Texas Science Teacher Resource Hub

Students hear "color change" and lock in on it. But stirring blue food coloring into water turns the water blue, and nothing chemical has happened. That's just mixing. The kind of color change that signals a chemical change is one that can't be explained by mixing two colors. A shiny metal turning dull and reddish from rust is a real chemical color change.

Bubbles come from any kind of gas, including a gas that was already in the liquid. When you open a soda, the bubbles are dissolved carbon dioxide escaping. No chemical change, just a physical release. When baking soda meets vinegar, however, the bubbles are brand-new carbon dioxide being created. Students have to ask: is gas being produced from a reaction, or released from something it was already in?

Students are used to thinking heat comes from a stove or a sun. The idea that a reaction itself can release or absorb heat is new. Mixing baking soda and vinegar gets colder without anyone adding ice. Steel wool dipped in vinegar gets warmer without a heater. The reaction itself is the source of that temperature change, and that's the clue they should catch.

Students sometimes assume every reaction can be undone. Most chemical changes are difficult to reverse with the simple methods used in a classroom. You can unburn toast only in very specialized ways, not by scraping or rinsing it. Physical changes (ice melting, water evaporating) are usually easy to reverse. Chemical changes usually aren't, which is another signal that something new has formed.