Elements, Compounds and Mixtures
2.1 Elements, Compounds and Mixtures
1. Differences between Elements, Compounds, and Mixtures
1.1 Definition of an Element
- An element is a pure substance that contains only one type of atom.
- Cannot be broken down into simpler substances by chemical means.
- Each element has its own unique symbol in the Periodic Table.
- Number of known elements: Over 118 (naturally occurring and synthetic).
- Examples: Hydrogen (H), Oxygen (O), Iron (Fe), Gold (Au).
1.2 Properties of Elements
- Have characteristic melting points, boiling points, densities, and colours.
- All atoms in an element have the same number of protons.
- Can exist as:
- Monatomic: Single atoms (e.g., noble gases like helium, He).
- Molecules: Two or more atoms bonded (e.g., oxygen, Oâ‚‚).
- Giant structures: Large networks of atoms (e.g., diamond, carbon).
- Cannot be separated into simpler substances by physical means.
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
1.3 Definition of a Compound
- A compound is a pure substance formed when two or more elements are chemically combined in a fixed ratio.
- Has a different set of properties from the elements that make it up.
- Elements in a compound are bonded together:
- Ionic bonds (e.g., sodium chloride, NaCl)
- Covalent bonds (e.g., water, Hâ‚‚O)
- Metallic bonds (in alloys considered chemically combined)
- Can only be broken down into its elements by chemical means (e.g., electrolysis, thermal decomposition).
1.4 Properties of Compounds
- Fixed composition (e.g., Hâ‚‚O always has 2 hydrogen atoms for every 1 oxygen atom).
- Uniform properties throughout (homogeneous).
- Physical and chemical properties differ from the original elements.
- Have a definite melting and boiling point (if pure).
1.5 Definition of a Mixture
- A mixture contains two or more substances (elements and/or compounds) not chemically bonded.
- Components retain their own properties.
- Can be separated by physical methods (filtration, distillation, chromatography, etc.).
- Composition can be variable (not fixed ratio).
- Can be homogeneous (uniform composition) or heterogeneous (visible separate components).
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
2. Key Differences Table
| Feature | Element | Compound | Mixture |
|---|---|---|---|
| Type of particles | One type of atom | Two or more elements chemically bonded | Two or more substances not chemically bonded |
| Composition | Fixed (one kind of atom) | Fixed (definite ratio) | Variable |
| Separation | Cannot be separated physically | Only chemical methods | Physical methods |
| Properties | Unique to element | Different from constituent elements | Combination of properties of components |
| Examples | Oâ‚‚, Fe, Au | Hâ‚‚O, NaCl | Air, saltwater, steel |
3. Particle Diagrams
Element (Oâ‚‚):
O O O O
O O O O
(Only oxygen molecules present)
Compound (Hâ‚‚O):
H O H H O H H O H
(Each molecule has fixed ratio: 2 hydrogen, 1 oxygen)
Mixture (Oâ‚‚ + Nâ‚‚):
O O N N O O N N
(Two types of molecules not bonded together)
4. Separation Methods for Mixtures
4.1 Filtration
- Separates insoluble solids from liquids.
- Example: Sand from water.
4.2 Distillation
- Separates liquids based on boiling points.
- Example: Water from saltwater.
4.3 Fractional Distillation
- Separates mixtures of liquids with close boiling points.
- Example: Separating components of crude oil.
4.4 Chromatography
- Separates substances based on different solubilities.
- Example: Separating dyes in ink.
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
5. Properties and Examples in Detail
5.1 Elements
- Metallic elements: Conduct electricity, malleable, ductile (e.g., copper).
- Non-metallic elements: Poor conductors, brittle (e.g., sulfur).
5.2 Compounds
- Water (H₂O): Clear liquid, extinguishes fire — properties very different from hydrogen and oxygen.
- Sodium chloride (NaCl): White crystalline solid — unlike sodium (a reactive metal) or chlorine (a poisonous gas).
5.3 Mixtures
- Air: Mixture of gases — nitrogen (~78%), oxygen (~21%), argon (~1%), CO₂ (~0.04%).
- Alloys: Brass (copper + zinc), steel (iron + carbon).
6. Exam Tips
- Element: Only one type of atom — cannot be separated.
- Compound: Chemically combined — fixed ratio.
- Mixture: Not chemically combined — separable physically.
- Always refer to particle diagrams to explain differences.
- Give real-life examples in answers.
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course