Question

Indicate whether each statement is true or false: (a) NaCl dissolves in water but not in benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$ because benzene is denser than water. (b) NaCl dissolves in water but not in benzene because water has a large dipole moment and benzene has zero dipole moment. (c) NaCl dissolves in water but not in benzene because the water-ion interactions are stronger than benzene-ion interactions.

Short answer

(a) False: NaCl's solubility in water and benzene is not directly determined by the density of the solvents but rather by the interaction between solute and solvent molecules. (b) True: NaCl dissolves in water due to its large dipole moment, while it does not dissolve in benzene due to benzene's zero dipole moment. (c) True: NaCl dissolves in water because water-ion interactions are stronger than benzene-ion interactions.

Step-by-step solution

Statement (a) Analysis:

For statement (a), we need to determine if the solubility of NaCl in water and benzene is dependent on the density of benzene compared to water. In general, solubility is determined by the interaction between solute and solvent molecules, not directly by the density of the solvent. This means that the given reasoning is incorrect, and statement (a) is false.

Statement (b) Analysis:

For statement (b), we should check if the solubility of NaCl in water and benzene is dependent on the dipole moment of the solvents. NaCl is an ionic compound, and it tends to dissolve better in polar solvents, which have significant dipole moments. Water is a polar solvent with a large dipole moment, while benzene is a nonpolar solvent with zero dipole moment. The difference in the dipole moment is responsible for their ability to dissolve NaCl. Therefore, statement (b) is true.

Statement (c) Analysis:

For statement (c), we're looking to see if the solubility of NaCl in water and benzene is related to the strength of the water-ion and benzene-ion interactions. Water molecules can form strong ion-dipole interactions with the ions in NaCl, which help dissolve the ionic compound. On the other hand, benzene molecules cannot form strong interactions with NaCl ions because they are nonpolar. As a result, NaCl doesn't dissolve well in benzene. This means that statement (c) is true. In summary: (a) False (b) True (c) True

Key concepts

Dipole Moment

The dipole moment is a measure of the separation of positive and negative charges within a molecule. It is a vector quantity, which means it has both magnitude and direction. Molecules with a significant dipole moment are polar; they have a partial positive charge on one end and a partial negative charge on the other. These polar molecules are attracted to ions and other polar molecules because of their electric charges.

In the context of solubility, the dipole moment of a solvent is critical in determining its ability to dissolve ionic compounds. For instance, water, with its high dipole moment, can dissolve sodium chloride (NaCl) efficiently. The positively charged end of water molecules is attracted to the chloride ions, and the negatively charged end to the sodium ions, facilitating the dissociation of NaCl into its component ions in solution.

Ion-Dipole Interactions

Ion-dipole interactions occur when an ion forms an electrostatic bond with a polar molecule that has a dipole moment. These interactions are strong and play a crucial role in the solubility of ionic compounds in polar solvents. When NaCl is introduced to water, the sodium ions (Na+) and chloride ions (Cl-) interact with the polar water molecules. The oxygen side of water, which has a slight negative charge, is attracted to the sodium ions, while the hydrogen side, which has a partial positive charge, is attracted to the chloride ions.

These interactions are so strong that they can overcome the ionic bond in NaCl, causing it to dissolve. Without ion-dipole interactions, the ionic compound would not dissolve as readily, thus emphasizing the importance of polar solvents like water in the dissolution process of ionic substances like table salt.

Polar and Nonpolar Solvents

Solvents can be classified as polar or nonpolar based on their ability to form dipole moments. Polar solvents, like water, have a significant dipole moment and can dissolve polar substances or ionic compounds through strong ion-dipole interactions. These solvents are well-suited for dissolving salts such as NaCl due to their ability to interact with the positively and negatively charged ions.

Nonpolar solvents, on the other hand, have little to no dipole moment and cannot form such interactions. Benzene (C6H6) is an example of a nonpolar solvent and is incapable of dissolving ionic compounds effectively. Because benzene molecules do not have a charge, they are unable to interact with the ions in NaCl meaningfully. This lack of interaction explains why NaCl would not dissolve in benzene, despite benzene's high density. The inability of nonpolar solvents to dissolve ionic compounds highlights the crucial role that polarity and dipole moments play in the solubility of substances.