Summary and Revision: Acids, Bases and Salts

📚 Chapter Overview

In this chapter, we've explored acids, bases, and salts—three classes of compounds that play crucial roles in our daily lives and in the world around us. Let's review the key concepts we've learned:

💡 Key Concepts: Acids

What are Acids?

• Definition: Substances that produce hydrogen ions (H⁺) in aqueous solutions
• General properties: Sour taste, turn blue litmus red, react with metals to produce hydrogen gas
• Examples: Hydrochloric acid (HCl), Sulfuric acid (H₂SO₄), Nitric acid (HNO₃), Acetic acid (CH₃COOH)

Chemical Reactions of Acids:

1. With metals:

   Metal + Acid → Salt + Hydrogen gas
   Example: Zn + 2HCl → ZnCl₂ + H₂

2. With metal carbonates and bicarbonates:

   Metal carbonate + Acid → Salt + Water + Carbon dioxide
   Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

3. With metal oxides:

   Metal oxide + Acid → Salt + Water
   Example: CuO + 2HCl → CuCl₂ + H₂O

💡 Key Concepts: Bases

What are Bases?

• Definition: Substances that produce hydroxide ions (OH⁻) in aqueous solutions
• General properties: Bitter taste, feel slippery, turn red litmus blue
• Examples: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Calcium hydroxide (Ca(OH)₂)
• Alkalis: Bases that are soluble in water

Chemical Reactions of Bases:

1. With certain metals:

   Base + Active metal → Salt + Hydrogen gas
   Example: 2NaOH + Zn → Na₂ZnO₂ + H₂

2. With non-metallic oxides:

   Base + Non-metallic oxide → Salt + Water
   Example: 2NaOH + CO₂ → Na₂CO₃ + H₂O

3. With acids (Neutralization):

   Base + Acid → Salt + Water
   Example: NaOH + HCl → NaCl + H₂O

4. With ammonium salts:

   Base + Ammonium salt → Salt + Water + Ammonia
   Example: NaOH + NH₄Cl → NaCl + H₂O + NH₃

💡 Key Concepts: pH Scale

Understanding pH:

• pH range: 0-14
• Acidic: pH < 7
• Neutral: pH = 7
• Basic: pH > 7
• Mathematical definition: pH = -log[H⁺]
• Each unit change: Represents a tenfold change in hydrogen ion concentration

pH Indicators:

• Litmus: Blue in bases, red in acids
• Phenolphthalein: Colorless in acids, pink in bases
• Methyl orange: Red in acids, yellow in bases
• Universal indicator: Shows different colors for different pH values

Importance of pH:

• Biological systems: Most function within narrow pH ranges
• Agriculture: Soil pH affects nutrient availability
• Industry: pH control is crucial in many manufacturing processes
• Environmental concerns: pH of water bodies affects aquatic life

💡 Key Concepts: Salts

What are Salts?

• Definition: Ionic compounds formed by neutralization reaction between acids and bases
• General formula: Metal + Non-metal (or polyatomic ion)
• Examples: Sodium chloride (NaCl), Calcium carbonate (CaCO₃), Potassium nitrate (KNO₃)

Types of Salts:

• Normal salts: Formed by complete neutralization (e.g., NaCl)
• Acid salts: Formed by partial neutralization of a polybasic acid (e.g., NaHSO₄)
• Basic salts: Formed by partial neutralization of a polyacidic base (e.g., Pb(OH)Cl)
• Double salts: Combination of two different salts (e.g., Alum - KAl(SO₄)₂·12H₂O)

pH of Salt Solutions:

• Neutral salts: Strong acid + Strong base (e.g., NaCl) - pH = 7
• Acidic salts: Strong acid + Weak base (e.g., NH₄Cl) - pH < 7
• Basic salts: Weak acid + Strong base (e.g., CH₃COONa) - pH > 7

Water of Crystallization:

• Definition: Fixed number of water molecules present in one formula unit of a salt
• Examples:
  • Copper sulfate pentahydrate (CuSO₄·5H₂O)
  • Gypsum (CaSO₄·2H₂O)
  • Washing soda (Na₂CO₃·10H₂O)

💡 Key Concepts: Important Chemicals from Common Salt

Chlor-Alkali Process:

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Important Products:

1. Sodium Hydroxide (NaOH):

   • Uses: Soap making, paper industry, textile processing, aluminum extraction

2. Bleaching Powder (CaOCl₂):

   • Preparation: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
   • Uses: Bleaching, water treatment, disinfection

3. Baking Soda (NaHCO₃):

   • Preparation: NaCl + NH₃ + CO₂ + H₂O → NaHCO₃ + NH₄Cl
   • Uses: Cooking, antacid, fire extinguishers

4. Washing Soda (Na₂CO₃·10H₂O):

   • Preparation: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂, then Na₂CO₃ + 10H₂O → Na₂CO₃·10H₂O
   • Uses: Cleaning agent, water softener, glass manufacturing

