St. Bernadette School
Drexel Hill, PA
Michael J. Gavaghan
Dr. Peter Yarnell
Edward M. Sioma
Rohm and Haas Company
Spring House, PA
6 to 8
- Introduce students to the ionic or
non-ionic nature of water
- Review water quality and
usefulness for various manufacturing purposes.
- Introduce students to the concept
of open and closed circuits.
- Students will construct a simple
- Students will observe that when
dipped in certain samples of water the LED on the
conductivity meter will light while it will not light
when placed in other samples that appear
- Students will observer that the
LED is brighter in some samples that in others that look
- Students will conclude that the
water samples may look similar but have different
- Students will discuss the
properities and usefullness of water and water
- Students will conduct a
semi-quantitative test of their own homes' tap water to
obtain a rough idea of its ionic content.
hot plate, immersion heater, or
droppers and bottles
beakers, jars, plastic cups, and/or glasses
quart jars for solutions
graduated cylinders or measuring jars
tincture of iodine
unsalted saltine crackers
baking soda (sodium bicarbonate)
I learned about this conductivity
meter during a three week long workshop called OPERATION
CHEMISTRY which was held at Widner University in the summer
of 1992. Since that time write ups of the device have
appeared various education journals. Despite variations in
configuration, I have found that this device serves best in
a very basic form, The meter is a rough and reasonably
accurate device which at 1.60 per copy is cheap and easy to
construct. Hard to break, construction takes about five
minutes. It is extremely safe for children, provided they
are careful with the 9 volt batteries.
The device is basically an open
circuit with two exposed leads. When the leads are touched
or come into contact with any material that will conduct
electricity the LED will light. The brightness of the bulb
will depend on the conductance of the material being
Pure water is a very poor conductor of
electricity. Yet ordinary household tap water will conduct
electricity reasonably well. If dilutions of tap water are
made, using pure or de-ionized water, and then tested with
the conductivity meter, the brightness of the bulb will
decrease in marked degrees with each diluton. What then is
the difference between pure, de-ionized water and ordinary
A clue to the answer is found in the
name given to pure or di-ionized water. Scientists know that
when materials like sodium, calcium, and magnesium, are
dissolved in water they dissociate. Dissociation means that
particles with electric charges are found in water that had
no charged particles prior to exposure to materials that
were electrically neutral before being placed in the water.
These dissociated atoms now an carry electric charge and are
When salt, sodium chloride (NaCl), is
dissolved in water the sodium atoms, which have lost one
electron apiece and represented by the symbol Na+, separate
or dissociate from the chlorine atoms. The chlorine atoms
each gain one electron in the process and are represented by
the symbol Cl-. These atoms exist in equal numbers When
solid sodium, magnesium, and calcium are dropped in water
their solid structure is broken up and charged ions float
all through the water molecules in the solution. When
electric current is introduced to the solution via the
conductivity meter the movement of the ions allows the
current to flow through the solution causing the LED to
light. The number of ions per liter of solution will affect
the flow of current through the now closed circuit.
Conductivity is dependent therefore on the presence of ions
as well as the concentration of ions per quantity of
It should be noted here that many of
the same elements which contributed to water hardness also
make it possible for water to conduct electric current. This
is not however an exclusive statement. Salt dissolved in
hard water continues to allow the water to conduct
electricity, but lowers the total hardness. A stock solution
of hard water made from conductive elements, along with
suitable dilutions, might provide a convenient tie in with
problems faced by certain manufacturing and power companies.
The problems associated with hard water should be familiar
to children who have completed the previous activity. It
seems reasonable to connect hardness with conductivity in
experimenting with a mystery sample, as long as the rough
and inconclusive nature of the activity is explained.
Students should understand that this test can not be used to
determine the exact hardness or make up of a mystery sample
but can only be used as a sort of guide. Conductivity of a
water sample, in a laboratory would lead to further testing
to determine the exact nature of the ionic material.
Students should be arranged in teams of 4 students. Each
team should provide one sample of tap, rain, or pond water.
