Acids Bases and Salts – Notes for CBSE Class 7 Science

Introduction to Acids Bases and Salts

Acids Bases and Salts- In our daily lives, we encounter many substances that taste sour, bitter, or neutral. Some of these substances can change color when tested with indicators, while others react in specific ways. This chapter introduces us to acids, bases, and salts, which are important types of chemical substances. We will explore their properties, how they react with each other, and their uses in everyday life.

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1. Acids

Definition: Acids are substances that taste sour and turn blue litmus paper red. They are usually corrosive and can react with metals, producing hydrogen gas.

Examples:

• Hydrochloric acid (HCl) – found in our stomach for digestion.
• Citric acid – present in citrus fruits like lemons and oranges.
• Sulphuric acid (H2SO4) – used in car batteries.
• Acetic acid – found in vinegar.

Properties of Acids:

1. Sour taste: Acids have a tangy or sour flavor (e.g., lemon juice).
2. Effect on indicators:
   • Turn blue litmus paper red.
   • No change with red litmus paper.
   • Phenolphthalein remains colorless.
   • Methyl orange turns red.
3. Reaction with metals: Acids react with metals like zinc or magnesium to release hydrogen gas. For example:
   • Zn + 2HCl → ZnCl₂ + H₂↑
4. Reaction with bases: Acids neutralize bases to form salt and water (neutralization reaction).
5. Conduct electricity: Acids conduct electricity when dissolved in water because they release ions.

Natural Sources of Acids:

• Lactic acid in curd.
• Tartaric acid in tamarind.
• Formic acid in ant stings.

Caution: Strong acids like sulphuric acid can burn skin and should be handled carefully.

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2. Bases

Definition: Bases are substances that taste bitter, feel slippery, and turn red litmus paper blue. They are often used in cleaning products.

Examples:

• Sodium hydroxide (NaOH) – used in soap-making.
• Calcium hydroxide (Ca(OH)₂) – found in lime water.
• Magnesium hydroxide (Mg(OH)₂) – used in antacids.
• Ammonium hydroxide (NH₄OH) – used in cleaning agents.

Properties of Bases:

1. Bitter taste: Bases have a bitter flavor (e.g., baking soda).
2. Slippery feel: Bases feel soapy when touched (e.g., soap).
3. Effect on indicators:
   • Turn red litmus paper blue.
   • No change with blue litmus paper.
   • Phenolphthalein turns pink.
   • Methyl orange turns yellow.
4. Reaction with acids: Bases neutralize acids to form salt and water.
5. Conduct electricity: Like acids, bases conduct electricity in water due to ion formation.

Natural Sources of Bases:

• Calcium hydroxide in lime water (used in whitewashing).
• Magnesium hydroxide in milk of magnesia (for acidity relief).

Caution: Strong bases like sodium hydroxide can cause burns and should be handled with care.

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3. Indicators

Definition: Indicators are substances that show whether a substance is acidic or basic by changing color.

Types of Indicators:

1. Natural Indicators:
   • Litmus: Extracted from lichens.
     • Blue litmus turns red in acids.
     • Red litmus turns blue in bases.
   • Turmeric: Turns red in bases, no change in acids.
   • Red cabbage juice: Turns red in acids, blue/green in bases.
2. Synthetic Indicators:
   • Phenolphthalein: Colorless in acids, pink in bases.
   • Methyl orange: Red in acids, yellow in bases.

Uses of Indicators:

• To test the nature of substances (acidic, basic, or neutral).
• To determine the pH of a solution in advanced studies.

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4. Neutral Substances

• Some substances are neither acidic nor basic; they are called neutral.
• Examples: Water, sugar solution, salt solution.
• Neutral substances do not change the color of indicators like litmus.

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5. Neutralization Reaction

Definition: When an acid reacts with a base, they neutralize each other to form salt and water. This is called a neutralization reaction.

