Comparing Attractive Forces Simulation Answer Key

Understanding the PhET Comparing Attractive Forces Simulation

The PhET interactive simulation titled "Comparing Attractive Forces" is a chemistry education tool that lets you visualize how different types of intermolecular forces behave as molecules move closer together. It plots force versus distance for four scenarios: nonpolar nonpolar, polar nonpolar, polar polar, and ion ion. The worksheet that goes with it asks students to identify which curve corresponds to which interaction type and answer questions about boiling points, molecular polarity, and the relative strength of those forces. I went through this simulation about three years ago with a group of AP Chemistry students, and the most common point of confusion wasn't the concept itself. It was that the simulation's force axis is drawn in a way that makes the attractive region go downward, which trips people up who expect "positive" to mean "stronger attraction." The deeper curves represent stronger attraction, but because they're plotted below the zero line, visually it looks inverted. I had students flip the interpretation themselves after the first graph, and once they caught that, everything else fell into place.

Comparing Attractive Forces Simulation Answer Key

Here is the breakdown of what each curve represents and the typical answers expected on the worksheet. Curve Identification: The steepest curve (the one that drops furthest below zero at close range) represents ion ion interactions. This makes sense because ion charges create the strongest electrostatic attraction of the four scenarios. The next curve down is polar polar, which includes dipole-dipole forces plus whatever London dispersion forces are present. Above that is polar nonpolar, representing dipole-induced dipole interactions, which are weaker still. The shallowest curve is nonpolar nonpolar, showing only London dispersion forces, sometimes called induced dipole-induced dipole interactions.

Why the order matters: The simulation reinforces a concept that students often miss on tests: the difference between intramolecular and intermolecular forces. The question that trips people up most asks something like "which substance has the highest boiling point" and lists HCl, F2, NaCl, and CH4. The answer is NaCl, but students will pick HCl because they're thinking about the covalent bond within the molecule instead of the forces between molecules. The simulation doesn't directly test this distinction, but it sets the foundation for it. Common worksheet questions and answers:

When asked to explain why the ion ion curve is steepest, the expected answer references Coulomb's law — the force is proportional to the product of the two charges, and full ionic charges produce much larger forces than partial dipole charges. For the question about what happens to the attractive force as distance increases, the answer is that it decreases, and it decreases faster for weaker interactions. The simulation shows this through the gradual flattening of each curve as you move right along the distance axis. A practical tip: Some versions of the worksheet ask students to rank substances by boiling point using the simulation. If the list includes something like H2O, H2S, and H2Se, the simulation alone won't show hydrogen bonding explicitly as a separate curve type. You have to infer it from the polar polar category and bring in prior knowledge that water has an unusually high boiling point for its molar mass because of hydrogen bonding. I found that pre-teaching this distinction before students access the simulation cut my explanation time in half. Instead of debugging their confusion during the activity, they just connected the dots themselves.

Where the simulation falls short: The main limitation is that the force-distance graphs are schematic, not quantitative. The axes aren't labeled with actual values in nanometers or nanojoules, so students sometimes treat the curves as if they represent real data rather than a conceptual model. I've seen learners argue over the exact shape of the nonpolar nonpolar curve as though it were experimental data. It's not. The curve is illustrative. If you need real quantitative data for intermolecular forces, you'd look at Lennard-Jones potential parameters or empirical boiling point tables instead. The simulation is designed for concept building, not data analysis. Another gap is that the simulation doesn't include hydrogen bonding as a separate category. It's lumped under polar polar, which is technically correct but pedagogically confusing when the subsequent unit treats hydrogen bonding as its own thing. I recommend pairing the simulation with a brief lecture or reading that explicitly calls out hydrogen bonding before students complete the worksheet. Otherwise you get answers like "water has strong polar polar forces" when the more precise answer the rubric is looking for is hydrogen bonding.

How to use it effectively: Run the simulation yourself before assigning it. The default view shows all four curves at once, but students can toggle them on and off individually, which is useful for focused comparison. The "Separate Atoms" mode is also worth using — it shows the transition from attraction to repulsion as atoms are pushed too close together, and that visual really helps with the question about why the curves shoot up sharply at very small distances. That sharp rise represents Pauli repulsion, and the simulation makes it concrete without needing a verbal explanation. The simulation is freely available through the PhET website at phet.colorado.edu. Search for "Comparing Attractive Forces" and it should be the first result. No download is needed; it runs in the browser. Some schools block it, so if you're distributing this to students, check access ahead of time or have a backup plan like a printed worksheet with pre-captured screenshots from the simulation.