Solubility depends on the type of substance the solute and solvent are. If the solute and solvent are similar in their molecular characteristics (i.e. one ionic and one polar; both are polar; or both are nonpolar), the solute will dissolve in the solvent. If the two are dissimilar in their molecular characteristics (i.e. one is either ionic or polar and the other is nonpolar), the solute will be insoluble in the solvent.
Physical properties allow us to identify an unknown substance, which is the purpose of this lab.