ChemixaLEARN CHEMISTRY

Splitting water with a 9V battery

Objective: electrolyze water with a 9V battery and graphite electrodes to produce hydrogen and oxygen gases in a 2:1 volume ratio. You will build the cell, collect both gases, and connect the volume ratio to the formula of water.

Safety:

Use a 9V battery ONLY: never use wall outlets, larger batteries, or phone chargers. An adult must build the circuit and supervise continuously. Work in a ventilated room and keep all flames and sparks away, because hydrogen is flammable. Use baking soda as the electrolyte; never use table salt, which can release chlorine gas. Disconnect the battery when finished.

Materials

  • 9V battery (maximum voltage allowed)
  • Two pencils with exposed graphite at both ends
  • Clear glass or cup
  • Water
  • Baking soda
  • Two small test tubes or narrow vials
  • Electrical tape or clips
  • Shallow dish

Procedure

1

Make the electrolyte

Dissolve a teaspoon of baking soda in a glass of water to make the electrolyte solution.

2

Prepare graphite electrodes

Strip both ends of two pencils to expose about 2 cm of graphite on each end.

3

Fill the collection tubes

Fill two small tubes completely with the solution, cover the openings with a finger, and invert them into a shallow dish of the solution so no air enters.

4

Connect the battery (adult)

An adult connects one pencil lead to each battery terminal with tape or clips, keeping the two electrodes apart.

5

Collect the gases

Slide one graphite tip up into each inverted tube and watch bubbles collect at the top of each tube.

6

Compare and disconnect

After several minutes, compare the gas volumes in the two tubes before disconnecting the battery.

What to observe

  • Bubbles stream from both pencil tips as soon as the circuit connects.

  • The tube over one electrode fills about twice as fast as the other.

  • The faster-filling tube holds hydrogen; the slower holds oxygen.

  • Bubbles stop immediately when the battery is disconnected.

Why it happens

Electric current forces water to split in a reaction that would not happen on its own: 2H2O -> 2H2 + O2. At the negative electrode (cathode), water molecules gain electrons and release hydrogen gas; at the positive electrode (anode), water loses electrons and releases oxygen gas. Because the balanced equation makes two H2 molecules for every O2, the hydrogen tube collects about twice as much gas, a visible proof of water's H2O formula. Baking soda dissolved in the water acts as an electrolyte, carrying charge through the solution so the current can flow. Graphite pencil leads work as inert electrodes that conduct electricity without being consumed.

Try variations

  • Time how long it takes to fill each tube and calculate the ratio.

  • Test distilled water with no baking soda and observe the difference.

  • Research how this same reaction runs in reverse inside a fuel cell.