Question

a teacher assigns the following experiment to a class. draw an outline of a bird’s feather, and color one half of the outline with a thick coat from a wax crayon. leave the other half uncolored. next, sprinkle a few drops of water on both halves of the feather. you will observe that the water soaks into the plain paper. the waxy coat, however, allows the paper to remain dry. water forms tiny beads on top of the wax instead of soaking through it. the results of the experiment most strongly support which conclusion about the polarity of lipid molecules?
lipid molecules are polar because they mix poorly with liquids, such as water.
lipid molecules are polar because they adhere tightly to smooth surfaces, such as paper.
lipid molecules are nonpolar because they repel water molecules, which also are nonpolar.
lipid molecules are nonpolar because they repel water molecules, which are polar.

Explanation:

1. Recall the principle of "like dissolves like": Polar substances interact favorably with polar substances, and nonpolar substances interact favorably with nonpolar substances. Water is a polar molecule (due to its bent shape and electronegativity difference between O and H, leading to a dipole moment).
2. Analyze the experiment: The wax (a lipid) repels water—water forms beads on wax, meaning they don't mix.
3. Evaluate each option:

• Option 1: Incorrect. If lipids were polar, they should mix with polar water. The experiment shows they don't mix.
• Option 2: Incorrect. The experiment is about interaction with water, not adhesion to paper. Also, polar/nonpolar is about molecular charge distribution, not surface adhesion.
• Option 3: Incorrect. Water is polar (not nonpolar). The "like dissolves like" principle would fail here if lipids were nonpolar and water was nonpolar (they should mix, but they don't).
• Option 4: Correct. Lipids (wax) repel water. Since water is polar, and nonpolar substances repel polar substances (due to "like dissolves like"—nonpolar and polar don't mix), this supports lipids being nonpolar.

Answer:

Lipid molecules are nonpolar because they repel water molecules, which are polar.