Gas collecting at the top of each tubeCarbon electrodes in water with a little acidSix 1.5 V cells and a plug keyElectrolysis of water. A current through water with a few drops of dilute sulphuric acid splits it: 2H2O → 2H2 + O2. Hydrogen bubbles off the cathode (the rod joined to −) and oxygen off the anode (joined to +), and each gas pushes the water down its tube. Twice as much hydrogen collects as oxygen, because each water molecule gives one H2 and half an O2. Faraday's law sets the rate: each ampere for one second gives 0.127 ml of hydrogen at room temperature, so more acid or more volts means more current and faster bubbling, while pure water hardly conducts at all. Below about 2 V nothing happens. Hydrogen held to a flame burns with a pop; oxygen relights a glowing splint. Time runs faster than life, as the readout shows.