The particle model gets an upgrade. Particles are atoms of different types, atoms join into compounds, and a chemical change makes new substances. Concept tutorials with atom-level simulations.
Particles are atoms; atom type decides the element; different atom types chemically joined make a compound (AC9S8U06).
The particles of the particle model are atoms. Atoms come in different types, one type for each element, and atom type decides what element a substance is.
An element is a pure substance made of only one type of atom. Its particles can be single atoms (helium) or atoms joined together (O₂), but every atom is the same type. Each element has a symbol of one or two letters, and the periodic table lists them all.
A compound is two or more types of atom chemically joined in a fixed ratio. It is a new substance with new properties, and a formula like H₂O tells you which atoms and how many.
A mixture is two or more substances mingled together but not chemically joined. Each keeps its own properties, the amounts can vary, and the parts can be separated by physical means. A compound is the opposite: atoms joined in a fixed ratio, with new properties.
Chemists show the same substance in different ways. A symbol names an element; a formula (H₂O) gives the types and numbers of atoms; a structural formula shows which atoms are joined; a 3D model shows the shape. Each shows something the others don't, so you pick the one that suits the job.
A chemical change makes a new substance: atoms rearrange into new particles (AC9S8U07).
In a physical change (melting, boiling, dissolving) the particles move, spread out or pack together, but every particle stays the same: no new substance is made. Steam is still made of water particles.
A chemical change makes a new substance, because atoms rearrange into new particles. A physical change only rearranges or spreads out the same particles. "Can it be reversed?" is not the test.
You can't see atoms rearrange, but you can see the signs: a colour change, a gas given off, a solid forming in a liquid, light, a smell, or a temperature change. Each is a clue that a new substance formed, not proof: boiling water bubbles too. The test is still a new substance.
Some reactions give out energy to their surroundings, so the surroundings get hotter: they are exothermic (burning, respiration, hand warmers). Others take in energy, so the surroundings get colder: they are endothermic (instant ice packs, photosynthesis, cooking).
To classify an unfamiliar change, ask the main question first: is a new substance made? Then check the clues: colour, gas, solid forming, light, and temperature or energy change. One clue can mislead (boiling makes bubbles and ice melting takes in heat, yet both are physical), so several clues pointing the same way give more confidence.
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