The volcano probe is a formative assessment tool for learning about how students think about choosing reactants and reaction conditions based on how they think about chemical control. It has been tested in middle school, high school, and university chemistry classes.
To use this formative assessment, first have your students view this three-minute silent video in which a student in a lab coat is preparing a chemical demonstration for a group of second graders. The student in the video intends to make a display that resembles a “volcano” eruption. Many chemical substances are available, as well as laboratory equipment, and she chooses to use sodium bicarbonate (baking soda) and citric acid. She measures 5 g of each of these, places both in a test tube with a red triangle (“volcano” shape) behind it, and then adds 20 mL of water to the test tube. A fizzing “eruption” occurs.
Three Minute Silent Video
After your students watch the video, give them the 
Volcano Probe Handout which recounts the activity in the video and lists available materials. The handout has two pages. On the first page, students are asked to propose and justify three ideas for how the student in the video could create a bigger “eruption”. On the second page, students are given the chemical equation for the baking soda and citric acid reaction and asked again to propose how to create a bigger “eruption”.