With the help of an easily constructed conductivity tester, the effects of concentration on conductivity can be shown. Strong and weak electrolytes can be demonstrated. The apparatus can also be used to show the conductivity associated with weak and strong acids and acid-base neutralization.

Conductivity tester (see figure)
Six 150 mL beakers
Stock solutions of sodium chloride, ammonia, acetic acid, sugar, and hydrochloric acid, all of roughly 2 mol/L strength.
Distilled or deionized water.

After discussing the idea of solution electrolytes with your class, demonstrate the conductivity tester in a "dry run." Show that when the two graphite rods touch, the circuit is completed and the light bulb glows.

Next, show the difference between a solution of a strong electrolyte (sodium chloride or hydrochloric acid) and a non-electrolyte (sugar). The former conducts as shown by the glowing of the bulb; the latter does not. Also show that distilled water does not conduct current in the same way.

Next, show that the ability of a solution to conduct current depends on the concentration of the electrolyte. Successively dilute the sodium chloride solution by pouring away half the solution and repeatedly replacing it with water. The glow of the bulb will diminish with each successive dilution until no light can be seen.

Now show that weak acids and bases are weak electrolytes by testing the ammonia and acetic acid solutions. Compare these with the hydrochloric acid (strong acid and strong electrolyte).

Finally, show that salts, which are formed by the neutralization of acids by bases, are always strong electrolytes irrespective of whether the "parent" acid and base were strong or weak. Mix the ammonia and acetic acid solutions (and note the mildly exothermic result). The resulting solution of ammonium acetate conducts current even though neither the acid nor the base was an electrolyte.

A 2 mol/L solution of a strong electrolyte like sodium chloride or hydrochloric acid will conduct electric current around 10% as well as a typical metal. It is the electric field caused migration of positive and negative ions that carries the current; there are no free electrons in the water solution. The apparatus shown in the above diagram is not particularly sensitive and will fail to show conduction after a strong electrolyte solution has been diluted down to less than 0.01 mol/L.

Starting with a 2 mol/L solution of a string electrolyte, one can successively dilute the solution by pouring away half and refilling the container with distilled water. In this way a series of solutions of concentrations 1 mol/L, 0.5 mol/L, 0.25 mol/L, 0.13 mol/L, 0.06 mol/L, etc. can be tested. After the first two or three, the bulb will become dimmer after each dilution until it finally fails to glow.

Weak acids and bases do not produce enough ions in water to conduct well enough for the apparatus to work. However, when ammonia and acetic acid neutralize each other, ammonium acetate is the product, which is a salt, and almost all salts are strong electrolytes in water. The solution conducts after acid-base neutralization has occurred.

Eye protection should be worn when preparing and handling all the solutions in this demonstration.