Acids, Bases and Salts

Overview

Acids, Bases and Salts form a foundational chemistry topic for GTET Paper-2, appearing consistently in the Science section. This topic connects everyday substances (lemon juice, soap, baking soda) to chemical principles, making it both practically relevant and conceptually important for upper primary teaching.

For the exam, you must understand the characteristic properties of acids and bases, how indicators help identify them, the neutralisation reaction that produces salts, and the common applications of these substances. Questions typically test identification of acidic/basic substances, indicator colour changes, salt formation reactions, and practical uses. Mastering this topic also builds the foundation for understanding chemical reactions and environmental chemistry.

The NCERT Class 7 and Class 8 Science textbooks cover this topic thoroughly, and GTET questions align closely with that treatment level—focusing on observable properties, simple reactions, and real-world connections rather than complex calculations.

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Key Concepts

• **Acids** are substances that taste sour, turn blue litmus red, and release hydrogen ions (H⁺) when dissolved in water. Examples: hydrochloric acid (HCl), sulphuric acid (H₂SO₄), citric acid in lemon.

• **Bases** are substances that taste bitter, feel soapy, turn red litmus blue, and release hydroxide ions (OH⁻) in water. Examples: sodium hydroxide (NaOH), calcium hydroxide (slaked lime), magnesium hydroxide.

• **Indicators** are substances that show different colours in acidic and basic solutions. Natural indicators include litmus, turmeric, and China rose; synthetic indicators include methyl orange and phenolphthalein.

• **Neutralisation** is the reaction between an acid and a base to form salt and water. This reaction releases heat (exothermic). General equation: Acid + Base → Salt + Water.

• **Salts** are ionic compounds formed from neutralisation reactions. The type of salt depends on which acid and base react. Example: HCl + NaOH → NaCl + H₂O.

• **pH Scale** measures how acidic or basic a solution is, ranging from 0 to 14. pH 7 is neutral, below 7 is acidic, above 7 is basic.

• **Strong vs Weak**: Strong acids/bases dissociate completely in water (HCl, NaOH); weak acids/bases dissociate partially (acetic acid, ammonia).

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Formulas / Key Facts

| Fact | Detail | |------|--------| | Litmus source | Lichen (a plant) | | Blue litmus + Acid | Turns red | | Red litmus + Base | Turns blue | | Turmeric + Base | Turns red/brown | | Phenolphthalein + Base | Turns pink | | Phenolphthalein + Acid | Remains colourless | | Methyl orange + Acid | Turns red | | Methyl orange + Base | Turns yellow | | pH of pure water | 7 (neutral) | | pH of lemon juice | Approximately 2 (acidic) | | pH of soap solution | Approximately 9-10 (basic) |

**Common Acids and Sources:**

• Hydrochloric acid (HCl) — stomach acid
• Acetic acid (CH₃COOH) — vinegar
• Citric acid — lemon, orange
• Lactic acid — curd, sour milk
• Oxalic acid — tomato, spinach
• Tartaric acid — tamarind, grapes

**Common Bases and Sources:**

• Sodium hydroxide (NaOH) — caustic soda, soap making
• Calcium hydroxide Ca(OH)₂ — slaked lime, whitewashing
• Magnesium hydroxide Mg(OH)₂ — milk of magnesia (antacid)
• Ammonium hydroxide (NH₄OH) — cleaning agents

**Important Salts:**

• Sodium chloride (NaCl) — common salt, food preservation
• Sodium bicarbonate (NaHCO₃) — baking soda, antacid
• Sodium carbonate (Na₂CO₃) — washing soda, glass making
• Calcium sulphate (CaSO₄) — Plaster of Paris, gypsum

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Worked Examples

**Example 1: Identifying Acid or Base**

*A solution turns blue litmus red and phenolphthalein remains colourless. Is the solution acidic or basic?*

Step 1: Blue litmus turning red indicates an acid. Step 2: Phenolphthalein staying colourless confirms acid (it turns pink only in bases). **Answer:** The solution is acidic.

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**Example 2: Neutralisation Reaction**

*Write the reaction when hydrochloric acid reacts with sodium hydroxide.*

Step 1: Identify the acid (HCl) and base (NaOH). Step 2: Apply neutralisation: Acid + Base → Salt + Water Step 3: HCl + NaOH → NaCl + H₂O

**Answer:** Sodium chloride (common salt) and water are formed.

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**Example 3: Choosing an Antacid**

*Why is milk of magnesia used to treat acidity in the stomach?*

Step 1: Stomach acidity is caused by excess hydrochloric acid. Step 2: Milk of magnesia contains magnesium hydroxide, which is a base. Step 3: The base neutralises the excess acid, providing relief.

**Answer:** Magnesium hydroxide neutralises excess stomach acid through a neutralisation reaction.

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Common Mistakes

• **Confusing litmus colour changes** → Remember: "Acid turns blue litmus to red" (A-B-R sequence). Base does the opposite.

• **Thinking all salts are neutral** → Not true. Salts like sodium carbonate are basic; ammonium chloride is acidic. The nature depends on the parent acid and base.

• **Mixing up baking soda and washing soda** → Baking soda is sodium bicarbonate (NaHCO₃), used in cooking. Washing soda is sodium carbonate (Na₂CO₃), used for cleaning. Different formulas, different uses.

• **Believing neutralisation always gives pH 7** → Only when a strong acid reacts with a strong base. Weak acid + strong base gives pH above 7; strong acid + weak base gives pH below 7.

• **Forgetting that neutralisation is exothermic** → Heat is always released during neutralisation. This is a common MCQ trap.

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Quick Reference

• Acids: sour taste, H⁺ ions, turn blue litmus red
• Bases: bitter taste, OH⁻ ions, turn red litmus blue
• Neutralisation: Acid + Base → Salt + Water + Heat
• pH scale: 0-14; below 7 acidic, 7 neutral, above 7 basic
• Turmeric turns red-brown in bases (used in turmeric paper test)
• Antacids are bases that neutralise stomach acid