a
solution. The electrodes must not touch each other.
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Note the relative conductivity of the solution by looking at the
brightness of the LED.
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Record your observation on the Microplate Data Form. Use the following
code: C = Conduction; NC = No Conduction; PC = Partial Conduction
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Test all the solutions. Wash the leads with distilled water after each
test. Record the results.
Part 3: Observing the Physical State
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Inspect the flasks containing each of the undissolved substances.
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In the corresponding box of your Microplate Data Form, record for each
compound "s" if the compound is a crystalline solid, "1" if the
material is liquid, and "g" if the substance is a gas.
Part 4: Finding the Melting Point
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Use a reference source, such as the Handbook of Chemistry and
Physics, to find the melting temperatures of all the substances
used in this activity.
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In the corresponding box of your Micro-plate Data Form record the
melting temperature (in degrees Celsius) for each compound.
DATA AND OBSERVATIONS
Refer to the instructions in step 4 of Part 2, step 2 of Part 3, and
step 2 of Part 4 to record your observations and information about each
substance in the following table. Also, indicate your predictions about
bond type.