Question

give reason why solution of NaCl is neutral but sodium carbonate is basic

Answer 1

 When sodium chloride is dissolved in water, its ions dissociate, meaning that they separate. As a sodium ion is positive and a chloride ion is negative, the the aqueous solution is neutral. No hydronium ions (that cause acidic behaviour) or hydroxide ions (that cause alkaline behaviour) are formed. 

Answer 2

Sodium ion is positive and on the other hand chlorine ion is negative.Due presence of opposite ions no hydronium ions are produced,thus,leaving the solution neutral....