A quick question about breaking H2O in twain with electricity

OK, so I decided to do some ‘Fun with Household Stuff’ with my nine-year-old, science obsessed daughter.

  1. Small cup of water
  2. Iodized salt
  3. Two pencils, graphite
  4. 9 Volt Battery

I put about a teaspoon of salt into the water and mixed it up. Then placed the pencils through some cardstock to make sure they stayed the same distance apart. One end of the pencils I placed in the cup, at the bottom of the water, the others stuck in the air. Place the two ends of the 9-volt battery on the two ends of the pencils.

Voila! Bubbles begin forming in the water at the tips of the pencils. We are using electricity to break the water molecules in half. That is, the hydrogen and the oxygen go their separate ways and head for the surface of the water, being less dense and all.

Two questions have occured following the amazement of the nine-year-old…

  1. Are we breaking the H[sub]2[/sub]O into H[sub2[/sub] and O or do all three atoms become independent?
  2. Lady Chance seems convinced that one of the pencils is generating the oxygen bubbles and the other is generating hydrogen bubbles. I say she is…um…less than correct in her assessment. This has caused some amateur science tension in the Chance houshold.

Can any chem dopers field these questions?

Oh, and science-kid wants to know if there’s a way to just capture the hydrogen atoms being released. I’d be interested to know a way but fear her undoubtedly explosive plans for the hydrogen, once captured.

  1. You are ending up with H[sub]2[/sub] and O[sub]2[/sub]. After you split the H[sub]2[/sub] and the O the Os don’t stay separate on their own.

  2. Go apologize to Lady Chance. Oxygen forms around one pencil and hydrogen around the other (oxygen forms around the one connected to the positive terminal on the battery).

ETA: (forgot the last question)

Instead of using pencils, use wires. Curve the ends of the wires upwards and place something over the top of them to collect the bubbles. In your typical chemistry class demonstration they’ll use test tubes.


If I remember correctly from my Physics class eons ago, one lead will be releasing oxygen and the other hydrogen, which is which probably depends on the direction of the current.

If you somehow put an inverted container over each pencil so that it traps the bubbles from only one, you could capture the oxygen and the hydrogen seperately.

The oxygen will gather over one electrode and the hydrogen over the other. That is why the apparatus used for this have a chamber over each electrode. This is also a safety feature as with one chamber you have an explosive mixture.

you can place a funnel over either electrode and capture the gas into a balloon.

IIRC the standard school lab method for testing for O[sub]2[/sub] is to extinguish a lighted taper and quickly plunge into the gas. If it is oxygen it should relight. To test for H[sub]2[/sub] plunge a lit taper into the gas and it should ignite with an audible pop.

Not exactly. In fact…

With just table salt and water you get hydrogen and chlorine plus sodium hydroxide in solution (which will eat at ordinary metal electrodes).

   2 NaCl + 2 H2O → Cl2 + H2 + 2 NaOH

Note that hydrogen atoms pair up quickly when formed. As does chlorine (and oxygen if you are forming that).

Nor do the Hs. In water, each hydrogen is bonded to the oxygen - the hydrogens aren’t bound to each other. So splitting the molecule releases free H atoms and free O atoms, which quickly - i.e. all but instantly - recombine into H2 and O2.

Most home experiments doing this don’t have much table salt in the water – just enough to make it conducting. With little NaCl in the mix, you don’t get sizeable amounts of chlorine. I know – I’ve done electrolysis of water with the shaved pencils and batteries.

Not exactly or each electrode would have a mixture of hydrogen and oxygen. Rather molecular oxygen and hydrogen cations are formed at one electrode and molecular hydrogen and hydroxide anions are formed at the other.

The side with about double the gas volume is the hydrogen.

Which reminds me of a question I have but have never though to ask when I was around people that might know.

Where is the molecule when it’s ripped apart? The cathode or anode? Somewhere in between?

And how do the atoms wind up at their designated collection points? Backstroke? Shuttle bus?

It’s not a single molecule being ripped apart. One molecule is oxidized at the anode, another is reduced at the cathode. It happens pretty close to the surface of the electrode.

Half and half.

In the case of water it’s always ripping itself apart and reforming. Simplistically there is a very very small amount of H+ and OH- in even the purest water, and it is these ions that react to form the H2 and O2 gasses. H+ at the anode and OH- at the cathode.

Since the concentration of H+ and OH- ions in pure water is so low you get almost no reaction in pure water and you need a salt (like NaCl) to help.

Thery’re already there.

I think you might be correct if you were using a stepped-down AC power source (such as a model train transformer [WARNING, COULD BE DANGEROUS, TRY AT YOUR OWN RISK]) instead of a battery. In that case the bubbles from either electrode would be generating hydrogen and oxygen alternating every 1/120 second.

Chem experts, please correct me if necessary.

ETA: the balanced equation is 2H[sub]2[/sub]O -> 2H[sub]2[/sub] + O[sub]2[/sub]