Question

why is washing soda also called basic salt

Answer 1

Because when it is dissloved in water it gives carbonic acid which is a weak acid.

Answer 2

Washing soda or the sodium carbonate is also known as the basic salt.

Because, sodium bicarbonate is a base itself.It behaves like an alkali in it's aqueous solution and produces very weak carbonic acid.

On the other hand, sodium bicarbonate is a water soluble salt variation of the sodium.

That's why we call washing soda as the "Basic Salt."