Lab: Conductivity
Objective
In this lab students will explore the difference between electrolytes and non-electrolytes. First,
they will determine whether a variety of substances are electrolytes by measuring the
conductivity of low concentration solutions. Second, they will learn about the idea of solute
concentration and how it affects conductivity. Finally, they will explore the differences in
conductivity between solutions containing compounds with different numbers of ions per
formula unit.
Background
The Nature of Electrolytes
When atoms combine to make compounds they bind to each other by electrical forces of
attraction. Positive charges attract negative charges and these strong forces create a bond
between atoms. These bonds can take different forms: some are covalent bonds in which atoms
share valence electrons. The negatively charged electrons occupy the space between the
positively charged nuclei and both nuclei are attracted to the electrons. Other bonds are ionic
bonds in which atoms gain electrons to become a negatively charged anion or lose electrons to
become a positively charged cation. These ions then attract each other directly with no electrons
between them.
An ionic compound in the act of dissolving in
water. See a full explanation with animations at
this page:
http://www.middleschoolchemistry.com/
multimedia/chapter5/lesson3
Chemical bonds are formed and broken during chemical reactions that change one substance into
another. Chemical bonds may also be temporarily broken by dissolving a compound in water.
Some compounds have bonds that can be broken to create two or more freely moving ions when
the compound dissolves in water. This is due to the fact that water is a strongly polar molecule. It
has two oppositely charged poles that strongly attract other atoms and molecules. In the
illustration at right water molecules are shown dissolving ions from a solid crystal. The water
molecules’ positively charged poles attract negative ions away from the crystal and then
surround them as the ions then become mobile and move around in the solution. The negative
poles of the water molecules do the same for the positive ions. The ions remain surrounded by
water molecules just as a famous pop star is always surrounded by an entourage. Chemical
compounds which break up into ions when dissolved in water are called electrolytes. There are
other compounds which dissolve well in water but which do not break into separate ions. Such
compounds are called non-electrolytes. Finally, a third class of compounds, called weak
electrolytes, are found to produce a small number of ions when dissolved in water. The majority
of the molecules of weak electrolytes remain as whole, uncharged molecules but a small fraction
break up into cations and anions. This lab is not concerned with weak electrolytes.
Concentration of Solutions
Solutions are homogeneous mixtures of two or more substances. For liquid solutions the solvent
is often water. Solvents are materials in which other substances dissolve. Solids, liquids and
gases may all dissolve in water and any of them may be called a solute. A solute is any substance
that dissolves in a solvent. Solutions may be made to have a variety of different strengths, or
concentrations. The concentration
The image on the left shows dissolved ions at
low concentration.
The image on the right shows dissolved ions
at high concentration.
of a solution is a quantitative measure of how much solute is dissolved in the solvent. Often
concentration is measure in moles per liter. Moles are a unit related to the number of molecules
of a substance and liters are a common unit of volume.
Concentration is higher for solutions containing more of the solute and lower when less material
has been dissolved. In the illustration at right there are two boxes showing different
concentrations. On the left there are very few particles of the solute and the image represents a
solution with a low concentration. On the right there are many more solute particles in a higherconcentration solution.
Conductivity
One interesting consequence of the fact that some compounds produce separate, mobile cations
and anions when dissolved in water is that the solution conducts electricity. Imagine a complete
circuit containing a light bulb and a battery. The light bulb lights because of the power supplied
by the battery. If a wire is cut then the bulb goes out. But if the two cut ends of the wire are
placed in an electrolyte solution of high enough concentration the bulb will light again. Very
often it is possible to demonstrate this in a classroom. There is also a high-quality simulation
available from the PhET project at the University of Colorado. It is called Sugar and Salt
Solutions and is available here: http://phet.colorado.edu/en/simulation/sugar-and-salt-solutions.
Electrons in a metal are mobile and can move among the atoms of the metal like water in a pool
full of pebbles. When a battery is used to make an electrical current it causes the electrons to
move in an organized way, all in one direction, turning the pool into a stream. This motion of the
electrons in a piece of metal wire is what we normally think of as an electrical current. The
electrical current in an electrolyte solution has a slightly different mechanism. The ions
themselves move and carry electrical charge from one wire to the other. Electrons do not leave
the metal and travel through the water to the other wire. Instead, cations move toward the
negatively charged wire and anions move toward the positively charged wire. Fundamentally, an
electrical current is only the movement of charged particles and it doesn’t matter whether the
particles are electrons or ions.
