Physical and chemical changes look like an easy unit until a student crushes a cracker, watches it crumble, and announces it is a chemical reaction. The instinct most middle schoolers walk in with is that any change they can see must be chemical. The real question is quieter and harder: did the change make a brand new substance, or just rearrange the same one?
Once students learn to ask that single question, the signs of a reaction stop being a list to memorize and become clues to investigate. Here is how I teach physical vs chemical changes, why the signs are evidence and not proof, and how conservation of mass ties it all together for MS-PS1-2 and MS-PS1-5.
What is the difference between a physical and a chemical change?
A physical change alters the form or appearance of a substance but makes no new substance, so the material keeps its identity and the change is often reversible. A chemical change produces one or more new substances with new properties. Melting, dissolving, cutting, and crushing are physical; rusting, burning, and reactions that fizz or change color are chemical.
I tell students the whole unit hinges on one word: identity. In a physical change the substance is still itself, just reshaped, dissolved, or in a new state. In a chemical change the original substance is gone and something new has taken its place.
- Physical change: form or appearance changes, no new substance, often reversible (melting ice, dissolving salt, cutting paper, crushing a can).
- Chemical change: one or more new substances form, with new properties, usually hard to reverse (iron rusting, wood burning, baking soda and vinegar reacting).
What are the signs of a chemical reaction?
Common signs that a chemical change may have happened include a color change, gas or bubbles forming, light or heat being released or absorbed, a new odor, or a precipitate (a solid) forming when two liquids mix. Any of these is a clue that new substances might have formed, which is exactly the kind of evidence MS-PS1-2 asks students to analyze.
- A color change that is not just mixing two colors together.
- Gas or bubbles forming when no gas was being added.
- Heat or light released or absorbed (the container warms up or cools down).
- A new odor that was not there before.
- A precipitate, a solid that appears when two liquids are combined.
Are the signs of a reaction proof that one happened?
No. The signs are evidence, not proof. Bubbles can come from boiling water, and color can change just by mixing dyes, with no new substance formed. To confirm a chemical change, students check whether the new material has new properties that the starting substances did not have. That property comparison is the heart of MS-PS1-2.
This is the distinction I protect most carefully, because it is what separates guessing from reasoning. A sign tells you to look closer; a new property is what confirms the reaction. So I have students compare the substance before and after, melting point, color, magnetism, how it reacts with water, and ask whether the after is genuinely a different material. That move is the scientific practice MS-PS1-2 is built on: analyzing data on properties before and after to decide if a reaction occurred.
What is conservation of mass in a chemical reaction?
Conservation of mass means the total mass stays the same before and after a chemical reaction. During a reaction atoms are only rearranged into new combinations, never created or destroyed, so every atom you start with is still there at the end. If you could trap all the products, including any gas, their mass would equal the mass of everything you started with.
The classic stumble is the disappearing-mass illusion: burn a log and the ash weighs less, so mass seems lost. It was not. Gases escaped into the air carrying atoms with them. I run reactions in a sealed container so nothing can leave, and the balance reads the same before and after, which makes MS-PS1-5 something students see rather than something I assert.
Why are chemical equations balanced?
Chemical equations are balanced because of conservation of mass. Since atoms are only rearranged and none are created or destroyed, there must be the same number of each kind of atom on both sides of the equation. Balancing is simply the bookkeeping that proves every atom present in the reactants is still accounted for in the products.
I connect balancing straight back to the sealed-container demo so it never feels like an arbitrary math rule. The same number of each atom on both sides is not a chemistry tradition, it is what conservation of mass looks like written down. Once students believe atoms cannot vanish, balancing equations becomes the natural way to model that, which is exactly what MS-PS1-5 asks them to do.
Teach the whole unit as one question, did a new substance form, and one rule, atoms are rearranged but never lost, and physical vs chemical changes and conservation of mass stop being separate topics and become a single story students can reason through.