L4. Physical and Chemical Changes

Matter and Measurement

R-report

L4. Physical and Chemical Changes

How can you tell if matter only changed shape or if it became a whole new substance?

Central Idea:

A physical change alters the form or state of a substance but does not make a new substance; a chemical change makes one or more new substances with different properties. For example, melting ice is a physical change: H₂O(s) → H₂O(l) — the same water molecules change state at 0 °C. Iron rusting is a chemical change because iron and oxygen form a new compound, Fe₂O₃ (balanced: 4 Fe + 3 O₂ → 2 Fe₂O₃). Ask three questions: Is a new substance present, did atoms rearrange into new bonds, what visible clues support this?

How To Observe Changes:

Name the starting materials (example: ice, iron nail) and write them down clearly. Record only observations first: temperature, color, bubbles, solid forming, mass change. Separate what you saw from the explanation you give. Identify relationships by asking whether atoms must rearrange to make what you saw; write a simple equation when possible to check atoms. Test your idea by trying a reverse change (can you easily undo it?) or measuring mass before and after.

Apply To New Situations:

Compare what stays the same and what changes when you move from one situation to another. For a new example, ask: does the sample keep the same chemical name (same atoms)? If yes, it is likely physical; if no, a chemical change probably happened. Test your reasoning with one observation that could falsify it: try reversing the change, check for gas or a new solid (precipitate), or measure mass. Prompt: Is melting chocolate like rusting iron? Which parts stay the same, which form new substances?

Synthesize Ideas:

Physical changes rearrange matter without making new substances; chemical changes make new products with new properties. Use vocabulary like reactant, product, precipitate, and state when you describe what happened, and write a simple equation to check that atoms balance. Example: melting ice — write H₂O(s) → H₂O(l) and note no new compound appears; rusting — write 4 Fe + 3 O₂ → 2 Fe₂O₃ and count atoms.

Before you accept a conclusion, check units (°C for temperature), tell the micro-level story of atoms and bonds, and make sure your visible evidence fits the story. Use the same reasoning steps on the next problem: name materials, record observations, propose a micro-level explanation, and test it.