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Q & A: Electrical resistance of Table Salt

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Most recent answer: 10/22/2007
Q:
What is the electrical resistance of dry table salt at room temperature?
- Jonathan
waltham
A:
Hi Jonathan,

I did some scouting around for this particular quantity, and the more I looked around the more skeptical I got. This is a very difficult question!

The simplest answer is that dry table salt at room temperature is an insulator, with a very high resistivity. Sodium chloride is an "ionic solid", in which the sodium and chloride ions alternate in a grid in space. The sodium atoms have each donated an electron, and the chlorine atoms have each received an extra electron, and the mutual attraction between the positive and negative charges keeps the crystal together. All of the available electron states are filled, and the next ones up require so much energy that free electrons cannot get there at room temperature because they don't have enough energy.

One reason why I am skeptical of quotes of measurements of the resistivity of this particular insulator is that adding water makes a conductive solution. Water dissolves the solid and allows the sodium and chloride atoms to travel freely. NaCl is "hygrosopic", in that it spontaneously absorbs water from humid air, and so to do the experiment properly, one would need to prepare a dry sample of NaCl (possibly by baking it in an atmosphere with no water in it). If water gets in the cracks in a crystal, it can form little channels in which the ions may travel. Even if there is very little water and the channels don't connect very well, it can still change the resistivity by a large fraction because there is so little to begin with. Or the water may sit on the surface of the crystal, forming a conductive path (which may not be all that conductive, but it'll be better than going straight through the crystal).

Another feature is that sodium chloride crystals may not be perfect, but they are often riddled with tiny cracks. Table salt is probably the worst example because it consists of many tiny grains of salt. Electricity flowing through such a material must hop from one grain to the next, through the corners where the crystals touch. A big crystal with a crack in it will affect the electrical conductivity. In this case, it is a bit of a toss-up: the electrical current flowing in the air around the crystals may actually be bigger than the current flowing through the crystals because table salt is such a good insulator.

Table salt melts at 801 C, and becomes an electrical conductor at that temperature because the ions may travel through the liquid.

Tom J.

(published on 10/22/2007)

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