Grade 8 Chapter 12: Introduction to Acid and Base

Introduction

Acids and bases are fundamental chemical substances that play a crucial role in our daily lives, from the food we eat to the products we use for cleaning. They are also essential in various industrial processes and biological systems. This chapter will introduce you to the basic properties of acids and bases, how to identify them using indicators, the concept of the pH scale, and the important reaction of neutralization. Understanding these concepts will help you appreciate the chemistry behind many everyday phenomena.

Acids

Acids are substances that typically taste sour and are corrosive. They release hydrogen ions (H⁺) when dissolved in water.

• Examples of common acids:
  • Hydrochloric acid (HCl) - present in stomach acid
  • Sulfuric acid (H₂SO₄) - 'King of Chemicals', used in batteries
  • Nitric acid (HNO₃) - used in fertilizers
  • Acetic acid (CH₃COOH) - present in vinegar
  • Citric acid - present in lemons, oranges
  • Lactic acid - present in curd

Properties of Acids

• Taste: Sour (e.g., lemon, vinegar).
• Corrosive Nature: Strong acids are corrosive and can cause burns to skin and damage materials.
• Effect on Litmus Paper: Turn blue litmus paper red.
• Reaction with Metals: Acids react with active metals to produce hydrogen gas and a salt.
  Metal + Acid → Salt + Hydrogen gas
  Example: Zn (s) + 2HCl (aq) → ZnCl₂ (aq) + H₂ (g)
• Reaction with Metal Carbonates and Bicarbonates: Acids react with metal carbonates and bicarbonates to produce carbon dioxide gas, water, and a salt.
  Metal Carbonate/Bicarbonate + Acid → Salt + Water + Carbon dioxide gas
  Example: Na₂CO₃ (s) + 2HCl (aq) → 2NaCl (aq) + H₂O (l) + CO₂ (g)
• Reaction with Bases (Neutralization): Acids react with bases to form salt and water.

Bases

Bases are substances that typically taste bitter and feel soapy or slippery to touch. They release hydroxide ions (OH⁻) when dissolved in water.

• Examples of common bases:
  • Sodium hydroxide (NaOH) - caustic soda, used in soap making
  • Potassium hydroxide (KOH) - caustic potash
  • Calcium hydroxide (Ca(OH)₂) - lime water
  • Ammonium hydroxide (NH₄OH) - used in window cleaners

Properties of Bases

• Taste: Bitter (e.g., soap).
• Feel: Soapy or slippery to touch.
• Corrosive Nature: Strong bases are also corrosive.
• Effect on Litmus Paper: Turn red litmus paper blue.
• Reaction with Acids (Neutralization): Bases react with acids to form salt and water.
• Reaction with Nonmetal Oxides: Bases react with nonmetal oxides (which are acidic) to form salt and water.
  Nonmetal Oxide + Base → Salt + Water
  Example: CO₂ (g) + Ca(OH)₂ (aq) → CaCO₃ (s) + H₂O (l)

Indicators

Indicators are substances that show a change in color when added to an acidic or basic solution. They help us identify whether a substance is acidic or basic.

• Litmus Paper:
  • Blue litmus turns red in acidic solutions.
  • Red litmus turns blue in basic solutions.
  • No color change in neutral solutions.
• Methyl Orange:
  • Turns red in acidic solutions.
  • Turns yellow in basic solutions.
• Phenolphthalein:
  • Remains colorless in acidic solutions.
  • Turns pink in basic solutions.
• Turmeric: (Natural indicator)
  • Remains yellow in acidic/neutral solutions.
  • Turns reddish-brown in basic solutions.
• Red Cabbage Juice: (Natural indicator)
  • Red/Pink in acidic solutions.
  • Green/Yellow in basic solutions.

pH Scale

The pH scale is a scale used to measure the acidity or alkalinity (basicity) of a solution. It ranges from 0 to 14.

• pH < 7: Acidic solution (lower pH, stronger acid).
• pH = 7: Neutral solution (e.g., pure water).
• pH > 7: Basic (alkaline) solution (higher pH, stronger base).

Neutralization Reaction

The reaction between an acid and a base to form salt and water is called a neutralization reaction. This reaction typically releases heat (exothermic).

Acid + Base → Salt + Water + Heat

• Example: HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l) + Heat
  (Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water + Heat)

Salts

Salts are ionic compounds formed when an acid reacts with a base during a neutralization reaction. They are typically neutral, but some can be acidic or basic depending on the strength of the acid and base from which they are formed.

• Examples: Sodium chloride (NaCl), Potassium sulfate (K₂SO₄), Calcium carbonate (CaCO₃).

Importance of pH in Everyday Life

• In Digestion: Our stomach produces hydrochloric acid (HCl) with a pH of about 1.5-3.5, which helps in digesting food and killing harmful bacteria. Excess acid causes acidity.
• pH of Soil: Plants require a specific pH range for optimal growth. If the soil is too acidic or too basic, it affects crop yield. Farmers often add lime (basic) to acidic soil or organic matter to basic soil to adjust pH.
• Acid Rain: Rainwater with a pH less than 5.6 is considered acid rain, caused by air pollutants (SO₂, NOₓ). It harms aquatic life, damages buildings, and affects soil fertility.
• Tooth Decay: When the pH inside the mouth falls below 5.5 (due to acids produced by bacteria breaking down food particles), tooth enamel starts to corrode, leading to tooth decay. Using basic toothpaste helps neutralize these acids.
• Insect Stings: Ant stings contain formic acid, which causes pain and irritation. Rubbing baking soda (a mild base) provides relief. Wasp stings are alkaline, so vinegar (a mild acid) can be used.

Summary

• Acids: Sour, corrosive, turn blue litmus red, react with metals (H₂ gas), carbonates (CO₂ gas), and bases.
• Bases: Bitter, soapy, turn red litmus blue, react with acids and nonmetal oxides.
• Indicators: Substances like litmus, methyl orange, phenolphthalein, and turmeric change color to identify acids/bases.
• pH Scale: Measures acidity/basicity (0-14). pH < 7 (acidic), pH = 7 (neutral), pH > 7 (basic).
• Neutralization Reaction: Acid + Base → Salt + Water + Heat.
• Salts: Formed from neutralization reactions.
• Importance of pH: Crucial in digestion, soil fertility, acid rain, tooth decay, and treating insect stings.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 12: Introduction to Acid and Base.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.