* Toluene is a nonpolar solvent: Toluene is a hydrocarbon, and its molecules are nonpolar.
* HCl is a polar molecule: HCl is a polar molecule, with a positive charge on the hydrogen atom and a negative charge on the chlorine atom.
* "Like dissolves like": Polar molecules tend to dissolve in polar solvents, while nonpolar molecules dissolve in nonpolar solvents.
Since toluene is nonpolar, it can't effectively solvate the polar HCl molecules. This means that HCl doesn't dissociate into H+ and Cl- ions in toluene, as it would in a polar solvent like water.
For litmus paper to turn red, it needs to come into contact with H+ ions. Because HCl in toluene doesn't produce free H+ ions, the litmus paper will remain blue.
