Introduction
Acids, Bases, and Chemical Reactions form the core of everyday Chemistry tested in Railway exams — from
the pH scale to the four fundamental reaction types, this chapter covers the concepts that appear most
frequently.
Acids — Properties
| Property |
|---|
| Sour taste |
| Turn blue litmus paper red |
| pH less than 7 |
| React with metals to release hydrogen gas |
| React with bases to form salt and water (neutralisation) |
Bases — Properties
| Property |
|---|
| Bitter taste, soapy/slippery feel |
| Turn red litmus paper blue |
| pH greater than 7 |
| A water-soluble base is called an Alkali |
The pH Scale
📌 Key Concept: The pH scale ranges from 0 to 14 and measures how acidic or basic (alkaline) a solution is.
| pH Range | Nature |
|---|
| 0-6 | Acidic (lower = more acidic) |
| 7 | Neutral (pure water) |
| 8-14 | Basic/Alkaline (higher = more basic) |
Common Acids and Bases — Everyday Examples
| Substance | Type | Chemical Name |
|---|
| Lemon/Citrus fruits | Acid | Citric acid |
| Vinegar | Acid | Acetic acid |
| Curd/Yogurt | Acid | Lactic acid |
| Stomach acid | Acid | Hydrochloric acid (HCl) |
| Soap | Base | Sodium/Potassium salts of fatty acids |
| Baking soda | Base | Sodium bicarbonate |
| Antacid tablets | Base | Magnesium hydroxide / Calcium carbonate |
Indicators
| Indicator | In Acid | In Base |
|---|
| Litmus Paper | Red | Blue |
| Phenolphthalein | Colourless | Pink |
| Methyl Orange | Red/Pink | Yellow |
Neutralisation Reaction
Q. What happens when hydrochloric acid reacts with sodium hydroxide?
Acid + Base → Salt + Water
HCl + NaOH → NaCl + H₂O
This is a classic neutralisation reaction, producing common salt and water.
Types of Chemical Reactions
| Type | Description | Example |
|---|
| Combination Reaction | Two or more substances combine to form a single product | 2H₂ + O₂ → 2H₂O |
| Decomposition Reaction | A single compound breaks down into two or more simpler substances | CaCO₃ → CaO + CO₂ |
| Displacement Reaction | A more reactive element displaces a less reactive one from a compound | Fe + CuSO₄ → FeSO₄ + Cu |
| Double Displacement Reaction | Two compounds exchange ions to form two new compounds | AgNO₃ + NaCl → AgCl + NaNO₃ |
Q. Identify the type of reaction: Fe + CuSO₄ → FeSO₄ + Cu
Iron (more reactive) displaces copper (less reactive) from copper sulphate
This is a Displacement Reaction
Oxidation and Reduction (Redox Reactions)
| Process | Definition |
|---|
| Oxidation | Loss of electrons / gain of oxygen |
| Reduction | Gain of electrons / loss of oxygen |
✅ Exam Focus: pH scale ranges (acidic <7, neutral=7, basic >7) · Common acid/base everyday examples · Litmus and phenolphthalein indicator colour changes · Four reaction types (combination, decomposition, displacement, double displacement) · Basic oxidation/reduction concept.