Physical vs Chemical Change: Six Kitchen Experiments (and Where the Mass Goes)

“Is it a chemical change?” sounds like a vocabulary question, but it’s really an evidence question: what would you observe to be sure that a new substance has formed? Kitchen experiments make good test cases.

Six quick changes

Use clear cups and record the properties of each material before and after: colour, smell, texture, temperature, whether it can be undone.

Dissolving sugar in water. The sugar seems to vanish, but taste (not in the lab, only with food-safe materials and permission) or evaporating the water brings it back. A physical change.
Melting an ice cube. Freezing it again returns the same substance. Physical.
Baking soda and vinegar. Fizzing, a gas forms and the mixture changes temperature slightly. Chemical.
Curdling milk with vinegar. Lumps form that weren’t there before. Chemical.
Tearing paper. Same material in smaller pieces. Physical.
Toasting bread (adult only). Colour and smell change permanently. Chemical.

No single clue is proof

Students often say “bubbles mean chemical change” or “colour change means chemical change.” Test the rule. Boiling water makes bubbles, but it’s still water, so it’s physical. Dissolving food colouring changes colour, but you haven’t made a new substance.

Better evidence for a chemical change includes several clues together, plus the key test: can you get the original substance back easily? A new substance has new properties, such as a different smell, a gas that wasn’t there, a solid that forms from clear liquids, or a permanent change that can’t be undone by cooling or drying.

Where does the mass go?

Here’s a classic puzzle. Weigh a cup with baking soda and vinegar before mixing and after fizzing in an open cup. The reading drops. Did mass disappear?

Now repeat in a capped plastic pop bottle (a fizzy-drink bottle only). Use goggles, an adult standing by, and no more than 1 level teaspoon of baking soda and 100 mL of vinegar. Weigh the sealed bottle before and after the reaction. The mass stays the same. Open the cap and listen to the gas rush out, and the reading drops.

Explanation

The reaction makes carbon dioxide gas. In the open cup, gas escapes into the air, so the cup gets lighter. In the sealed bottle, the gas stays inside and the total mass is unchanged. Atoms are rearranged into new substances, but they aren’t created or destroyed. This is the law of conservation of mass.

Mass isn’t lost in a reaction, only moved, and that’s a strong idea worth building toward the particle model of chemical reactions.

Safety

Use goggles. Never seal a bottle with more baking soda and vinegar than the quantities above, because pressure builds. Use only a plastic fizzy-drink bottle, never glass, and point it away from people.

The six changes are Is it a new substance? and the weighing activity is Where does the mass go? Both are in Kitchen Chemistry for Grades 7 and 8. Families can try Kitchen Chemistry for Families. Sign up for three lessons free.