Pure Substances
Understanding the fundamental building blocks of matter with constant composition and fixed properties
Pure substances are materials that have a constant composition and uniform properties throughout. They are the foundation of chemistry, consisting of either elements or compounds.
Constant composition
Fixed properties
Cannot be separated by physical means
Variable composition
Variable properties
Can be separated by physical means
1. Definition of a Pure Substance
Constant Composition
A pure substance has a fixed chemical composition, containing only one type of particle. This uniformity ensures consistent properties throughout the entire sample.
All particles identical
Fixed Melting/Boiling Points
Pure substances have sharp, specific melting and boiling points - a key identifier of purity.
Impure substances melt/boil over a range of temperatures. For example, salt water boils above 100°C and over a range, not at a single sharp point.
Quick Check
A substance melts sharply at 0°C and boils sharply at 100°C. What can you conclude?
2. Elements
Cannot Be Broken Down
Elements are the simplest form of pure substances. They cannot be separated into simpler substances by physical or chemical means.
Atomic number: 8
Periodic Table
Elements are systematically organized in the Periodic Table based on atomic structure and properties.
3. Compounds
Fixed Ratios
Elements in compounds are always present in the same fixed mass ratio.
2 Hydrogen : 1 Oxygen (by atoms)
11.2% H : 88.8% O (by mass)
Chemical Formulae
Compounds are represented by chemical formulas showing elements and quantities.
NaCl - Sodium chloride
CO₂ - Carbon dioxide
CH₄ - Methane
Different Properties
Compounds have properties different from their constituent elements.
Chemical Means of Separation: Compounds can only be separated into their constituent elements through chemical reactions (e.g., electrolysis, thermal decomposition). Physical methods like filtration or distillation will NOT work.
Compound Builder
Click to see how elements combine in fixed ratios:
4. Differentiating from Mixtures
Homogeneous Mixtures
Uniform composition throughout, but still a mixture.
Example: Salt dissolved in water
NaCl(aq)
Heterogeneous Mixtures
Visibly different substances or phases.
Example: Salad, sand and water
Key Differences: Pure Substances vs Mixtures
| Feature | Pure Substances | Mixtures |
|---|---|---|
| Composition | Constant / Fixed | Variable |
| Separation Methods | Chemical means only | Physical means (filtration, distillation, etc.) |
| Properties | Consistent throughout | Can vary with composition |
| Melting/Boiling Points | Sharp, fixed temperatures | Range of temperatures |
| Examples | Water (H₂O), Oxygen (O₂), Salt (NaCl) | Salt water, Air, Soil |
Classify These: Pure or Mixture?
Test Your Understanding
1. Which of the following is a pure substance?
2. A compound can be separated into its elements by:
3. Which is true about elements?
4. The chemical formula of water shows that:
Key Terms
Key Takeaways
- Pure substances have constant composition and fixed melting/boiling points
- Elements are the simplest pure substances - cannot be broken down further
- Compounds are made of two or more elements in fixed ratios (e.g., H₂O always 2H:1O)
- Compounds have different properties from their constituent elements
- Pure substances cannot be separated by physical means - only chemical reactions
- Mixtures have variable composition and can be separated physically
- Homogeneous mixtures are uniform; heterogeneous mixtures are visibly different
