Elements, Compounds & Mixtures
All matter can be classified as either a pure substance (element or compound) or a mixture. Understanding the differences is fundamental to all of chemistry.
A. Classification of Matter
Element vs Compound vs Mixture
| Category | Definition | Key Properties | Examples |
|---|---|---|---|
| Element | Pure substance made of only ONE type of atom. Cannot be broken down by chemical means. | Fixed composition; unique set of physical and chemical properties | O₂, Fe, Au, Na, H₂, N₂, C |
| Compound | Two or more elements chemically combined in a FIXED ratio. Has properties completely different from its constituent elements. | Fixed composition; can only be separated by CHEMICAL means (not physical); properties differ from elements | H₂O, NaCl, CO₂, H₂SO₄, CH₄, NH₃ |
| Mixture | Two or more substances PHYSICALLY combined. Each substance retains its own properties. | Variable composition; components retain individual properties; separated by PHYSICAL means | Air, seawater, soil, alloys, milk, crude oil |
Element
DefinitionPure substance made of only ONE type of atom. Cannot be broken down by chemical means.
Key PropertiesFixed composition; unique set of physical and chemical properties
ExamplesO₂, Fe, Au, Na, H₂, N₂, C
Compound
DefinitionTwo or more elements chemically combined in a FIXED ratio. Has properties completely different from its constituent elements.
Key PropertiesFixed composition; can only be separated by CHEMICAL means (not physical); properties differ from elements
ExamplesH₂O, NaCl, CO₂, H₂SO₄, CH₄, NH₃
Mixture
DefinitionTwo or more substances PHYSICALLY combined. Each substance retains its own properties.
Key PropertiesVariable composition; components retain individual properties; separated by PHYSICAL means
ExamplesAir, seawater, soil, alloys, milk, crude oil
⚡ MCQ Tip Element = one atom type. Compound = chemically combined, fixed ratio, properties differ from elements. Mixture = physically combined, variable ratio. A compound's properties are DIFFERENT from its constituent elements — e.g. H₂ (flammable gas) + O₂ (supports combustion) → H₂O (liquid, extinguishes fire).
B. Types of Mixtures
Solution vs Colloid vs Suspension
| Type | Particle Size | Appearance | Settles? | Tyndall Effect | Examples |
|---|---|---|---|---|---|
| Solution (Homogeneous) | < 1 nm | Transparent, clear | Never | Absent | Saltwater, sugar in water, vinegar, air |
| Colloid | 1 – 1000 nm | Often cloudy/translucent | Never (stable) | Present ✅ | Milk, fog, gelatin, blood, smoke, mayonnaise |
| Suspension (Heterogeneous) | > 1000 nm | Cloudy/opaque, visible particles | Yes, on standing | Present | Muddy water, chalk in water, oil in water |
Solution (Homogeneous)
Particle Size< 1 nm
AppearanceTransparent, clear
Settles?Never
Tyndall EffectAbsent
ExamplesSaltwater, sugar in water, vinegar, air
Colloid
Particle Size1 – 1000 nm
AppearanceOften cloudy/translucent
Settles?Never (stable)
Tyndall EffectPresent ✅
ExamplesMilk, fog, gelatin, blood, smoke, mayonnaise
Suspension (Heterogeneous)
Particle Size> 1000 nm
AppearanceCloudy/opaque, visible particles
Settles?Yes, on standing
Tyndall EffectPresent
ExamplesMuddy water, chalk in water, oil in water
Tyndall Effect
Colloid particles scatter a beam of light — visible as a bright path (Tyndall beam)
True solutions do NOT show Tyndall effect. Used to distinguish solutions from colloids.
⚡ MCQ Tip Tyndall effect is shown by COLLOIDS (and suspensions), NOT by true solutions. Milk = colloid. Saltwater = solution. Muddy water = suspension (settles).
C. Separation Techniques
Technique → What It Separates → Example
| Technique | Principle / Used For | Example |
|---|---|---|
| Filtration | Separates insoluble solid from a liquid using filter paper | Sand from water; chalk from water |
| Evaporation | Removes solvent by heating to recover dissolved solid | Salt from saltwater (halite recovery) |
| Distillation | Separates liquids with significantly different boiling points — liquid vaporises, condenses, collected separately | Alcohol from water; pure water from seawater |
| Fractional Distillation | Separates a mixture of liquids with close boiling points using a fractionating column | Crude oil into fractions; separation of liquid air |
| Chromatography | Separates mixtures based on different solubilities in a solvent and different attractions to the stationary phase | Separating ink dyes; food colorings; amino acids |
| Crystallisation | Dissolves a substance in hot solvent, then cools slowly — pure crystals form and separate | Purifying sugar; growing salt crystals |
| Centrifugation | Rapid spinning — denser particles settle to bottom (pellet); used for colloids and suspensions | Separating blood components; cream from milk |
| Decantation | Carefully pouring off the upper liquid after a suspension has settled | Muddy water — pour off clear water |
⚡ MCQ Tip Distillation separates by boiling point. Filtration separates insoluble solids. Chromatography separates by solubility. Fractional distillation = crude oil fractions. Centrifugation = blood components, milk.
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Element definition | Pure substance with only one type of atom — cannot be chemically broken down |
| Compound vs mixture | Compound: fixed ratio, chemical combination. Mixture: variable ratio, physical combination. |
| Tyndall effect seen in | Colloids (and suspensions) — NOT in true solutions |
| Milk is a | Colloid |
| Muddy water is a | Suspension — settles on standing |
| Saltwater is a | Homogeneous solution |
| Separating crude oil fractions | Fractional distillation (by boiling point) |
| Separating sand from water | Filtration |
| Separating blood components | Centrifugation |
| Separating ink dyes | Chromatography (by solubility) |