Be sure to wash off the bare ends of the meter between
samples to avoid contamination. The conductivity meter once
constructed can be used for a wide range of additional
TO CONSTRUCT THE
METER - (part numbers from the
1991 Radio Shack catalogue)
Alligator clips (no soider
black electrical tape
craft sticks or tong depressors ( one
1.4 grams (1/4 tbs) Calcium or Calcium
Chloride ice melt material
3 grams (1/2tbs) of Magnesium Chloride
or Epsom Salts
4 empty two liter soda bottles
1 empty one liter soda bottles
100ml graduated cylinder
several empty baby food jars - 4 per
samples of students' tap water from
de-ionized water- obtain from high
school or purchase from food
DE-IONIZED WATER IS DIFFERENT FROM DISTILLED WATER
A. CONSTRUCTION OF CONDUCTIVITY
- The LED and the resistor are
pigtailed together and placed flat on stick, near the
middle of the stick
- Wrap tape around the pigtailed
joint attaching it to the craft stick.
- Strip both ends from 2 pieces of
wire leaving about 1/4 inch of exposed copper at each
- Pigtail an end of one wire to one
end of the resistor/LED combination so that the bare end
extends over the end of the stick about 1/2 inch.
- The battery with cap and
alligators can be attached to the end of stick, opposite
the bare wire, with a rubber band.
** CAUTION - TO AVOID DISCHARGE OF THE BATTERY DO NOT
HOOK BOTH ENDS OF THE BATTERY CAP UNTIL THE LAST
CONNECTION HASBEEN MADE.
- Clip one of the alligators to the
unattached end of the LED/resistor.
- Take the second piece of wire and
attach one end to the second alligator clip, allowing the
bare end of the wire to extend next, but not touching the
- Tape all exposed joints securely
to the stick with electricians tape.
- Connect the battery cap and touch
the bare ends of the wire together. The LED should light
brightly, if not check all connections.
**NOTE ** Since the LED only conducts electricity in one
direction try reversing the alligator connections
B. TO MAKE A STOCK HARD WATER
SOLUTIONS AND DILUTIONS OF KNOWN CONCENTRATION.
**NOTE-- ONLY DE-IONIZED WATER CAN BE
- Mass out 1.4 grams of calcium
chloride or 3 grams of magnesium chloride.
- Add to two liters of water, a soda
bottle will work well. Just make a mark on the outside to
indicate when the bottle is full. label this bottle
- Take 100ml or one 4oz baby food
jar of A and place in a second two liter bottle.
- Fill this with one liter (1000ml)
of de-inozed water and label it B. This is a 0.1 dilution
of the stock. For a 0.2 dilution use 200ml of A.
- Take 100 ml of B and add to 1
liter (1000ml) of de-inozed water and label this C. This
is a 0.01 dilution of the stock.
- 4 baby food jars per team, labeled A.B.C.and D.
- Add 25 ml of the appropriate solution to each jar.
- Place 25 ml of de-ionized water in D and use as a
- Students should test each sample with their
conductivity meter, being careful wash off the exposed
leads with de-ionized water between each sample to avoid
- Students should compare the intensity of the LED as
the meter is placed in each sample.
- Each group should agree on the intensity of the LED
and arrange the samples in order from least to greatest.
- Test a sample of tap water from one of the students.
Find which test comes closest to the unknown in the
intensity of the LED.
- Refer to the hardness chart. How hard is the sample
of unknown water?
- Did all water samples look the same?
- How did the conductivity meter react when placed in
- Did the LED indicate the same intensity for each
- Did The LED not light in any of the solutions?
- What was different about each solution and the
control that caused the conductivity meter to respond
- Describe how the meter functioned in the unknown
- Which of the dilutions came closest to the unknown in
intensity of the LED?
- What might be implied about the unknown and the
sample that made the meter behave in a similar fashion?
Materials other than water can be
tested for conductivity. 1k, 2k or .5k resistors could be
obtain and used in place of the 330 ohm resistor. The
results could be compared with the 330ohm version