5. Plaster of Paris (CaSO₄·½H₂O):

   • Preparation: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O
   • Uses: Casts for broken bones, decorative items, smooth wall surfaces

🧮 Important Chemical Equations

Acids:

1. With metals: Zn + 2HCl → ZnCl₂ + H₂
2. With carbonates: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
3. With bicarbonates: NaHCO₃ + HCl → NaCl + H₂O + CO₂
4. With metal oxides: CuO + 2HCl → CuCl₂ + H₂O

Bases:

1. With metals: 2NaOH + Zn → Na₂ZnO₂ + H₂
2. With non-metallic oxides: 2NaOH + CO₂ → Na₂CO₃ + H₂O
3. With acids: NaOH + HCl → NaCl + H₂O
4. With ammonium salts: NaOH + NH₄Cl → NaCl + H₂O + NH₃

Common Salt Derivatives:

1. Chlor-alkali process: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
2. Bleaching powder: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O
3. Baking soda decomposition: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂
4. Plaster of Paris setting: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O

⚖️ Quick Comparison Table: Acids vs. Bases

Property	Acids	Bases
Taste	Sour	Bitter
Feel	-	Slippery/Soapy
Litmus test	Turn blue litmus red	Turn red litmus blue
Phenolphthalein	Remains colorless	Turns pink
Methyl orange	Turns red	Turns yellow
Ions produced in water	H⁺ (hydrogen ions)	OH⁻ (hydroxide ions)
pH value	Less than 7	Greater than 7
Reaction with metals	Produce H₂ gas with active metals	Some produce H₂ gas with certain metals
Reaction with carbonates	Produce CO₂ gas	No reaction
Electrical conductivity	Conduct electricity	Conduct electricity

🧠 Memory Tricks

1. For pH scale: "A for Acid" (0-7) and "B for Base" (7-14)

2. For acid-base indicators:

   • "RED for ACID, BLUE for BASE" (litmus paper)
   • "RED is ACID" (methyl orange)
   • "PINK is not ACID" (phenolphthalein)

3. For neutralization products:

   • "SWAN" - Salt + Water from Acid and base Neutralization

4. For water of crystallization colors:

   • "Blue CuSO₄, White CaSO₄, Green FeSO₄"

⚠️ Common Misconceptions Clarified

1. Not all hydrogen-containing compounds are acids

   • Only those that produce H⁺ ions in water

2. Not all bases are alkalis

   • Only bases that dissolve in water are alkalis

3. Not all salts have a neutral pH

   • pH depends on the strength of the parent acid and base

4. Neutralization doesn't always result in pH 7

   • The final pH depends on the strengths of the reacting acid and base

🔍 Exam Tips

1. Pay attention to balanced equations

   • Ensure coefficients are correct for atoms on both sides

2. Memorize the pH ranges

   • Acidic: 0-7
   • Neutral: 7
   • Basic: 7-14

3. Practice pH calculations

   • Remember pH = -log[H⁺]

4. Focus on real-life applications

   • Antacids for stomach acidity
   • Soil treatment in agriculture
   • Common salt derivatives in daily life

5. Understand neutralization reactions

   • Identify the salt formed
   • Predict the pH of the resulting solution

🤔 Questions to Test Your Understanding

1. What happens when zinc metal is added to sodium hydroxide solution?

2. Explain why aqueous solutions of acid conduct electricity.

3. A farmer finds that the soil in his field is too acidic. How can he reduce the acidity of the soil?

4. What would happen when a solution of an acid is diluted? Will the pH increase or decrease? Explain.

5. Name the salt that has the formula CaSO₄·½H₂O. How is it prepared and what happens when it is mixed with water?

6. What is the role of water of crystallization in a hydrated salt?

7. Why is washing soda used for softening hard water?

8. When carbon dioxide is passed through lime water, it turns milky. Explain why.

9. Explain why Plaster of Paris should be stored in a moisture-proof container.

10. What is the chemical formula of baking soda? Give two uses of baking soda.

🌍 Real-World Connections

Remember that acids, bases, and salts are all around us:

• The citric acid in lemons and oranges
• The acetic acid in vinegar
• The sodium hydroxide in soaps
• The calcium carbonate in chalk and limestone
• The sodium chloride on your dining table
• The hydrochloric acid in your stomach
• The sodium hydrogen carbonate in baking powder

Understanding the chemistry of these substances helps us appreciate the science behind many everyday phenomena and products.

🔜 Moving Forward

As you proceed to more advanced chemistry, the concepts of acids, bases, and salts will form a foundation for understanding:

• Chemical equilibrium
• Ionic bonding
• Redox reactions
• Organic chemistry
• Biochemistry

The knowledge you've gained in this chapter connects to nearly every area of chemistry you'll study in the future!