The conductivity probes you will use in this lab activity measure how well a solution conducts
electricity. It gives a low reading for a low concentration of ions and a higher reading for higher
concentrations of ions. In effect, the reading produced by the probe is a measure of the number
of ions per liter. The number of ions per liter depends on two things. First, it depends on how
much solid is dissolved. The more material that is present in solution, the more conductive the
solution will be. Second, it depends on the chemical formula of the dissolved material. Some
compounds have more ions in their chemical formulas than others. Consider the following
compounds and the number of ions each one makes when they dissolve:
sodium chloride
NaCl —> Na+ + Cl–
calcium chloride CaCl2 —> Ca2+ + Cl– + Cl–
sodium phosphate Na3PO4 —> Na+ + Na+ + Na+ + PO43–
Each equation shows how many ions are produced from each formula when they dissolve in
water. Sodium chloride produces two ions, calcium chloride produces three ions, and sodium
phosphate produces four ions. Even when these salts are made into solutions with the same
concentration, measured as whole formula units per liter, they produce different levels of
conductivity. For example, a solution of sodium phosphate will be twice as conductive as a
solution of sodium chloride with the same concentration.
Pre-Lab Questions
Before handing this lab in, write your answers to these questions.
1. What is an electrolyte and how is it different from a non-electrolyte?
2. Define the terms solute, solvent, and solution in your own words.
3. In your own words, describe the process by which an ionic solute dissolves in water to
make an electrolyte solution.
4. In your own words, describe how electrical current is moved through an electrolyte
solution and how this is different from how electrical current moves through a metal
wire.
5. Do electrolytes carry electricity or produce it? Justify your answer by thinking about the
example of the light bulb and the battery from the introduction.
6. How many ions does one formula unit of MgBr2 break up into when it dissolves? How
many ions per formula unit does AlCl3 make?
7. How does the number of ions per formula unit affect the conductivity of a solution?
8. How does changing the concentration of a solution of the same electrolyte change the
conductivity? In other words, what is more conductive, a solution with a high or a low
concentration?
9. Based on your knowledge of ionic compounds, which of the following two compounds
will produce a solution with higher conductivity if the same number of formula units are
dissolved? AlCl3 or MgBr2? Justify your answer.
Materials
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Labquest software on a computer
Vernier Conductivity Probe
ring stand
clamp
stirring rod
several 100-mL beakers
distilled water
wash bottle with distilled water
100-ml graduated cylinder
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dropper bottle of 0.1 M NaCl
dropper bottle of 0.1 M CaCl2
dropper bottle of 0.1 M Na3PO4
dropper bottle of 0.1 M C12H22O11 (sugar)
dropper bottle of 3% H2O2 (hydrogen peroxide)
Sharpie marker for labelling
Safety
The chemicals used in this activity are all of low toxicity and are used in dilute solutions. Even
so, it is best to wear standard laboratory safety equipment such as splash goggles, long pants, and
nitrile gloves, if desired. Tie back long hair and do not wear dangling jewelry in the lab.
Part I: Electrolytes vs. Non-electrolytes
Procedure
In this part of the lab you will measure the conductivity of six different substances: distilled
water, hydrogen peroxide (H2O2) solution, sucrose (C12H22O11) solution, sodium chloride (NaCl)
solution, calcium chloride (CaCl2) solution, and sodium phosphate (Na3PO4) solution.
1. Wear your safety goggles while working in the lab.
2. On a separate piece of paper create a data table to record one number for each of the five
substances you are going to test in this part of the lab.
3. Set up the Conductivity Probe using the ring stand and clamp. Do not clamp the probe
too tightly and do not submerge the probe completely into water at any time.
4. Set the range selector on the Conductivity Probe to “0 - 200 μS/cm”.
5. Connect the probe to the computer and start the Logger Lite or Logger Pro software.
6. The software will immediately recognize the probe and produce a data collection screen.
7. Go to the “Experiment” menu, select Set Units, select the probe and choose mg/L. This is
milligrams per liter of equivalence to NaCl.
8. Obtain and label five 100-mL beakers with the formulas of the substances you will be
testing: NaCl, CaCl2, Na3PO4, C12H22O11, and H2O2. You will use these beakers
throughout this lab without emptying them until it is time to clean up.
