Sodium is a very reactive metal. Hence it reacts with water to form hydrogen gas and large aount of heat. This makes hydrogen gas to catch fire. Hence explosion could result. Thus to prevent explosion sodium piece should not be thrown in sink.

(a) Anodising is a process of forming a thick oxide layer of aluminium. During anodising, a clean aluminium article is made the anode and is electrolysed with dilute sulphuric acid. The oxygen liberated at the anode reacts with aluminium to produce a thick protective oxide layer on its surface. Also the oxide layer can be dyed easily to give an attractive finish to the aluminium articles.

(b) Aqua regia is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3: 1. It is also called royal water. It is highly corrosive and faming liquid. It can dissolve noble metals like gold and platinum.

Take copper sulphate solution in a test tube and place a silver few granule in it. After some time you observe that there is no change in the test tube, i.e., blue colour of copper sulphate does not fade away. This shows silver is less reactive than copper.

Ag(s) + CuSO₄(ag) → No reaction