Module 1: Introduction to Acids and Bases

 

🔬 Module 1: Introduction to Acids and Bases

(Class 10 CBSE Science – Chapter: Acids, Bases, and Salts)

🌟 1. What are Acids and Bases?

Acids and Bases are two important categories of chemical substances that show distinctly different chemical and physical properties.

📘 Definition:

• Acids: Substances that release hydrogen ions (H⁺) in an aqueous solution.
  ➤ Example: Hydrochloric acid (HCl) dissociates in water as:
  HCl → H⁺ + Cl⁻

• Bases: Substances that release hydroxide ions (OH⁻) in an aqueous solution.
  ➤ Example: Sodium hydroxide (NaOH) dissociates as:
  NaOH → Na⁺ + OH⁻

📌 Note: The presence of H⁺ or OH⁻ ions in water is responsible for the acidic or basic nature of the substance.

🔍 2. Physical Properties

Property	Acids	Bases
Taste	Sour (Don't taste in lab!)	Bitter (Don't taste in lab!)
Touch	Corrosive	Soapy or slippery
Effect on litmus	Turns blue to red	Turns red to blue
Electrical conductivity	Good conductor (due to H⁺ ions)	Good conductor (due to OH⁻ ions)

🧪 3. Common Examples from Daily Life

Acids	Bases
Lemon juice (citric acid)	Soap (sodium hydroxide)
Vinegar (acetic acid)	Baking soda (sodium bicarbonate)
Curd (lactic acid)	Toothpaste (mild base)
Tamarind (tartaric acid)	Limewater (calcium hydroxide)

🧫 4. Indicators: Detecting Acids and Bases

Indicators are substances that change color in acidic or basic media. They help to identify whether a given solution is acidic or basic.

🔹 Types of Indicators:

Indicator	Acidic Solution	Basic Solution
Litmus	Blue → Red	Red → Blue
Phenolphthalein	Colorless	Pink
Methyl Orange	Red	Yellow

✅ Natural Indicators: Litmus (from lichens), turmeric, red cabbage

✅ Synthetic Indicators: Phenolphthalein, Methyl orange

🔁 5. Acid-Base Reactions with Indicators (Visual Table)

Substance	Effect on Litmus	Effect on Phenolphthalein	Effect on Methyl Orange
Hydrochloric acid	Blue → Red	Colorless	Red
Sodium hydroxide	Red → Blue	Pink	Yellow
Lemon juice	Blue → Red	Colorless	Red
Soap solution	Red → Blue	Pink	Yellow

🎯 6. Key Scientific Concepts

• Arrhenius Theory:

  • Acids: Produce H⁺ in water.

  • Bases: Produce OH⁻ in water.

• Neutralisation Reaction:

  • Acid + Base → Salt + Water
    HCl + NaOH → NaCl + H₂O

📚 7. Misconceptions Cleared

• Acids are not always dangerous. Many are edible and found in food (like citric acid).

• Not all substances that feel slippery are bases (e.g., oil is slippery but not basic).

• Indicators are tools, not proof of concentration or strength—just the presence of H⁺ or OH⁻ ions.

🧠 8. Thinking Corner

Why do ant bites cause a burning sensation, and how does applying baking soda help?

Answer: Ant sting injects formic acid (acidic). Baking soda (a base) neutralizes the acid, relieving the pain.

🧪 Hands-on Activity (Home/Lab)

Name: Testing Acids and Bases Using Indicators
Materials: Red/blue litmus paper, lemon juice, baking soda solution, soap, vinegar, shampoo
Procedure: Dip litmus in each solution and record color changes
Conclusion: Identify acidic and basic substances

📌 Summary Points

• Acids and bases are classified based on the release of H⁺ and OH⁻ ions respectively.

• Physical characteristics help in identification but are not to be tested manually.

• Indicators help detect the acidic or basic nature of substances.

• Some substances can behave as acids or bases depending on the environment (amphoteric substances – to be studied later).