10.4 Characteristic Properties of Bases

This topic explains the general properties of bases — how they taste, how they feel, how they behave with litmus, and how they react with acids and ammonium salts.

Definition

• Taste bitter
• Feel slippery in aqueous solution
• Turn red litmus blue
• Neutralize acids to form salt and water
• Decompose ammonium salts on heating to liberate ammonia gas

Simple explanation: Bases have a set of characteristic properties that make them recognizable and useful.

Main Idea

The book lists five general properties of bases:

• Bases have bitter taste.
• Bases change the colour of red litmus paper to blue.
• Aqueous solutions of bases have a slippery touch.
• Bases neutralize acids to form salt and water.
• Bases decompose ammonium salts on heating and liberate ammonia gas.

Simple explanation: Bases taste bitter, feel slippery, turn red litmus blue, neutralize acids, and release ammonia from ammonium salts.

Core Concept

Summary Table

Detailed Explanation

Property 1: Bitter Taste

Bases have bitter taste.

• Soap — bitter
• Baking soda — bitter
• Antacid tablets — bitter

Simple explanation: The bitter taste of soap and baking soda is due to bases.

Property 2: Effect on Litmus Paper

Bases change the colour of red litmus paper to blue.

• Red litmus → blue in base
• Blue litmus → no change in base

Simple explanation: Red litmus paper is used to test for bases — it turns blue.

Property 3: Slippery Touch

Aqueous solutions of bases have a slippery touch.

• Soap solution — slippery
• Sodium hydroxide solution — slippery

Simple explanation: Bases feel soapy or slippery when dissolved in water.

Property 4: Neutralization of Acids

Bases neutralize acids to form salt and water.

General equation:

Acid + Base → Salt + Water

Examples from the book:

Sulphuric acid reacts with sodium hydroxide:

H₂SO₄ + NaOH → Na₂SO₄ + H₂O

Simple explanation: Bases cancel out acids to form salt and water — this is neutralization.

Property 5: Decomposition of Ammonium Salts

Bases decompose ammonium salts on heating and liberate ammonia gas.

General equation:

Base + Ammonium salt → Salt + Ammonia + Water

Examples from the book:

Sodium hydroxide reacts with ammonium chloride:

NaOH + NH₄Cl → NaCl + NH₃ + H₂O

Potassium hydroxide reacts with ammonium chloride:

KOH + NH₄Cl → KCl + NH₃ + H₂O

Simple explanation: When bases react with ammonium salts, they release ammonia gas (which has a characteristic smell).

Intermediate Level

Why Bases Have These Properties

Bases produce OH⁻ ions in water (Arrhenius theory) or accept protons (Bronsted-Lowry theory). These properties are responsible for the characteristic behaviour:

• Bitter taste — OH⁻ ions interact with taste receptors.
• Slippery feel — OH⁻ ions react with oils on the skin, forming soap-like substances.
• Litmus change — OH⁻ ions change the dye colour.
• Neutralization — OH⁻ ions react with H⁺ ions to form H₂O.
• Ammonia release — OH⁻ ions react with NH₄⁺ to form NH₃ and H₂O.

Simple explanation: It's the OH⁻ ions (or proton-accepting ability) that give bases their characteristic properties.

Balancing the Reactions

Neutralization:

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

Ammonium salt decomposition:

2NaOH + (NH₄)₂SO₄ → Na₂SO₄ + 2NH₃ + 2H₂O

Note: Always balance the equations to satisfy the law of conservation of mass.

Industrial and Daily-Life Applications

Simple explanation: These reactions are used in medicine, agriculture, industry, and everyday products.

Comparison of Acids and Bases

Important Points

• - Bases have bitter taste.
• - Bases turn red litmus blue.
• - Aqueous solutions of bases have a slippery touch.
• - Bases neutralize acids to form salt and water.
• - Bases decompose ammonium salts on heating, liberating ammonia gas.
• - General equations:
• - Acid + Base → Salt + Water
• - Base + Ammonium salt → Salt + Ammonia + Water
• - Examples:
• - H₂SO₄ + NaOH → Na₂SO₄ + H₂O
• - NaOH + NH₄Cl → NaCl + NH₃ + H₂O
• - KOH + NH₄Cl → KCl + NH₃ + H₂O

Remember

• - Bases = bitter taste.
• - Red litmus → blue in bases.
• - Aqueous solutions of bases = slippery touch.
• - Acid + Base → Salt + H₂O (neutralization).
• - Base + Ammonium salt → Salt + NH₃ + H₂O.
• - Examples:
• - H₂SO₄ + NaOH → Na₂SO₄ + H₂O
• - NaOH + NH₄Cl → NaCl + NH₃ + H₂O
• - KOH + NH₄Cl → KCl + NH₃ + H₂O

Common Mistakes

• 1. Thinking bases turn blue litmus red. Bases turn red litmus blue; acids turn blue litmus red.
• 2. Forgetting that bases feel slippery. Aqueous solutions of bases have a slippery touch.
• 3. Confusing the products of neutralization. Products are salt + water.
• 4. Forgetting that bases release ammonia from ammonium salts. This is a key test for ammonium salts.
• 5. Thinking all bases are soluble in water. Some bases (like Al(OH)₃) are insoluble.
• 6. Thinking bases are always dangerous. Many bases (like baking soda) are safe and used in food.

Quick Revision

• General properties of bases:
• Acid + Base → Salt + Water
• H₂SO₄ + NaOH → Na₂SO₄ + H₂O
• Base + Ammonium salt → Salt + Ammonia + Water
• NaOH + NH₄Cl → NaCl + NH₃ + H₂O
• KOH + NH₄Cl → KCl + NH₃ + H₂O

60-Second Revision

• Bases = bitter taste.
• Red litmus → blue in bases.
• Aqueous solutions of bases = slippery touch.
• Acid + Base → Salt + H₂O.
• Base + Ammonium salt → Salt + NH₃ + H₂O.
• Warning: bases turn red litmus blue, not blue litmus red.