Sodium chloride is very soluble in water but insoluble in liquid hexane, \(\mathrm{C}_{6} \mathrm{H}_{14} .\) Why is this so?

Understanding why certain substances dissolve in one solvent but not in another can be a bit like figuring out social dynamics in the school cafeteria. In the same way that people often group together with those who share similar interests, molecules prefer to mingle with others that have related properties, especially when it comes to polarity. The adage 'like dissolves like' is a simplified principle that guides us in predicting solubility.

For example, imagine you're trying to mix oil and water for a salad dressing. No matter how vigorously you shake the bottle, the oil always separates from the water. That's because oil is nonpolar and water is polar, and their distinct properties keep them from mixing well. Similarly, polar solutes, substances to be dissolved, blend better with polar solvents, the liquids that do the dissolving. This is why table salt (NaCl), a polar compound, goes well into water, a polar solvent, just like friends with shared hobbies. On the other hand, when we try to mix it with hexane, a nonpolar solvent, it's like a jock trying to discuss Shakespeare with a group of theater buffs—not a great match.

Picture a magnet—there's a clear north and south pole. Each end has a specific charge, positive or negative, much like how polar molecules have areas of different charges. Water is the classic example: it has a negative end around the oxygen atom and a positive end near the hydrogen atoms. These differences in charges create an uneven distribution, making the molecule polar.

Now imagine a pack of neutral batteries, all lined up. They don’t have clear positive or negative ends. That's akin to nonpolar molecules, like hexane. Because hexane molecules don't have an imbalance of charges, they aren't drawn to the charged particles in a salt crystal. This explains why trying to dissolve sodium chloride in hexane is like attempting to get a cat to enjoy swimming; their inherent natures just don't mix.

Diving deeper into the molecular world, we encounter various types of forces that explain how substances interact. An essential one for understanding solubility is the ion-dipole interaction. It's a bit like social networking where opposites attract: positive charges seek out negative ones. In the context of solubility, when you stir table salt into water, the positively charged sodium ions (Na⁺) are attracted to the slightly negative oxygen ends of the water molecules, while the negatively charged chloride ions (Cl⁻) are drawn to the slightly positive hydrogen ends.

These attractions help pry apart the sodium and chloride ions from the solid crystal, allowing them to disperse throughout the water and thus, dissolve. It's akin to a group of friends encouraging two shy people at a party to start talking to each other, facilitating a new connection. These connections between opposites end up being strong enough to keep the salt particles in solution, demonstrating the power of ion-dipole interactions.