Surface Tension in AP Biology

Step 1: Polar O-H bonds give water charged ends

Everything about surface tension starts inside a single water molecule. Oxygen is more electronegative than hydrogen, so it pulls the shared electrons in each O-H bond closer to itself. That makes each bond a polar covalent bond, with a partial negative charge on the oxygen and partial positive charges on the two hydrogens.

Because the molecule is bent, those charges don't cancel out, so the whole molecule is polar. The partial positive hydrogen of one molecule is attracted to the partial negative oxygen of an adjacent molecule. That attraction is a hydrogen bond.

This chain is the core of Unit 1: Chemistry of Life, Topic 1.1 (Structure of Water and Hydrogen Bonding). The topic asks you to explain how polarity leads to hydrogen bonding, and how hydrogen bonding produces the properties living things rely on: high specific heat, high heat of vaporization, cohesion, adhesion, and surface tension.

Step 2: Surface molecules get pulled inward and sideways

Deep in the liquid, each water molecule is hydrogen-bonded to neighbors on every side, so the pulls roughly balance. A molecule at the surface has no water above it, only air. Its hydrogen bonds pull it sideways and down, so there's a net inward pull.

That uneven pull packs the surface molecules tightly into a film that resists being pushed through. Small, light objects that press on the surface without breaking it get held up. A carefully placed steel needle can rest on water even though steel is much denser than water, because the film supports it.

A multiple-choice item could describe a scenario like this and ask which property explains it. The strongest answer choices trace the observation back to hydrogen bonds between adjacent water molecules, rather than just naming the property.

Step 3: Detergent breaks the film (an investigation setup)

Detergents and other surfactants have a polar region and a nonpolar region. They collect at the surface and get between water molecules, which reduces hydrogen bonding at the surface and lowers surface tension. Water striders, which rely on surface tension and water-repellent legs to stay up, start sinking.

This setup works well for the investigation-style free-response questions. On Question 3 (Scientific Investigation) you might identify the independent variable (detergent concentration), the dependent variable (mass needed to break the surface film, or number of striders supported), and the null hypothesis. A solid null hypothesis states no effect: "Adding surfactant has no effect on the number of water striders the pond surface supports." Controlled variables include water temperature and the volume and source of the water.

Data questions (Question 1 and Question 6) can add a calculation. If pure water's surface tension is 72 mN/m and a pollutant lowers it to 24 mN/m, the ratio of original to polluted is 72 ÷ 24 = 3. The polluted water has one-third the surface tension.

On Question 4 (Conceptual Analysis), the disruption is the surfactant. A top answer moves through the full chain: fewer hydrogen bonds between surface molecules, then weaker surface tension, then the surface can no longer support the insect's weight, so fewer striders stay up. You may also be asked to evaluate an alternative explanation, such as a claim that the surfactant sinks striders by lowering water's density. Measuring both density and surface tension in treated and untreated water can check whether density actually changed. To test which property causes the sinking, though, you'd need a design that varies one relevant factor while controlling for the other.

Three surface tension mix-ups that cost points

Watch for these mix-ups in written explanations about surface tension:

• Covalent vs. hydrogen bonds. Surface tension comes from hydrogen bonds between water molecules. Detergent doesn't break the polar covalent O-H bonds inside each molecule. If your answer says covalent bonds hold the surface together, it's wrong.
• Cohesion vs. surface tension. Cohesion is water-to-water attraction anywhere in the liquid. Surface tension is what that cohesion produces at the air boundary. Adhesion is different again, since it's water attracted to other polar or charged surfaces.
• Floating vs. being held up by the surface. A strider isn't buoyed up because it's less dense than water. It's held up by the surface film, which is why lowering surface tension sinks it even though the water's density barely changes.