General Equation:

• Acid + Base → Salt + Water

Examples:

1. HCl + NaOH → NaCl + H₂O
   • Hydrochloric acid + Sodium hydroxide → Sodium chloride (common salt) + Water
2. H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O
   • Sulphuric acid + Potassium hydroxide → Potassium sulphate + Water

Observations:

• The reaction releases heat (exothermic).
• The sourness of the acid and bitterness of the base disappear.

Applications:

• Treating indigestion: Antacids (bases like Mg(OH)₂) neutralize excess stomach acid (HCl).
• Soil treatment: If soil is too acidic, bases like lime (Ca(OH)₂) are added to neutralize it.

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6. Salts

Definition: Salts are ionic compounds formed when an acid reacts with a base. They usually do not affect litmus paper (neutral), though some salts can be acidic or basic.

Examples:

• Sodium chloride (NaCl) – Common salt used in food.
• Potassium nitrate (KNO₃) – Used in fertilizers.
• Copper sulphate (CuSO₄) – Used in agriculture.

Properties of Salts:

1. Most salts are neutral, but some can be acidic (e.g., ammonium chloride) or basic (e.g., sodium carbonate).
2. Soluble in water (e.g., NaCl), while some are insoluble (e.g., calcium carbonate).
3. Conduct electricity when dissolved in water or melted due to free ions.

Types of Salts:

1. Neutral Salts: Formed from strong acids and strong bases (e.g., NaCl).
2. Acidic Salts: Formed from strong acids and weak bases (e.g., NH₄Cl).
3. Basic Salts: Formed from weak acids and strong bases (e.g., Na₂CO₃).

Uses of Salts:

• Sodium chloride: For cooking and preserving food.
• Sodium bicarbonate (baking soda): For baking and as an antacid.
• Calcium carbonate: In making chalk and as a building material.

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7. Acids, Bases, and Salts in Daily Life

1. In the Kitchen:
   • Acids like vinegar and lemon juice are used in cooking.
   • Baking soda (a base) is used to make food fluffy.
2. In Medicine:
   • Antacids neutralize excess stomach acid.
3. In Agriculture:
   • Lime is added to acidic soil to make it suitable for crops.
4. In Industries:
   • Sulphuric acid is used in batteries and fertilizers.
   • Sodium hydroxide is used to make soap and paper.

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8. pH Scale (Brief Introduction)

• The pH scale measures how acidic or basic a substance is.
• Range: 0 to 14.
  • pH < 7: Acidic (e.g., lemon juice, pH ≈ 2).
  • pH = 7: Neutral (e.g., water).
  • pH > 7: Basic (e.g., soap solution, pH ≈ 10).
• Universal indicator or pH paper is used to measure pH by showing different colors.

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Key Points to Remember

1. Acids are sour, turn blue litmus red, and react with metals to release hydrogen.
2. Bases are bitter, turn red litmus blue, and feel slippery.
3. Indicators like litmus, turmeric, and phenolphthalein help identify acids and bases.
4. Neutralization produces salt and water when an acid reacts with a base.
5. Salts have various uses in food, medicine, and industries.
6. Neutral substances do not affect indicators.

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Practical Activities (From NCERT Textbook)

1. Testing with Litmus:
   • Take lemon juice, soap solution, and water. Dip blue and red litmus paper in each. Record the changes.
2. Neutralization Experiment:
   • Mix dilute HCl with NaOH in a beaker. Test the mixture with litmus. It remains neutral, indicating salt and water formation.
3. Natural Indicators:
   • Use turmeric or red cabbage juice to test household items like vinegar and baking soda.

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Conclusion

Acids, bases, and salts are all around us, playing vital roles in our lives. Understanding their properties and reactions helps us use them wisely, whether in cooking, cleaning, or treating health issues. This chapter lays the foundation for further studies in chemistry by explaining these fundamental concepts in a simple way.

Ch 5 Acids Bases And Salts