2.12   explain, in terms of dissociation, why hydrogen chloride is acidic in water but not in methylbenzene

Methylbenzene is a hydrocarbon, similar to alkanes found in petrol. It is a non-aqueous solvent. The HCl molecules dissolve but the H-Cl bonds do not break like they do in water. This means that it does not form H⁺ ions so the solution is not acidic.  It does not change the colour of dry blue litmus paper.

A solution of HCl in water (hydrochloric acid) DOES change blue litmus paper to red because there are H⁺ ions present.