Lesson focus: elementA substance made of only one type of atom (all with the same atomic number), which cannot be broken down into anything simpler by chemical reactions. contain one type of atom, compoundA substance containing two or more elements chemically bonded together in fixed proportions, such as water, H₂O. contain chemically bonded elements in fixed ratios, and mixtureTwo or more substances that are not chemically combined, which can be separated by physical methods such as filtration or distillation. contain substances not chemically bonded.
Classify samples as elements, compounds or mixtures using particle and property evidence.
Build on earlier learning
Compounds and moleculesYear 7 distinguished elements and compounds. This lesson adds purity evidence and separation consequences.
You will be successful when you can…
- Define element, compound and mixture.
- Interpret particle diagrams.
- Use melting and boiling evidence.
- Choose chemical or physical separation.
Words that carry the science
- element
- A substance containing atoms with one proton number.
- compound
- A substance containing atoms of different elements chemically bonded in fixed proportions.
- mixture
- Two or more substances together but not chemically bonded.
- pure substance
- A single element or compound with a fixed composition.
- chemical formula
- Symbols showing the elements and relative numbers of atoms in a substance.
- melting range
- Temperature interval over which a sample melts.
Composition defines the classification
01Composition defines the classification
An elementA substance made of only one type of atom (all with the same atomic number), which cannot be broken down into anything simpler by chemical reactions. contains one kind of atom, defined by proton number. A compoundA substance containing two or more elements chemically bonded together in fixed proportions, such as water, H₂O. contains different elementA substance made of only one type of atom (all with the same atomic number), which cannot be broken down into anything simpler by chemical reactions. chemically bonded in a fixed ratio. A mixtureTwo or more substances that are not chemically combined, which can be separated by physical methods such as filtration or distillation. contains at least two substances whose proportions can vary.
A compoundA substance containing two or more elements chemically bonded together in fixed proportions, such as water, H₂O. is not simply a “mixed element”. Its properties can differ greatly from those of its constituent elements because bonding and structure have changed.
02Particle diagrams encode bonds and identity
Identical unjoined atoms or identical same-element molecules represent an element. Identical particles containing different atom types bonded together represent a pure compound. More than one particle type represents a mixtureTwo or more substances that are not chemically combined, which can be separated by physical methods such as filtration or distillation..
A diagram should distinguish a touching or linking convention for bonds from accidental closeness. A key is essential.
03Purity has measurable consequences
A pure substanceA single element or compound with nothing else mixed in; it melts and boils at fixed temperatures. has characteristic fixed melting and boiling points at a stated pressure. A mixture commonly melts or boils over a range and may shift the observed temperatures.
Mixtures can be separated by physical methods because their components retain chemical identities. Separating a compound into elements requires chemical change.
Elements, compounds and mixtures
An element contains only one type of atom. A compound contains two or more elements chemically joined in a fixed ratio. A mixture contains different substances that are not chemically joined, so they can be separated by physical methods. Look at the particles in each sample and decide which it is.
Controls
Each circle is an atom; touching atoms are chemically joined. In a gas the particles spread out and move; in a solid they vibrate in place.
Classify a particle sample
Every particle contains one blue atom bonded to two identical white atoms.
- Only one particle type is present, so the sample is pure.
- Each particle contains more than one atom type.
- The atom types are chemically bonded in a fixed ratio.
- The sample is therefore a pure compound.
How to make the reasoning visible: Different atom colours do not create a mixture when every identical particle is one bonded unit.
Use converging evidence
Classify each sample from composition and thermal evidence.
| Sample evidence | Particle types | Thermal behaviour | Classification |
|---|---|---|---|
| only Cu atoms | one atom type | fixed melting point | element |
| identical CO₂ molecules | one molecule type | fixed sublimation point | compound |
| air sample | several molecule types | components condense differently | mixture |
| impure wax | several substances | melting range | mixture |
Can a pure substance contain more than one element?
Why can filtration not separate sodium from chlorine in sodium chloride?
Check your understanding
Answer from memory, then check against the suggested answer.
What defines an element?
What defines a compound?
What thermal behaviour suggests purity?
A clear liquid boils from 78–84 °C. What cautious conclusion can be made?
Why can an alloy be a mixture despite appearing uniform?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A student has a white solid. It may be a pure compound or a mixture of two compounds.
Describe what is meant by a pure compound.
(2)Explain how melting point evidence could tell the two apart.
(3)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., combine particle-level and measurable evidence.