Question

how does dry hcl do not turn blue litmus red?

Answer 1

Acids are those substances which release H⁺ ions in water and turn turn blue litmus red. HCl being a Covalent compound doesn't dissociate to furnish H⁺ ions. But when water is added to it heat energy is released which is sufficient to break the H-Cl bond and so H⁺ ions are released. Hence aqueous solution of HCl is acidic in nature and turns blue litmus red.