Modelling matter & classifying substances
1.1 · Matter
Modelling matter & classifying substances
By the end of this section:
- I can explain that all matter is made of atoms
- I can sort matter into mixtures, compound substances and elemental substances
1 · Read
The notes, the key idea and the words to knowMatter is the ‘stuff’ that everything is made of. We define it as anything that has mass and volume. All matter is ultimately built from atoms. To explain how it behaves, scientists use the particle model: matter is made of tiny particles in constant motion, with forces of attraction between them. How much energy those particles have decides the state — solid, liquid or gas.
- Solid — fixed, packed
- Liquid — close, can move
- Gas — far apart, fast
Slide the temperature to heat or cool the particles.
The particles do not change — only their arrangement and movement.
A model: real matter has far more particles.
We also sort matter by what its particles are made of. Three categories cover everything:
- Elemental substances — made of one type of atom only. Some are simple: single atoms, like helium (He) or gold (Au). Some are molecular: the same atoms joined in molecules, like oxygen (O2) and hydrogen (H2).
- Compound substances — two or more types of atom chemically combined in fixed ratios, e.g. H2O, NaCl.
- Mixtures — different substances mixed but not chemically combined, in no fixed ratio, e.g. air, salt water.
A homogeneous mixture looks the same throughout (salt solution, air); a heterogeneous mixture has visibly separate parts (oil and water, a rock).
Why these categories actually differTap to open
The dividing line between a compound and a mixture is the words chemically combined. In a compound, the atoms are joined by chemical bonds, so the only way to separate them is another chemical reaction. In a mixture nothing is bonded, so the parts keep their own properties and can be separated physically — by filtering, evaporating or letting them settle.
That single idea explains a lot:
- Fixed ratio? Compound yes (always H2O). Mixture no (salt water can be weak or strong).
- New properties? A compound behaves like a new substance, unlike the elements it formed from. A mixture keeps each part’s properties.
- Separated how? Compound — by reaction only. Mixture — by physical means.
The same reasoning sits underneath the states of matter: heating gives particles more energy, so they break free of their fixed positions (solid → liquid) and then of each other (liquid → gas). The particles do not change — only their arrangement and movement do.
Is salt water a compound or a mixture? It contains water molecules and sodium chloride, but they are simply mixed — the salt is dissolved, not bonded to the water, and the ratio can vary. You can get the salt back by evaporating the water (a physical change, no reaction). So it is a mixture — and a homogeneous one, because it looks uniform throughout.
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Not thisA mixture and a compound are the same kind of thing.
ThisNo — a compound’s atoms are bonded in a fixed ratio; a mixture’s substances are just mixed, in any ratio, and keep their own properties.
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Not thisAir is a compound because it is “one thing”.
ThisAir is a mixture of gases (mostly nitrogen and oxygen). They are not bonded and the proportions can vary.
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Not thisAn element can only be a single atom.
ThisAn elemental substance can be a molecule (O2, H2) — what matters is that all the atoms are the same type.
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Not thisMelting or boiling makes a new substance.
ThisThese are physical changes — the particles gain energy and move apart, but ice, water and steam are all still H2O.
- matter
- anything that has mass and volume; all matter is made of atoms
- particle model
- the idea that matter is made of tiny moving particles with forces between them
- elemental substance (element)
- a substance made of only one type of atom
- compound substance (compound)
- two or more types of atom chemically combined in fixed ratios
- mixture
- different substances mixed but not chemically combined, in no fixed ratio
- homogeneous / heterogeneous
- uniform throughout / made of visibly separate parts
- chemically combined
- joined by chemical bonds; separable only by another chemical reaction
2 · Watch
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We try to help you in other languages. Learn the idea here in your own language, then learn the science words from the English or French video. Not all of these videos have been independently checked. If something does not make sense, or does not match what we did in class, ask your teacher.
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- Booklet pp. 3–6
- Knowledge Book pp. 3–6
- In your class workbook: Chapter 1.1 Substances, states of matter and the particle model (pp. 6–7) · Revise this section
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