Aqueous solution of sodium sulphate gets hydrolysed to form sodium hydroxide and sulphuric acid which are strong base and strong acid respectively. So, its aqueous solution is neutral. When sodium carbonate is dissolved in water, it gets hydrolysed to some extent and forms sodium hydroxide and carbonic acid. Now sodium hydroxide is a strong base which is fully ionised and gives a large amount of hydroxide ions [OH−(aq)]. On the other hand, carbonic acid is a weak acid which is only slightly ionised and hence gives a small amount of hydrogen ions [H+(aq)]. The mixture contains more of hydroxide ions than hydrogen ions, so it is basic in nature.