Elements, compounds and mixtures
Classify a sample by its particles, bonding and composition rather than its appearance.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Know that particles can represent atoms or groups of bonded atoms.
- Distinguish a substance from a mixture of substances.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
O₂ contains how many types of atom?
What you will learn
- Distinguish elements, compounds and mixtures.
- Interpret particle models without confusing a molecule with a compound.
Count atom types and look for bonds
An element contains only one type of atom, identified by its proton number. An element can consist of individual atoms or molecules: oxygen gas contains O₂ molecules but remains an element because every atom is oxygen. A compound contains atoms of at least two different elements chemically bonded in a fixed proportion.
A water molecule contains two hydrogen atoms bonded to one oxygen atom. Pure water is a compound. It does not behave like a physical mixture of hydrogen gas and oxygen gas; bonding has produced a substance with its own properties.
A mixture has variable composition
In a mixture, two or more substances are present without being chemically joined to make a single new substance. Their proportions can vary. Air is a mixture of gases; salt water is a mixture even when it looks uniform because the amount of dissolved salt can change.
Physical separation methods can separate a mixture by exploiting properties such as solubility or boiling point. Separating a compound into its elements requires chemical change. A diagram with several different particle types is not automatically a compound: inspect which atoms are bonded within each particle.
Pause and explain
Which sample is a compound?
Worked example
A diagram shows only identical pairs of one red atom bonded to one blue atom. Red and blue represent different elements. Classify the sample.
Show the worked solution
- There is one repeating kind of particle.
- Each particle contains two different atom types chemically bonded.
- The fixed bonded composition identifies a pure compound.
Answer A pure compound.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Classify pure helium, pure water and air. Give the particle reason for each.
Give me a hint
Ask whether there is one atom type, a fixed bonded combination or several substances.
Compare my reasoning
- Helium contains one atom type, so it is an element.
- Water contains hydrogen and oxygen bonded in a fixed ratio, so it is a compound.
- Air contains different gases in variable proportions, so it is a mixture.
Helium: element. Water: compound. Air: mixture.
Look for these in your work
- Each classification has a particle reason.
- Variable composition is distinguished from fixed bonding.
A model contains separate O₂ molecules and separate N₂ molecules. Is it one compound? Explain.
Give me a hint
Inspect bonds within each molecule, rather than counting colours across the whole box.
Compare my reasoning
- O₂ and N₂ are each molecules of an element.
- The oxygen and nitrogen molecules are not bonded together.
- Two substances are present, so the box represents a mixture.
A mixture of two elements, rather than one compound.
Look for these in your work
- The bonding within particles is examined.
- The whole-sample classification follows from separate substances.
Two clear salt solutions have different salt concentrations. Explain why both are mixtures and suggest a physical separation method.
Give me a hint
Different concentrations demonstrate variable composition.
Compare my reasoning
- Each solution contains salt and water in variable proportions.
- A uniform appearance does not make a single compound.
- Crystallisation can recover salt; simple distillation can recover water.
Both are mixtures; crystallisation recovers salt or distillation recovers water.
Look for these in your work
- Variable proportions support the classification.
- The chosen method matches the substance to recover.
Common mistakes
- Calling O₂ a compound because it contains two atoms.
- Assuming a clear, uniform liquid cannot be a mixture.
What can you explain now?
Can salt water be a mixture even when no salt crystals are visible?
Compare with the explanation
Yes
Dissolving disperses the salt through water; the proportions remain variable and the substances are not a new single compound.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Draw particles for an element, a compound and a mixture. Include a molecular element so you cannot rely on counting atoms per particle.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.