Clues from the Bike Shop Mixing Bench

A Safe Test at the Community Bike Shop

At a community bike shop, volunteers sometimes teach visitors how scientists test materials. Rowan and Priya wanted to compare mixtures that might be used in a demonstration. An adult supervisor provided clear cups, a graduated cylinder, crushed chalk, water, lemon juice, and clean, empty gas-collection pouches. The pouches were attached to the cups before each test. No one tasted or touched the mixtures.

For each trial, the students put 5 grams of crushed chalk into a cup. They added 20 milliliters of liquid, stirred for 10 seconds, and watched for one minute. They used a new pouch and fresh materials each time. The water-and-chalk mixture was a comparison. The lemon-juice-only mixture checked what happened when lemon juice was stirred without chalk.

The students recorded how many seconds steady bubbles continued after stirring stopped. They also measured the width of each pouch after one minute. Before a trial, each pouch was flat and measured 0 centimeters across. At the end, they looked at the chalk. They did not use the names of any gases because these observations alone did not identify a gas.