Water Properties

As long as the applied votage exceeds about 1.23v., the the passage of a current through water will cause electrolysis i.e. the water is decomposed into hydrogen at the cathode (negative electrode) and oxygen at the anode (positive electrode). This occurs with or without the presence of most dissolved salts. Dissolved salts will only be decomposed themselves by higher voltages, so as long as water is present, it will be preferentially discharged and the salts will merely act as conductors for the electrical current. A better result is obtained by adding an acid or alkali to the solution because the H+ (acid) and the OH- ions (alkali) have the highest conductances of all, thus lowering the resistance of the cell. In practice you don't see much gas evolution until the applied voltage exceeds about 1.5v due to a phenomenon called "hydrogen overvoltage."

Trying to electrolyse absolutely pure water is a pointless exercise. The concentration of H+ and OH- ions present is very, very small (10 to the minus 7, each). If you measure the conductivity of ultra pure water ("deionised"), you find it has a specific resistance of a massive 20 Megohms per cm. Think of it this way, the ions in solution carry the current to the electrodes, but it is the solvent (water) that gets decomposed.

Pure distilled water is an insulator.

to aid seperatio you need to add chemicals to aid the flow of electricty

http://cr4.globalspec.com/search/sitesearch?do=show&sort=textmatchrank&srch=catalyst&srchobjs=t%2Cbe%2Cc%2Ctg%2Cu&order=asc

been covered i beleive

http://cr4.globalspec.com/search/sitesearch?do=show&sort=textmatchrank&srch=hho&srchobjs=t%2Cbe%2Cc%2Ctg%2Cu&order=asc

<...Pure distilled water is an insulator....>

Only, distilled water ain't so pure.

Ultrapure water production for the semiconductor industry might use typically 17 stages of processing starting from town mains water before being distributed to users. A resistivity of 18.2MΩcm at 20degC would be typical. It still conducts electricity, only very little of it.

i wouldnt like to try it. but

http://www.google.co.uk/search?q=distilled+water+conductivity&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7GZEU_en&redir_esc=&ei=mtyATPWkDZCTjAf3lK2BAQ

There is a good article on electrolysis in Wikipedia.