You can differentiate between ionic, covalent and molecular compounds by the fact that ionic compounds contain elements that include both a metal and a nonmetal. Molecular compounds contain both non metals covalently bonded to each other. While acids most often on their chemical formula start with the element of Hydrogen - H.
For Eg - sulphuric acid
H2SO4
hydrochloric acid - HCl.
Changing the volume increases the area that the molecules collide with so the force is spread over a larger area.
