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1.2.1Principles of Chemistry

Elements, Compounds or Mixtures — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 8 of 50

📚 Edexcel International GCSE Science Double Award 4SD0 - Chemistry Hub / Topical Revision
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Part 1 of 4Three Types of Substance

1.2.1 Elements, Compounds or Mixtures

Every substance can be classified as an element, a compound or a mixture.

Element
  • A substance made of only one type of atom, in which every atom has the same number of protons, such as copper, iron or magnesium.
Compound
  • A pure substance containing two or more different elements chemically combined in a fixed ratio, such as calcium carbonate or carbon dioxide.
Mixture
  • Two or more substances, which may be elements or compounds, that are not chemically combined and can be separated by physical methods.

Part 2 of 4Compounds and Mixtures Compared

Comparing the two helps to decide which category a given substance belongs to.

Compounds and Mixtures Compared
Fixed ratio
  • A compound always contains the same ratio of elements, so every sample of water contains two hydrogen atoms for each oxygen atom.
Variable ratio
  • A mixture can contain any proportion of its components, such as sand and water in different amounts, so its composition varies.
Chemical bonding
  • The elements in a compound are chemically bonded, so the compound has new properties that differ from those of its elements.
Physical separation
  • Mixtures can be separated by physical methods because their components are not bonded together, unlike the atoms of a compound.
FeatureCompoundMixture
Chemical bondingAtoms are chemically bondedNo chemical bonds between components
CompositionFixed ratio of elementsVariable proportions
PropertiesDifferent from the elements that form itEach component keeps its own properties
SeparationOnly by a chemical reactionBy physical methods such as filtration

Part 3 of 4Classifying a Substance

1
Ask whether the substance contains only one kind of atom, in which case it is an element such as sulfur or iron.
2
If it contains two or more elements chemically combined in a fixed ratio, it is a compound such as sodium chloride.
3
If it contains two or more substances that are not chemically joined together, it is a mixture such as seawater.
4
Remember that air is a mixture rather than a compound, because its nitrogen and oxygen are not chemically combined.

Part 4 of 4Exam Checklist & Tips

1.2.1 – Key Points for the Exam

  • Element A substance containing only one type of atom.
  • Compound A substance made of two or more elements chemically combined in a fixed ratio.
  • Mixture Two or more substances that are not chemically combined.
  • Chemical bonding The attraction that holds atoms together in a compound by sharing or transferring electrons.
  • Pure substance A single element or compound with no other substances present.

Common Mistakes

  • Calling a mixture a compound because it contains several substances;
    • a compound must have its elements chemically combined.
  • Saying a compound can be separated into its elements by filtration or evaporation;
    • only a chemical reaction can break its bonds.
  • Describing an element as containing different types of atoms; an element contains only one type of atom.
  • Forgetting that a mixture has a variable composition, so it does not have a fixed chemical formula.

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