Using the PhET Chemical Formulas Simulation in Practice
The PhET simulation for chemical formulas is a browser-based tool from the University of Colorado that lets students build molecules interactively. You drag atoms onto a workbench, connect them with bonds, and the tool tells you the resulting chemical formula. It covers ionic compounds, molecular compounds, and some introductory stoichiometry. The worksheets that accompany it are typically distributed by teachers as printed sheets or PDFs, and the answer keys exist separately so educators can check student work quickly. You can find the official simulation at phet.colorado.edu by searching for "Chemical Formulas" or navigating through the Chemistry section. It opens directly in your browser. The worksheets vary by school district, but most follow a standard set of problems where students are shown a molecule and asked to write the correct formula, or shown a formula and asked to build the corresponding molecule in the simulation. The answer keys that circulate online are usually compiled by teachers and shared through educational forums or school websites. They are not always perfectly aligned with every version of the worksheet, so cross-reference carefully. I ran into a specific issue last year when a worksheet listed water as H2O and expected students to build it in the simulation, but the simulation's default behavior shows the structural arrangement with the oxygen atom central and two hydrogens bonded at an angle. Some students wrote H2O but the worksheet answer key also accepted OH2 because technically both represent the same atoms. This created confusion during grading because the answer key had been typed from memory and didn't account for the fact that a single molecule can be written in multiple valid notations depending on the teaching convention being used. My workaround was to tell students to stick with the standard convention taught in their textbook and to flag any borderline cases for manual review instead of trying to automate grading for those.
The simulation itself has some behaviors that aren't obvious at first. When you're building ionic compounds, the tool doesn't actually show lattice structures the way it shows individual molecules. It simplifies things by asking you to balance charges and produce a neutral formula unit. This means the simulation is teaching you the empirical formula concept, not the full crystal structure. That distinction matters if your students are later going to encounter concepts like unit cells or coordination numbers, because the simulation intentionally hides that complexity. One thing most teachers don't mention: the simulation has a "Check" button that gives immediate feedback, but it also has a "New Question" function that randomly generates compounds. If you are using this for independent practice, the random generator can produce compounds that haven't been covered in your lessons yet, like transition metal compounds with variable oxidation states. Students will get stuck on those and the answer key won't help because the worksheet they're working from only covers the basics. I disable the random generator and pre-select a fixed set of problems that match my curriculum scope. The worksheets are typically one to three pages depending on the teacher's design. They usually contain between fifteen and twenty-five problems mixing ionic and molecular compound types. The answer keys I've seen range from simple lists of correct formulas to detailed explanations showing charge balancing steps. The detailed versions are more useful for struggling students because they walk through why MgCl2 is correct instead of MgCl, which addresses a common misconception about subscript.
There are limitations to be aware of. The simulation does not handle polyatomic ions elegantly. You can build ammonium nitrate, but the process is clunky and the visual feedback isn't as smooth as with simple binary compounds. Also, the tool doesn't validate names—only formulas. So if a worksheet asks students to name the compound from its formula, the simulation can't help with that part of the problem. You need a separate naming worksheet or quiz for that. Another issue is that the simulation runs on modern browsers but can lag or freeze on older school computers or Chromebooks with limited RAM. If your students are using devices from three or four years ago, expect about thirty percent slower load times and occasional crashes when switching between questions. A practical workaround is to have students complete the simulation portion in pairs on a single device rather than individually, which also reduces the total number of devices needed in your classroom setup. If you need the PDF answer keys, they are most reliably found through your school district's internal resource portal or through legitimate educator sharing platforms like Teachers Pay Teachers where the creators have verified the accuracy against current curriculum standards. Be cautious with random file-sharing sites because some of those documents contain outdated information or incorrect formulas that have been copied and recopied through multiple uploads without correction.
Get the Full Details
The simulation itself is free and requires no download. Everything runs online. The worksheets and answer keys are what you need to source separately. Most of the time, the answer key matches the worksheet within a few minutes of work if you read both documents carefully before distributing anything to students. The biggest time sink is not the tool itself but aligning the worksheet questions with the specific compound coverage your class needs.
