Question: (a) In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode?
(b) (i) What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube?
(ii) What type of reaction is this?
(iii) Write a balanced chemical equation to represent the above reaction. [CBSE, Term 1, Set 2, 2015]
Answer: (a) In electrolysis of water (H₂O), the hydrogen goes to one test tube and oxygen goes to another. The two electrodes collect H and O separately. Since water (H₂O) consists of 2 parts of hydrogen and 1 part of oxygen, so the volume of hydrogen gas (H₂) collected over cathode (negative electrode) is double the volume of oxygen gas (O₂) 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.
(iii) Pb(NO₃)₂ (aq.) + 2KI (aq.) → PbI₂(s) + 2KNO₃ (aq.) (yellow ppt)