Which of the following is less soluble in water, and why?

• A. A phospholipid, because it contains a polar phosphate group.
• B. A fat, because it is mostly nonpolar and lacks hydrophilic groups.
• C. A fat, because it contains a polar head group.
• D. A phospholipid, because it is entirely nonpolar.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the molecular structure of fats and phospholipids. Fats (triglycerides) are primarily composed of long hydrocarbon chains, making them mostly nonpolar and hydrophobic (water-repelling). Phospholipids, on the other hand, have both nonpolar tails and a polar phosphate-containing head group, making them amphipathic (having both hydrophobic and hydrophilic parts).
2. Step 2: Recall the principle 'like dissolves like' in chemistry, which means polar substances tend to dissolve well in polar solvents like water, while nonpolar substances do not dissolve well in polar solvents.
3. Step 3: Analyze the solubility of fats in water. Since fats are mostly nonpolar and lack hydrophilic (water-attracting) groups, they do not interact favorably with water molecules and thus have low solubility in water.
4. Step 4: Analyze the solubility of phospholipids in water. The polar phosphate head group of phospholipids can interact with water molecules through dipole-dipole interactions and hydrogen bonding, increasing their solubility compared to fats.
5. Step 5: Conclude that fats are less soluble in water than phospholipids because fats are mostly nonpolar and lack hydrophilic groups, while phospholipids have a polar head group that enhances their interaction with water.