Discover Questions

Why are HCl (s) and HCl (aq) named differently and why do they act differently? What experimental observations indicate that a chemical reaction have occured during the experimental? Prove that Sin( [π/2] - x) = cos(x)? Help, double integrals, I cant get answer:
∫[from 0 to infinity] (tan−1(5x)−tan−1(2x))/(x) dx. I got:
∫[from 2 to 5] (1/(1+(xy)^2)) dy.? I have blocked my sink with caustic soda will pouring hydrochloric acid down dissolve it?