Question

In order to carry out the decomposition reaction of water, a student passes electric current from a battery through water containing a few drops of dilute sulphuric acid. In this process, two volume of gas X are collected in a test-tube inverted over cathode (negative electrode) whereas one volume of gas Y is collected in the test-tube inverted over anode (positive electrode).
(a) Why do we add a few drops of dilute sulphuric acid to water before passing electric current?
(b) Name the gas X.
(c) Name the gas Y.
​(d) What does this decomposition reaction tell us about the composition of water?

Answer 1

(a) Decomposition of water is a very slow process and addition of dilute sulphuric acid increases the rate of decomposition of water.

$2{\mathrm{H}}_2\mathrm{O}\stackrel{{\mathrm{H}}^{+}}{\to }2{\mathrm{H}}_2+{\mathrm{O}}_2$

(b) Gas X is hydrogen, which will accumulate at the cathode, as positively charged hydrogen ions will move towards the negatively charged cathode electrode.

(c) Gas Y is oxygen, which will accumulate at the anode, as negatively charged oxygen ions will move towards the positively charged anode electrode.

(d) When electricity is passed in water, it splits water into two hydrogen ions and one oxygen ions.