Counting spoonfuls of salt

Demonstration

This demonstration shows how a circuit can be set up to detect a familiar event that produces ions. A simple circuit with a beaker of water can be made to conduct by adding a heap of salt. This is a useful step towards understanding the spark counter and the Geiger-Müller tube.

Apparatus and Materials

• Lamp holder on base
• Lamp, 12 V 36 W and power supply
• Beaker, 1000cm3
• Copper electrodes, 2
• Crocodile clips, 2
• Distilled or de-ionised water
• Table salt, supply
• Teaspoon
• Retort stands, bosses and clamps, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

Photo courtesy of Mike Vetterlein

1. Set up a series circuit consisting of the power supply, the lamp and the 2 copper electrodes separated by the width of the beaker into which they dip.

2. Switch the circuit on. You could try touching the electrodes together to show that the light comes on.
3. Add distilled water to half fill the beaker.
4. Throw a spoonful of common salt into the water. The lamp will come on.

5. Empty the beaker and wash it out including the copper electrodes. Alternatively, have a water flow through the beaker to flush away the ions.
6. Repeat (1-5) and ask the students what is going on.

Teaching Notes

• You could introduce the experiment as a mystery without any reference to Geiger counters. At the end, you could say: "You think that this is an experiment to show electrolysis. Not this time! This is a SALT COUNTER. It is a method for counting spoonfuls of salt. I throw in a spoonful of salt and the lamp lights. I clean out the beaker and refill it and throw in another spoonful of salt: the lamp lights. You count how many times the lamp lights. This corresponds with the number of spoonfuls of salt I throw in."
• The potential difference between the wires creates an electric field across the beaker of water. That field is ready to tug electric charges; positive charges (sodium ions) to the negative electrode and negative charges (chlorine ions) to the positive electrode. If there are charges there, then a current will flow. The changes are put in when the salt is added.
• Unlike the spark counter, this counter measures ions without any breakdown of the medium. The dissolved ions move sedately to their respective electrode carrying a current without a cascade effect.
• There is no amplification of ions. The only ones that move are the ones that come from the salt. With advanced students, you can make this point and suggest that, to detect ionisation from, say, a flame in the air, it will help if the number of ions is multiplied.
This links in to the demonstration

Counting matches with a Van de Graaff generator

This experiment was safety-tested in June 2007