Yes, CAUSTIC is a Scrabble word!

Letter values in Scrabble US: C 3, A 1, U 1, S 1, T 1, I 1, C 3.

Is it a valid word?

CAUSTIC is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

harsh or corrosive in tone
of a substance, especially a strong acid; capable of destroying or eating away by chemical action