Elements, Compounds and Mixtures
1. Pure Substances
A pure substance is made up of ONLY ONE kind of particle and has FIXED composition.
Types of pure substances:
- Elements (one kind of ATOM)
- Compounds (two or more elements CHEMICALLY combined)
2. Elements
An element is a pure substance that CANNOT be broken down into simpler substances by ordinary chemical methods.
Symbols of Elements:
| Element | Symbol | Element | Symbol |
|---|---|---|---|
| Hydrogen | H | Sodium | Na |
| Helium | He | Magnesium | Mg |
| Carbon | C | Aluminium | Al |
| Nitrogen | N | Silicon | Si |
| Oxygen | O | Phosphorus | P |
| Fluorine | F | Sulphur | S |
| Neon | Ne | Chlorine | Cl |
| Iron | Fe | Potassium | K |
| Copper | Cu | Calcium | Ca |
| Silver | Ag | Zinc | Zn |
| Gold | Au | Mercury | Hg |
| Iodine | I | Lead | Pb |
'Many element symbols come from LATIN names: Fe (Ferrum), Na (Natrium), K (Kalium), Ag (Argentum), Au (Aurum), Pb (Plumbum), Hg (Hydrargyrum).'
Classification of Elements
| Category | Properties | Examples |
|---|---|---|
| METALS | Malleable, ductile, good conductors, lustrous | Iron, copper, gold, aluminium |
| NON-METALS | Brittle (if solid), poor conductors, dull | Oxygen, carbon, sulphur |
| METALLOIDS | Properties of BOTH metals and non-metals | Silicon, germanium, arsenic |
3. Compounds
A compound is a pure substance formed by the CHEMICAL combination of TWO or MORE elements in a FIXED ratio by mass.
Characteristics:
- Can be BROKEN DOWN into elements by chemical methods
- Has FIXED composition
- Properties are DIFFERENT from constituent elements
- Homogeneous (uniform throughout)
Examples: Water: H₂O (2:16 = 1:8 by mass). Decomposes into H₂ and O₂. Carbon dioxide: CO₂ (12:32 = 3:8 by mass). Sodium chloride: NaCl (23:35.5 by mass).
4. Mixtures
A mixture is a substance formed by the PHYSICAL mixing of two or more substances.
| Aspect | Homogeneous Mixture | Heterogeneous Mixture |
|---|---|---|
| Uniformity | UNIFORM throughout | NON-UNIFORM |
| Components visible? | NOT visible | VISIBLE |
| Examples | Sugar solution, air, brass | Sand and water, oil and water |
Characteristics of mixtures:
- NO chemical reaction between components
- Components can be in ANY ratio
- Components RETAIN their original properties
- Can be SEPARATED by physical methods
5. Comparison: Element vs Compound vs Mixture
| Aspect | Element | Compound | Mixture |
|---|---|---|---|
| Composition | ONE kind of atom | TWO+ elements FIXED ratio | TWO+ substances ANY ratio |
| Separation method | Cannot be broken | CHEMICAL methods | PHYSICAL methods |
| Properties | Element's properties | DIFFERENT from elements | Same as components |
| Purity | PURE | PURE | IMPURE |
6. Separation of Mixtures
| Mixture Type | Separation Method | Principle |
|---|---|---|
| Solid + Solid (one magnetic) | MAGNETIC separation | Magnet attracts magnetic substance |
| Solid + Solid (different size) | SIEVING | Particle size difference |
| Solid + Solid (one sublimes) | SUBLIMATION | One solid sublimes, other does not |
| Solid + Liquid (insoluble) | FILTRATION | Particles too large to pass through filter |
| Solid + Liquid (soluble) | EVAPORATION / CRYSTALLISATION | Liquid evaporates, solid remains |
| Two immiscible liquids | SEPARATING funnel | Difference in density |
| Two miscible liquids (different BP) | DISTILLATION | Different boiling points |
| Coloured components | CHROMATOGRAPHY | Different rates of movement |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| 'Air is a compound' | Air is a MIXTURE of gases (N₂, O₂, Ar, CO₂) — components can have ANY ratio |
| 'Alloys are compounds' | Alloys are HOMOGENEOUS MIXTURES (solid solutions), not compounds |
| 'Compounds can be separated by physical methods' | Compounds can only be separated by CHEMICAL methods |
| 'Gold is a compound' | Gold (Au) is an ELEMENT — it consists of only gold atoms |
ICSE Exam Focus (5–7 marks)
- 2-mark questions: Define element, compound, mixture — give examples
- 3-mark questions: Differences between compounds and mixtures
- 4-mark questions: Separation techniques — which method for which mixture
- 5-mark questions: Classification with reasons — element, compound, or mixture
Self-Test
Q1. What is an element? Give three examples. A1. An element is a pure substance that CANNOT be broken down into simpler substances. Examples: Hydrogen, oxygen, iron, gold.
Q2. Why is water a compound and not a mixture? A2. Water has a FIXED composition (H₂O). Hydrogen and oxygen are chemically combined. Water's properties are DIFFERENT from H₂ and O₂. It can only be separated by chemical methods (electrolysis).
Q3. How would you separate a mixture of sand and salt? A3. Add water (salt dissolves). FILTER the mixture (sand is retained on the filter paper). EVAPORATE the filtrate to recover salt.
Q4. Distinguish between a compound and a mixture. A4. A compound has a FIXED composition and can only be separated by chemical methods. A mixture has VARIABLE composition and can be separated by physical methods.
Q5. Name the method to separate: (a) Alcohol from water. (b) Iron filings from sulphur powder. A5. (a) DISTILLATION (different boiling points). (b) MAGNETIC separation (iron is magnetic, sulphur is not).
Q6. Classify: (a) Brass (b) Sodium chloride (c) Oxygen gas A6. (a) Brass → homogeneous MIXTURE (alloy of Cu and Zn). (b) Sodium chloride → COMPOUND. (c) Oxygen gas → ELEMENT.
