Investigating the Reaction's Products

Day 2's investigations are more quantitative than the first day's explorations. As MOVIE 1 shows, students do timed measurements of the falling temperature as water, sodium acetate and carbon dioxide gas result from mixing acetic acid and sodium bicarbonate. Listen to Nancy's questions about the reaction and students' beginning conceptions about what a chemical reaction is and what is happening when the materials mix. Nancy uses a class demo to help students determine what gas is released.

The gist of this challenge is this -- students must have enough vinegar so that all of the baking soda reacts, thus releases the carbon dioxide. Since they working with a fixed total amount of ingredients, they must choose a ratio with as much baking soda (maximize gas) with just enough vinegar to react with all of the baking soda.

A good design challenge should have very clear, unambiguous ways for students to measure their product's performance. (Read about this and other criteria for What Makes for a Good Design Challenge.) Harvard's Phil Sadler and others who developed the Challenges In Physical Science curriculum worked hard to produce design tasks where measurements were central to determining the success of a design solution. This is unlike many products that designers make, where customer satisfaction rules.