Elements, Compounds and Mixtures
1. Elements
An ELEMENT is a PURE substance that CANNOT be broken down into simpler substances by ordinary chemical means.
Examples: Hydrogen (H), Oxygen (O), Iron (Fe), Gold (Au), Carbon (C).
Classification of Elements
| Type | Properties | Examples |
|---|---|---|
| Metals | Good conductors, malleable, ductile, lustrous | Iron, copper, gold, aluminium |
| Non-metals | Poor conductors, brittle, non-lustrous | Oxygen, carbon, sulphur, nitrogen |
| Metalloids | Properties of BOTH metals and non-metals | Silicon, germanium, arsenic |
Symbols of Elements
Elements are represented by SYMBOLS (from their English or Latin names).
| Element | Symbol | Latin Name |
|---|---|---|
| Hydrogen | H | — |
| Helium | He | — |
| Carbon | C | — |
| Nitrogen | N | — |
| Oxygen | O | — |
| Sodium | Na | Natrium |
| Iron | Fe | Ferrum |
| Copper | Cu | Cuprum |
| Gold | Au | Aurum |
| Lead | Pb | Plumbum |
| Mercury | Hg | Hydrargyrum |
| Potassium | K | Kalium |
2. Compounds
A COMPOUND is a PURE substance formed by the CHEMICAL combination of two or more elements in a FIXED ratio by mass.
Examples: Water (H₂O) — 2H : 1O by atoms, 1:8 by mass. Carbon dioxide (CO₂), Sodium chloride (NaCl), Sugar (C₁₂H₂₂O₁₁).
Characteristics of Compounds
- Formed by CHEMICAL reaction between elements.
- Elements combine in FIXED PROPORTION by mass.
- Properties are DIFFERENT from the constituent elements.
- Can be separated into elements ONLY by chemical means.
- Homogeneous (uniform throughout).
Writing Chemical Formulae
- Write the symbols of the constituent elements.
- Use CRISS-CROSS method for valency.
- Water: H⁺ O²⁻ → H₂O.
- Calcium chloride: Ca²⁺ Cl⁻ → CaCl₂.
- Magnesium oxide: Mg²⁺ O²⁻ → MgO.
3. Mixtures
A MIXTURE is formed by PHYSICALLY mixing two or more substances (elements or compounds) in ANY proportion.
Types of Mixtures
| Type | Definition | Examples |
|---|---|---|
| Homogeneous | Uniform composition throughout | Salt solution, air, alloy |
| Heterogeneous | Non-uniform composition | Sand and water, oil and water, soil |
Comparison: Compound vs Mixture
| Feature | Compound | Mixture |
|---|---|---|
| Nature | PURE substance | IMPURE (two or more substances) |
| Formation | Chemical reaction | Physical mixing |
| Ratio | FIXED ratio by mass | ANY ratio |
| Properties | Different from constituents | Similar to constituents |
| Separation | Chemical methods only | Physical methods |
| Homogeneity | Always homogeneous | Can be homogeneous or heterogeneous |
4. Separation of Mixtures
Physical Methods of Separation
| Method | Used For | Principle |
|---|---|---|
| Handpicking | Large visible particles | Manual sorting |
| Threshing | Separating grains from stalks | Beating |
| Winnowing | Lighter husk from heavier grain | Difference in weight/wind |
| Sieving | Particles of different sizes | Difference in size |
| Sedimentation & Decantation | Insoluble solid from liquid | Gravity settling |
| Filtration | Insoluble solid from liquid | Filter paper |
| Evaporation | Soluble solid from liquid | Liquid evaporates, solid remains |
| Distillation | Two miscible liquids with different boiling points | Difference in boiling point |
| Chromatography | Coloured components of a mixture | Different speeds on medium |
| Magnetic separation | Magnetic from non-magnetic materials | Magnetic attraction |
| Separating funnel | Immiscible liquids | Difference in density |
Worked Example (ICSE 2024, 2 marks)
'How would you separate a mixture of iron filings and sulphur?'
Solution: Use a MAGNET. Iron filings are attracted to the magnet. Sulphur is NOT attracted and remains behind.
5. Solutions
A SOLUTION is a HOMOGENEOUS mixture of two or more substances.
| Term | Definition | Example |
|---|---|---|
| Solute | The substance that DISSOLVES (smaller amount) | Salt in salt water |
| Solvent | The substance that DOES the dissolving (larger amount) | Water in salt water |
| Solution | The mixture formed | Salt water |
Types of Solutions (Based on Amount of Solute)
- Saturated: Cannot dissolve more solute at given temperature.
- Unsaturated: Can dissolve more solute.
- Supersaturated: Contains more solute than normally possible (unstable).
6. ICSE Exam Focus
| Topic | Marks | Frequency |
|---|---|---|
| Element symbols (name → symbol) | 2 marks | Very High |
| Compound vs mixture differences | 3 marks | Very High |
| Separation techniques | 2-3 marks | High |
| Classification of elements | 2 marks | Medium |
| Solution terms (solute, solvent) | 1-2 marks | Medium |
Common Mistakes
- Writing CO for cobalt (CO is carbon monoxide; Cobalt is Co).
- Thinking compounds and mixtures are the same (compounds have fixed ratio, mixtures do not).
- Confusing distillation and evaporation (distillation involves condensation too).
- Forgetting that ALL compounds are homogeneous, but mixtures can be homogeneous or heterogeneous.
Self-Test (5 Questions)
Q1. The symbol of gold is: (1 mark)
- A) Go
- B) Gd
- C) Au
- D) Ag
Q2. 'Is air an element, compound, or mixture?' (1 mark)
Q3. Differentiate between a compound and a mixture. (Give 2 points) (2 marks)
Q4. 'How would you separate a mixture of sand and salt?' (2 marks)
Q5. 'In a sugar solution, identify the solute and solvent.' (2 marks)
Answers
A1. C) Au (from Latin 'Aurum'). A2. Air is a MIXTURE of gases (nitrogen, oxygen, CO₂, noble gases, etc.). A3. Compound: Fixed ratio by mass, properties different from constituents, chemical formation. Mixture: Any ratio, properties similar to constituents, physical mixing. (Any two points.) A4. Add water to dissolve salt. Filter to separate sand (undissolved). Evaporate filtrate to obtain salt. A5. Solute = Sugar. Solvent = Water.
