Final answer:
The best way to determine whether a solid dissolved in water is a molecular or an ionic substance is by observing its conductivity and dissolution behavior. Ionic compounds have higher melting points, dissociate into ions in water, and the resulting solution conducts electricity. Molecular compounds do not conduct electricity and dissolve as neutral molecules; their water solubility varies with intermolecular forces.
Step-by-step explanation:
The question of whether a solid that dissolves in water is a molecular substance or an ionic substance can be best determined by observing a few key properties of the substance when it dissolves. Ionic compounds, which are made up of positively and negatively charged ions, tend to have higher melting points than molecular compounds due to the strong ionic bonds that must be broken. Additionally, ionic compounds are strong electrolytes because they dissociate into ions when they dissolve in water, resulting in a solution that can conduct electricity. Molecular compounds, which are made up of molecules held together by covalent bonds, generally do not conduct electricity when dissolved because they do not break into ions but rather separate into individual neutral molecules.
For example, when sodium chloride (NaCl) dissolves in water, it dissociates into sodium ions (Na+) and chloride ions (Cl−), creating a solution that is able to conduct an electrical current. On the other hand, molecular compounds like glucose (C6H12O6) dissolve as entire molecules and do not increase the conductivity of the solution significantly. Solubility in water and the ability to conduct electricity when dissolved are the key indications of whether a substance is ionic or molecular.
Furthermore, molecular compounds tend to have lower solubility in water compared to ionic compounds, although this can vary widely depending on the intermolecular forces present in the molecular compounds. Substances like sugar, which form hydrogen bonds with water molecules, are generally water-soluble, while those with only London dispersion forces tend to be less soluble.