When you heat it on the pan, does it melt or does it burn?
Drop a lump of ghee on a hot grill. It melts and slides across the metal, but it is still the same fat. Drop raw flatbread dough on the same grill. It puffs, browns and smells completely different. The first change leaves every molecule alone. The second breaks bonds and builds new substances. By the end, you can tell the two apart at the particle scale and show that atoms are conserved.
Core Claims
- Physical change: The particles change state or spacing. Bonds inside each particle stay intact, so the substance keeps its identity (melting, dissolving).
- Chemical change: Bonds break and new bonds form. Atoms regroup into new substances (reactants become products).
- Conservation of mass: Every atom is kept in any change. The count of each atom type stays the same.
Physical vs. chemical (particle view)
Retrieval Checklist
- Tell a physical change (spacing) from a chemical change (bonds broken).
- Spot the mistake in a particle diagram that loses atoms.
- Draw solids, liquids and gases with the particle conventions.
The distinction lies entirely in the bonds.
Every sample of matter is a collection of particles. A change can move those particles around, or it can rebuild them from the inside.
Before you label a change, trace the bonds inside each molecule. If those bonds stay untouched, the change is a physical change. The particles may speed up, spread out, slide past neighbors or mix during dissolving. Each one is still the same substance.
If bonds break and new ones form, the atoms end up in new combinations. That is a chemical change. The starting substances (reactants) disappear. New substances (products) appear with different properties.
A physical change moves particles apart or together and leaves each particle intact. A chemical change breaks the bonds inside particles and builds new ones.
Calling a change chemical because it looks dramatic or cannot be undone. Boiling makes bubbles, and torn foil stays torn, yet the particles are unchanged. Ask one question: is it the same substance afterward?
How the two changes read in particle panels.
Compare a phase change with a combustion reaction. In the physical change, every molecule stays intact. In the chemical change, the atoms regroup, but the count of each atom stays the same.
Particle diagrams show state and motion with three conventions.
- Solid: Brackets beside the particles show vibration. The particles stay in fixed positions.
- Liquid: Particles stay in contact with their neighbors. They slide past each other along the bottom of the container.
- Gas: Particles are far apart. They move fast in straight lines until they collide.
Physical change: ice melting (H₂O)
Chemical change: hydrogen combustion (2H₂ + O₂ → 2H₂O)
Thinking a dissolved solid has disappeared. Sugar in water is still sucrose. The molecules are spread out between the water molecules, and stirring did not break any bond inside them.
Watch the particles rearrange.
Pick a change, then drag the slider from start to finish. The dashboard counts every atom that is drawn.
What happens to the atoms when a change runs from start to finish?
The explanation appears after you predict.
- Hydrogen atom
- Oxygen atom, or chloride ion (Cl⁻)
- Carbon atom, or sodium ion (Na⁺)
- Vibration bracket
- Motion trail, or the surface of the liquid
- Collision
- Load water at 0%, then press Record. Look for: nine water molecules in a lattice with brackets, and 18 hydrogen and 9 oxygen on the dashboard.
- Load water at 100%, then press Record. Look for: the same nine molecules, far apart. Each one is still H₂O, and the counts have not moved.
- Load salt at 100%, then press Record. Look for: water molecules around each ion. The ions are separate, and no new substance formed.
- Load methane at 100%, then press Record. Look for: the CH₄ and O₂ are gone, and CO₂ and H₂O have taken their place. The counts still match.
Use your evidence
Where does the mass go in an open system?
The Law of Conservation of Mass says atoms are never created or destroyed. In a closed container, the mass before a change equals the mass after it.
But a cooking pan, an oven and an open beaker are open systems. Bread dough loses weight as it bakes. That does not break the law. Heat boils off water and releases carbon dioxide from the leavening agent. In an open system, those gas molecules drift into the room. The atoms still exist, but they are no longer on your scale.
To test conservation, run the change in a sealed flask (a closed system). Mix vinegar and baking soda in the flask and the gas stays trapped. The scale shows no change in mass, even though bubbles form and a new solution appears. Every atom stays inside.
Saying the bread lost atoms. The pan lost mass because gas escaped into the room. In a sealed flask, the same change keeps its mass.
Commit to a prediction first.
Practice sorting everyday changes
Same skill, faster reps. Commit each of the six scenes to a bin before you see why.
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
A change that moves or spreads particles but keeps every bond is a . A change where bonds break and atoms regroup into new substances is a . The starting substances in a chemical reaction are the . The new substances that form are the . A system where gas can escape into the room is an .
Explain why a flatbread loses mass as it bakes on an open pan. Use the Law of Conservation of Mass.
Writing it out forces your brain to organize the idea. Write your answer first, then peek and compare.
Baking is a chemical change. The dough releases carbon dioxide and steam. The pan is an open system, so the gas molecules escape into the air. The mass on the scale drops. That does not break the Law of Conservation of Mass. The escaping gas carries atoms and mass into the room, and the total number of atoms stays the same.
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Sample A (solid paraffin wax, C25H52) is gently warmed until it liquefies. Sample B (solid sucrose, C12H22O11) is heated strongly in an open crucible. It chars, releases water vapor and leaves a black carbon residue. Classify each change. Describe the covalent bonds and intermolecular forces at the particle level. Apply the Law of Conservation of Mass.
- Classification (1 mark): You call the melting wax physical and the charring sugar chemical.
- Particle level (1 mark): You say melting overcomes weak intermolecular forces, while charring breaks and forms covalent bonds.
- Conservation (1 mark): You say atoms are conserved in both, so the total mass of atoms stays constant.
- Open system (1 mark): You say water vapor escapes the open crucible, so the mass on the scale drops.
Rust on a bridge is a chemical change. Iron atoms bond with oxygen and form iron oxide, a new solid with new properties. Rust is brittle and crumbly, so it slowly weakens the steel.
Adaptive Competency Practice: HS-PS1-2
Distinguish physical phase changes from chemical bond rearrangements with diagnostic questions. The system analyzes your evidence-based claims and issues an authenticated mastery certificate.