(a) In electrolysis of water $(\mathrm{H}_2\mathrm{O})$, the hydrogen goes to one test tube and oxygen goes to another. The two electrodes collect $\mathrm{H}$ and $\mathrm{O}$ separately.Since water $(\mathrm{H}_2\mathrm{O})$ consists of 2 parts of hydrogen and 1 part of oxygen, so the volume of hydrogen gas $(\mathrm{H}_2)$ collected over cathode (negative electrode) is double the volume of oxygen gas $(\mathrm{O}_2)$ collected over anode (positive electrode).(b) (i) When potassium iodide solution is added to lead nitrate solution, then a yellow precipitate of lead iodide is produced along with potassium nitrate solution. (ii) This is double displacement reaction.