9. Fill each beaker with 50 mL of distilled water by measuring with a graduated cylinder.
10. Test the conductivity of one of the beakers containing distilled water by submerging the
electrode at the bottom completely. Water alone will be the first material you test. Hold
the beaker up to the probe and submerge the end of the probe in the water. Record the
conductivity value on your screen.
11. Now add just one drop of each solution your teacher has provided for you to each labeled
beaker.
12. Stir the NaCl solution with the glass rod. Rinse the rod and dry it before stirring the next
solution. Stir each one with the stirring rod, rinsing and drying in between each one.
13. Use the Conductivity Probe to measure the conductivity of the NaCl solution. Record the
value in a data table you write down. Don’t forget to include distilled water as one of the
substances you tested.
14. Rinse the end of the probe carefully using the wash bottle full of distilled water. This
prevents the probe from giving inaccurate readings when you test the next solution.
15. Now use the probe to measure the conductivity of each of the other solutions. Always be
sure to rinse the probe between tests!
Part II: Electrolyte Conductivity vs. Concentration
Procedure
In this part of the lab you will collect data to show the proportion between concentration and
conductivity for three strong electrolytes. You will not be testing the conductivity vs.
concentration for the non-electrolytes.
1. On the “Experiment” menu, click “Data Collection…”. Choose “Events with Entry”.
2. For “Name” enter “Number of Drops” and for “Units” enter “Drops”. Click OK
3. Right-click in the middle of your graph. Go to the Axes tab. Set the maximum value for
the x-axis to 10 so your data will fill your graph.
4. Click the “Collect” button to start data collection.
5. Raise the beaker containing 50 mL distilled water and one drop of NaCl solution up over
the end of the probe. Click the Keep button and enter 1 for the volume in drops. Do not
rinse off the conductivity probe between tests.
6. Add one drop of NaCl solution and stir it using your clean stirring rod.
7. Test the conductivity of the solution by holding it so the end of the probe is submerged.
Swirl it gently, without spilling and watch the conductivity value for about 5 seconds.
Once the reading reaches a steady value, click “Keep” and enter 2. Do not rinse off the
conductivity probe between tests.
8. Repeat this process until you have reached a total of 8 drops.
9. Stop data collection by clicking the “Stop” button.
10. Next you will use the software to analyze your result by producing a linear regression.
This will calculate the equation of the line and plot a best-fit line for your data.
1. On the “Analyze” menu choose “Linear Fit”.
2. When it plots the line and gives you the equation it will be in the form y = mx +
b. The y variable stands for conductivity, the x value stands for volume in drops.
3. Record the value of the slope in a data table you create yourself on a separate
piece of paper.
4. Click the File Cabinet icon to “Store” the data from the first run.
11. Rinse the electrode with the wash bottle.
12. Repeat the data collection procedure with the CaCl2 solution.
13. Remember to store the data from Run 2 using the File Cabinet icon.
14. Repeat the data collection process with the Na3PO4 solution.
15. As instructed by your teacher, print this graph and/or export the data for further analysis
and formatting in a spreadsheet program. In your report you must include a wellformatted graph with all three runs properly labeled and showing the slope of each line.
page break
Post-lab Questions
Answer the following questions in a typed document on a separate paper. As part of your report
you must include the answers to the Pre-lab questions, your formatted and labeled graph, and the
answers to the questions below. This lab does not require a formal lab report.
1. Which of the substances you tested in part one were electrolytes? Which of them were
non-electrolytes? Use your data to justify your answers and recall that distilled water
should have essentially zero conductivity. If your distilled water had a reading slightly
above zero then take this value as equivalent to zero conductivity.
2. Create a data table to compare the conductivity of each different solution with only one
drop of the stock solution.
3. Using the chemical formulas of each compound explain why the conductivity values
follow the pattern that you see.
4. Why do some compounds show essentially zero conductivity?
5. Why are some more conductive than others despite the fact that you added the same
amount of each one?
6. What kind of proportion exists between the conductivity of a solution and the
concentration of ions in the solution? Describe what happens to conductivity as
concentration increases.
7. Compare the slopes of the three lines and put them in order from smallest to largest.
What explains the difference in the slopes since all three were generated starting with
solutions with the same starting concentration?
8. Write equations for when the following salts dissolve in water: Na2SO4, Mg3(PO4)2, and
Al(OH)3.
9. Which of the salts in the previous problem would have the steepest slope if it were used
in Part II of this lab? Which would have the smallest slope? Justify your answers.