With respect to solubility in water, which type of molecules generally dissolve best in water: polar molecules or nonpolar molecules?

Introduction / Context:
Solubility is a key concept in chemistry and biology. Whether a substance dissolves in water or not depends on the nature of both the solute and the solvent. Water is often called the universal solvent because it dissolves many substances, but it does not dissolve everything. The general rule like dissolves like states that polar solvents tend to dissolve polar solutes, while nonpolar solvents dissolve nonpolar solutes. This question asks which type of molecules, polar or nonpolar, generally dissolve best in water and why.

Given Data / Assumptions:

• Water is a highly polar solvent, with a bent molecular shape and a significant dipole moment.
• Polar molecules have uneven charge distribution and can interact with water through dipole dipole or hydrogen bonding interactions.
• Nonpolar molecules have little or no permanent charge separation.
• Ionic compounds dissociate into positive and negative ions in water.

Concept / Approach:
The essential idea is that polar solvents stabilise charged or polar solutes through electrostatic interactions. Water molecules surround ions or polar solute molecules, forming hydration shells and lowering the energy of the system. As a result, many polar compounds and ionic salts are very soluble in water. Nonpolar molecules, such as many hydrocarbons, do not interact strongly with water and tend to separate, leading to poor solubility. The approach is to recall that water is polar and therefore favours dissolution of polar and ionic substances rather than nonpolar ones.

Step-by-Step Solution:
Step 1: Recognise that water has a bent shape and a strong dipole, with partial negative charge near the oxygen and partial positive charges near the hydrogens. Step 2: Polar molecules, such as alcohols and sugars, also have regions of partial positive and negative charge, allowing them to form hydrogen bonds or dipole interactions with water. Step 3: Ionic compounds, like sodium chloride, dissociate into ions (Na⁺ and Cl⁻), which are stabilised by water molecules that orient around them according to charge. Step 4: Nonpolar molecules, such as oils and many hydrocarbons, lack strong charge separation and cannot form favourable interactions with water, so they tend to be insoluble and may form separate layers. Step 5: Therefore, the type of molecules that generally dissolve best in water are polar molecules and ionic compounds, not nonpolar molecules.

Verification / Alternative check:
Everyday examples support this rule. Table salt (ionic) and sugar (polar covalent) both dissolve readily in water, forming clear solutions. Ethanol, a polar molecule, is completely miscible with water. In contrast, vegetable oil, petrol and many waxes, which are nonpolar, do not dissolve in water and often float on the surface. Biology texts also explain that water dissolves many polar nutrients and ions in blood and cytoplasm, while nonpolar lipids aggregate into membranes or fat droplets. These observations confirm that polar and ionic species are the ones that dissolve best in water.

Why Other Options Are Wrong:
Option B incorrectly states that water is nonpolar and therefore dissolves nonpolar molecules, which contradicts the well known polar nature of water. Option C claims that both polar and nonpolar molecules dissolve equally well, which is clearly false; oils and hydrocarbons are largely insoluble in water. Option D suggests that water dissolves only metals, which is not correct; many metals are not very soluble in water, while nonmetallic ionic salts dissolve easily. Option E states that only large nonpolar molecules dissolve, but in fact large nonpolar molecules tend to be even less soluble in water than smaller ones. Only option A correctly states that polar molecules and ionic compounds are most soluble in polar water.

Common Pitfalls:
A common mistake is to think that because water is so important in biology, it must dissolve everything equally well. Another pitfall is to confuse solubility of ionic solids with metallic substances, assuming that all solids behave alike. To avoid these errors, remember the like dissolves like guideline and focus on polarity. Water dissolves ionic and polar substances well by stabilising charges, but it does not dissolve nonpolar substances such as oils, which instead form separate phases or micelles when surfactants are present.

Final Answer:
The type of molecules that generally dissolve best in water are Primarily polar molecules and ionic compounds, because water is a polar solvent that stabilises charges and dipoles effectively.