Becausem in the process, it undergoes the electrolysis process. In the positive terminal of the setup, i.e. the anode, it attracts the anions towards there. As in the solution, it contains the carbonate ions and the hydroxide ions. While the hydroxide ion has a higher position so it's preferentially discharged to give out the Oxygen, the ionic half equation is shown in what follows:
While in the negative terminal, i.e. the cathode, it attracts the cations. In the solution, it contains the Sodium ions and the Hydrogen ions. As the hydrogen ion has a higher tendncy to accept electrons than that of the Sodium ions, so it discharges and give out the Hydrogen. the ionic half equation is shown in what follows:
2 H+(aq) + 2e− → H2(g)
As a result, the product mixture contains the Hydrogen and Oxygen gas. When a burning splint is given to it, it gives a mild explosion and give a 'pop' sound.