h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Hmmm... we were always told, at highschool, that pure water doesn't conduct electricity at all... it's the impurities in the water which lead to its ability to conduct electricity, what with ions being free to move around and everything.

Whether this is true or not, I've no idea.

26199

I've always understood water to be a better conductor when pure. Hence distilled water in batteries, but then what do I really know anyway.


Runs off to fill bath with distilled water, apply electric fire and jump in. (Always willing to experiment)

oh my stars! i do not know if i want to find out how that experiment turned out but i did want to pop over to tell you that i did greatly enjoy your article i must sadly admit that i never thought much about how soap worked, i only used it and expected it to do what the container promised (which does not always happen) but after reading this article i think i might find myself looking a little closer at what is happening right under my nose
AA

mmmmmm... soap

Tis not so, sir Hieroglyphic. The main ingredient in a car battery is a fairly concentrated solution of sulphuric acid. The reason that you top up car batteries with distilled water is that, under the conditions found in the engine bay, water evaporates and acid does not. Thus, as water evaporates, the acid gets more and more concentrated; you add distilled water to put the mixture back to rights. Incidentally, the reason you shouldn't use tap water is that the acid will react with the salts in the water and fill your battery full of sulphate crystals.

this i feel is one of the reasons i so enjoy my time in h2g2, you learn something new everyday
AA

Pure water is not a very effective conductor because under normal conditions only a small percentage (about 0.001%) of water molecules will auto-ionize. Auto-ionization is the name of the mechanism that causes molecules of water with enough free energy to break up into H+ and OH-. This ionization is effectively a separation of charges and thus allows conduction of electrons (hence conduct, as electricity is a flow of electrons)

Ions in solution are a much better conductor, as they are free to move and facilitate the electron movement, hence allowing a current to flow. Ion movement (not ion migration, which is something to do with surface wetting of thin colloid wafers) is the principle behind car batteries.

Inpurities in water, such as salt are often termed "electrolytes." This name was coined because it is these impurities which conduct an electrical current. If you had a glass full of water that had no impurities and only contained hydrogen and oxygen it would not conduct electricity in the slightest. Sprinkle a bit of salt in this glass and you will be again able to conduct a current. If you are enterprizing you may test this by trying to complete the circuit of a small light bulb through a glass of distilled water. Poor salt in and see what happens.

Fascinating, boy you can learn some stuff.

PAY ATTENTION OUT THERE, THERE'S INTERESTING STUFF HERE!!!!!

Pure water does not conduct electricity. You add distilled water to car batteries to prevent changing the equilibrium already established in the battery - as it contains hydrochloric acid it contains many ions and hence is able to conduct electricity.

Only water with dissolved ions is able to conduct electricity.

Actually, pure water does lead a current although it is almost to weak to measure. In normal water, 1 out of 10.000.000 molecules will be ionized (split into H3O+ and OH-) and these ions can lead electricity. This will happen even in pure distilled water, but because the ratio is so low it is very hard to measure.

Yes, you've spotted a slight inaccuracy in the article.

Water doesn't tend to "split into positive and negative ions" very easily - but it does contain mini electrical fields because, as the article states, water is "polar", that is it's slightly electrically assymetric - one end of every (unsplit) water molecule is positively charged and the other negatively. IIRC, the Oxygen end should be negative. Whatever, doesn't matter much. The important thing is that this local separation of charge is enough to help dissolve ionic compounds more easily than a non-polar solvent (such as benzene) would manage.

But to clarify, even pure water does contain H+ and OH- ions (though H+ tends to join with another water molecule to make H3O+)

pH is the measure of the number of H+ (or H30+) ions in a solution - in fact, it is minus the log (base 10) of the concentration of H+ ions in moles per litre.

Pure water has a pH of about 7 - so it has an H+ concentration of 0.0000001 moles per litre. A litre of water is 55 moles (1000/18), so (in something approaching English) around one molecule in 550 million is split (I think - it's about 20 years since I did this sort of thing).