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Name: Rebecca
Status: educator
Age: 4-5
Location: CA
Country: N/A
Date: 1/14/2005


Question:
Hi, I am looking for a way to explain to fourth graders why water conducts electricity. Would it be accurate to say that water makes the electrons in dissolved materials (especially salts) act like the electrons in metals, in the sense that the electrons in the salts are free to "swim" around once they are in the water?


Replies:
Rebecca,

Your approach is a good one for grades 4-5. When salts dissolve in water, they form ions (NaCl becomes Na+ and Cl-, for example) and it is these ions that carry charge.

Greg (Roberto Gregorius)


Electricity is the flow of charges. Many minerals and gases will readily dissolve in water. It is the presence of this minerals and gases that allow water to conduct electricity. Perfectly pure water without these impurities will not conduct electricity. This is why salt water is a good conductor of electricity.

Grace Fields


Rebecca,

This is not quite right. The electrons in salt water are localized on the salt ions and are not free to move through the water like electrons move through a metal.

When salts are dissolved in water they break up into electrically charged atoms (ions) which can then move through the water. If you put two wires in the salt water and charge them (+ and -) with a battery, negatively charged ions (anions) will move towards the positively charged wire and positively charged atoms (cations) will move towards the negatively charged wire. The movement of the ions carries charge between the wires.

Best, Dr. Topper


I think you are on the right track. I would add the following points:

1. VERY PURE water is a poor conductor of electricity. However, very small amounts of impurities, especially salts, greatly increase the electrical conductivity of water.

2. The main carriers of electrical current are dissolved ions. If a battery is connected to a container of water so that the (+) and (-) poles of the battery are immersed in the water, the positive ions will migrate toward the (-) pole and the negative ions will migrate toward the (+) pole (electrical opposites attract). This closes the "switch" and electric current flows through the solution.

3. Different ions can help carry the electric current, some more / some less, depending upon their electrical charge and their size. More highly charged, smaller ions are more efficient carriers of the electrical current.

4. The higher the concentration of ions, the greater is the conductance of the solution.

5. There is a complication however, the water and / or certain of the ions may undergo a chemical reaction to one or the other, or both of the electrodes. You may not want to introduce this in the beginning but it does happen.

6. None of the ion electrical current carriers of salts dissolved in water are as mobile as "free" electrons in a metal are. And the differences are very large. For example, if the conductivity of VERY PURE WATER is 0.000004 conductivity units, the conductivity of a saturated solution of common table salt NaCl is 23 conductivity units, and the conductivity of a typical metal such as copper wire is 60, 000, 000 conductivity units. So while the ions in a dissolved salt behave something like "free" electrons, they are very slow and sluggish moving. This is probably more than you wanted to know, but you can adapt the details to fit your lesson plan on the subject.

Vince Calder


Completely demineralized water actually does not conduct electricity, and is widely used as a coolant in high powered radio amplifiers. However, when exposed to even small amounts of impurities, (even normal air), It becomes a fair conductor. It is not the water conducting this electricity so much as the dissolved minerals in it. Some of these minerals are decent insulators themselves, but when dissolved in water still allow conduction because the molecules are now free to move around.

(For a more thorough, and technically accurate answer you would have to ask a chemist, but this should suffice for a 4th grade level.)

Ryan Belscamper


I suppose it is fair enough. I hope you can afford to add that in ionic liquids these electrons remain stuck to big, round atoms, and these atoms do the swimming around, relatively slowly, dragged by their electric charge. Sort of a ball and chain thing. Electrons in metal are "free"; they slip rather freely across all the atoms, and do not need to drag one around with them. So, carrier mobilities in ionic liquids are some orders of magnitude lower than carrier mobilities in metals. So their resistance is higher.

If you had a snail-trail of salt-water or molten salt with the same length and cross-section as a metal wire, it would use up half the voltage and get hot trying to pass one tenth the current the wire can handle.

In a metal, each metal atom contributes about one mobile electron that helps the current flow. In molten salts like NaCl, every atom has a + or - charge and contributes to the current flow. So there are just as many carriers per unit volume as in a metal. But in water-solutions of salts, the salt is a minority. Usually less than 10% of the solution is ions, so that makes the conductivity even poorer. That is part of why we will never make our power lines out of seawater in pipes.

Jim Swenson



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