Elements, mixtures and compounds (AQA GCSE Chemistry Combined Science): Revision Notes
Elements, mixtures and compounds
What substances can be
All substances around us can be put into three main groups:
- Pure elements - made of only one type of atom
- Pure compounds - made of different atoms joined together
- Mixtures - two or more substances mixed together but not joined
Understanding these three categories is essential for studying chemistry as they form the foundation for how we classify all matter around us.
What are atoms?
Everything around you is made up of tiny particles called atoms. Think of atoms as the building blocks of matter - they are incredibly small but make up everything you can see and touch.
An atom is the smallest piece of an element that can exist on its own. If you kept cutting up a piece of gold into smaller and smaller bits, an atom would be the tiniest piece you could get before it stopped being gold.
This concept of atoms as "building blocks" helps explain why we can't see individual atoms with our eyes - they are far too small! Yet everything from the paper you're reading to the air you breathe is made from these tiny particles.
Elements
An element is a pure substance made from only one type of atom. For example:
- Oxygen gas contains only oxygen atoms
- Iron metal contains only iron atoms
Key facts about elements:
- There are about 100 different elements
- Each element has its own chemical symbol (like a short name)
- Symbols start with a capital letter, sometimes followed by a small letter
- Examples: N = nitrogen, Na = sodium, O = oxygen
- All elements are organised in the periodic table
The periodic table is like a giant filing system that organises all known elements based on their properties and atomic structure.
Compounds
Compounds form when atoms of different elements join together during a chemical reaction. The atoms are held together by chemical bonds.
Critical points about compounds:
- Different elements combine in fixed proportions (exact amounts)
- The compound has different properties from the original elements
- You can only separate compounds back into elements using chemical reactions
- Compounds have chemical formulas that show which atoms they contain
Worked Example: Water ()
Every water molecule contains exactly 2 hydrogen atoms and 1 oxygen atom:
- The formula tells us this proportion never changes
- Water has completely different properties from hydrogen gas and oxygen gas
- Hydrogen is explosive, oxygen supports burning, but water extinguishes fires!
This transformation of properties is one of the most important concepts in chemistry - when elements combine to form compounds, something entirely new is created.
Mixtures
A mixture contains two or more elements or compounds that are mixed together but not chemically joined.
Key features of mixtures:
- The substances are not chemically bonded together
- Each substance keeps its own properties unchanged
- You can separate mixtures using physical methods (like filtering or heating)
- The amounts of each substance can vary
Worked Example: Iron and Sulphur Mixture
Before heating: Iron and sulphur powder mixed together
- Iron stays magnetic, sulphur stays yellow
- The iron can be removed with a magnet
- Each substance maintains its original properties
After heating: They react to form iron sulphide compound
- The compound is no longer magnetic and has new properties
- This shows the difference between a mixture and a compound!
Telling the difference
Understanding how to distinguish between elements, compounds, and mixtures is crucial for chemistry success.
Quick Reference Guide:
Elements: One type of atom only → Can't be broken down further
Compounds: Different atoms chemically joined → Can be separated by chemical reactions → Fixed proportions
Mixtures: Substances mixed but not joined → Can be separated by physical methods → Variable proportions
Remember!
Key Points to Remember:
- Atoms are the smallest building blocks of everything
- Elements contain only one type of atom and have chemical symbols
- Compounds form when different atoms join together in fixed amounts
- Mixtures contain substances that are mixed but not chemically bonded
- Elements and compounds keep their properties in mixtures, but compounds have different properties from their original elements