The Language of Chemistry
Introduction
Chemistry has its own language consisting of symbols, formulae, and equations. ICSE Class 9 requires students to become fluent in this language to describe chemical substances and reactions accurately.
Symbols of Elements
Each element has a unique symbol, usually the first one or two letters of its name.
| Element | Symbol | Element | Symbol |
|---|---|---|---|
| Hydrogen | H | Sodium | Na |
| Carbon | C | Chlorine | Cl |
| Nitrogen | N | Calcium | Ca |
| Oxygen | O | Iron | Fe |
| Helium | He | Copper | Cu |
| Aluminium | Al | Zinc | Zn |
Rules for Writing Symbols:
- First letter is always capital
- Second letter (if any) is always small
- Symbols are derived from Latin or Greek names (e.g., Na from Natrium)
Valency
Valency is the combining capacity of an element, measured by the number of hydrogen atoms it can combine with or displace.
| Valency | Elements |
|---|---|
| 1 | H, Na, K, Cl, F, Br, I, Ag |
| 2 | Mg, Ca, Zn, Ba, O, S, Cu |
| 3 | Al, N, P, Fe |
| 4 | C, Si, Pb |
Variable Valency: Some elements exhibit more than one valency.
- Iron: Fe²⁺ (ferrous), Fe³⁺ (ferric)
- Copper: Cu⁺ (cuprous), Cu²⁺ (cupric)
Radicals (Ions)
A radical is an atom or a group of atoms with a charge.
| Positive Radicals (Cations) | Formula | Valency |
|---|---|---|
| Hydrogen | H⁺ | 1 |
| Sodium | Na⁺ | 1 |
| Calcium | Ca²⁺ | 2 |
| Aluminium | Al³⁺ | 3 |
| Negative Radicals (Anions) | Formula | Valency |
|---|---|---|
| Chloride | Cl⁻ | 1 |
| Sulphate | SO₄²⁻ | 2 |
| Nitrate | NO₃⁻ | 1 |
| Carbonate | CO₃²⁻ | 2 |
| Hydroxide | OH⁻ | 1 |
| Phosphate | PO₄³⁻ | 3 |
Writing Chemical Formulae
Criss-cross Method:
- Write symbols side by side (positive first)
- Write valencies above each symbol
- Criss-cross the valencies
- Simplify if possible
Balancing Chemical Equations
A balanced equation has equal numbers of atoms of each element on both sides.
Steps:
- Write the skeleton equation
- Count atoms of each element
- Balance elements one at a time (leave O and H for last)
- Use coefficients (never change subscripts)
- Verify both sides are equal
Balance O: 2H₂ + O₂ → 2H₂O Now H: 4 on left, 4 on right ✓ O: 2 on left, 2 on right ✓
Balanced: 2H₂ + O₂ → 2H₂O </Solution> </ICSEExample>
Types of Chemical Reactions
| Type | Description | Example |
|---|---|---|
| Combination | A + B → AB | 2H₂ + O₂ → 2H₂O |
| Decomposition | AB → A + B | CaCO₃ → CaO + CO₂ |
| Displacement | A + BC → AC + B | Zn + H₂SO₄ → ZnSO₄ + H₂ |
| Double displacement | AB + CD → AD + CB | AgNO₃ + NaCl → AgCl + NaNO₃ |
| Redox (Oxidation-Reduction) | Transfer of electrons | CuO + H₂ → Cu + H₂O |
Common Mistakes With Fixes
| Mistake | Correction |
|---|---|
| Changing subscripts when balancing | Only change coefficients; never change subscripts |
| Writing CaOH₂ instead of Ca(OH)₂ | Use brackets for polyatomic ions |
| Confusing valency and oxidation number | Valency is combining capacity; oxidation number is charge |
| Forgetting to simplify criss-cross results | Mg₂O₂ simplifies to MgO |
ICSE Exam Focus
| Topic | Marks (approx.) | Frequency |
|---|---|---|
| Writing chemical formulae | 3-4 marks | Very common |
| Balancing equations | 4-5 marks | Very common |
| Identifying reaction types | 3 marks | Common |
| Valency and radicals | 2-3 marks | Very common |
Self-Test
Q1: Write the formulae of: (i) Sodium carbonate (ii) Aluminium sulphate (iii) Calcium phosphate
Q2: Balance: Fe + H₂O → Fe₃O₄ + H₂
Q3: What is the valency of: (i) Calcium (ii) Aluminium (iii) Carbonate radical
Q4: Distinguish between combination and decomposition reactions with examples.
Q5: Write the symbols of: (i) Iron (ii) Sodium (iii) Potassium
