Lab: Conductivity of Solutions
Electricity is "created" when certain chemicals react together. We use chemically- made electricity to
power many machines from flashlights to a watch or even cars! The devices that store electricity are
called batteries. Electricity can also be used to produce chemical changes.
Water is a simple chemical made from two gases -- hydrogen and oxygen. Every molecule of water has
two atoms of hydrogen for every atom of oxygen. H2O is the chemical formula for a molecule of water.
If an electrical current is passed through water between electrodes (the positive and negative poles of a
battery), the water is split into its two parts: oxygen and hydrogen.
You can use electricity to split hydrogen gas out of the water similar to the process called electrolysis. If
an ionic compound is dissolved in water, it dissociates into ions and the resulting solution will
conduct electricity. Dissolving solid sodium chloride in water releases ions according to the
equation: NaCl(s) --> Na+(aq) + Cl–(aq)
When you pass an electric current through a solution, ions (charged particles) migrate towards the
electrode of the opposite charge.
In a salt solution (NaCl) the dominant species of ions are sodium (Na+) and chloride (Cl-), because only a
tiny amount of water (H2O) is ionised (to H+ and OH-) at pH 7, and that's why pure water is very difficult
to electrolyse.
So when you apply a current to the solution using copper electrodes, the chloride ions (Cl-), termed
anions, will move towards the positive electrode (the anode), whilst the positively-charged sodium ions
(the cations) will migrate towards the negative electrode (the cathode). The migrating ions carry charge
through the solution and hence help to complete the circuit.
At the anode, 2 chloride ions (Cl-) will each surrender an electron to the anode (which likes electrons
because it is positively charged) to form a molecule of chlorine gas, which you see fizzing off :
2Cl-(aq) -> Cl2(g) + 2eAt the same time, the copper (Cu) forming the electrode will also try to donate electrons :
Cu(s) -> 2e- + Cu2+(aq). When the copper (Cu) gives up 2 electrons it forms a copper ion (Cu2+) which
then goes into solution, turning the electrolyte blue / green, as you have observed.
At the negative electode (cathode) hydrogen ions (H+) from water pick up electrons to form hydrogen :
2H+(aq) + 2e- -> H2(g)
...and the copper ions (Cu2+) which were sent from the anode also pick up electrons to form metallic
copper which is deposited on the cathode :
Cu2+(aq) + 2e- -> Cu(s).
So if you weighed the 2 electrodes before and after the experiment, you should be able to demonstrate
that the anode (the positive electrode) gets lighter, and the cathode (the negative electrode) gets heavier.
In this lab, you will study the effect of adding an ionic compound to distilled water to observe the effects
on conductivity. Conductivity is present if there are bubbles present at the cathode and anode.
Questions for you to answer as you do this lab.
1. Will deionized water conduct electricity? Why or why not? What do you observe when
you do the experiment? Does it support your hypothesis?
2. When the solution bubbles, what are the bubbles? What gases do you think are
bubbling out of the solution?
3. What would happen if we let this solution continue to bubble? What is left? What will
this do to the pH of the solution? To the ion concentration of the solution?
4. Determine the conductivity of each of the following compounds. Dissolve about a teaspoon of
each compound in 100 mL of distilled water. Keep your data on this sheet. Hypothesize about
which ones you think will conduct electricity and which ones won’t.
Compound/Solution Formula
Distilled water
Tap Water
NaCl
Sugar
Hypothesis- Is it ionic?
Polar?
Conducts Electricity?
Results and
Observations
5. Draw the complete set up you used and label the cathode, anode, battery, solution, and gas (be
specific!). Explain what exactly you are doing in the experiment. Include an explanation about
what the cathode and anode are, and what is happening at each.