Class 7 Science Notes Chapter 5 (Acids; Bases and Salts) – Science Book

Okay, here are the detailed notes for NCERT Class 7 Science, Chapter 5: Acids, Bases and Salts, structured for government exam preparation.

NCERT Class 7 Science: Chapter 5 - Acids, Bases and Salts
(Detailed Notes for Government Exam Preparation)

1. Introduction

• Many substances we encounter daily can be classified based on their chemical nature as acidic, basic, or neutral.
• Taste is a preliminary indicator (Acids: Sour, Bases: Bitter), but never taste unknown substances. Chemical indicators are used for safe testing.

2. Acids

• Definition: Substances that are sour in taste and turn blue litmus paper red.

• The term 'acid' comes from the Latin word 'acere', meaning sour.

• Properties:

  • Sour taste.
  • Turn blue litmus red.
  • Do not change the colour of red litmus.
  • Do not change the colour of turmeric indicator.
  • Turn China rose indicator to dark pink (magenta).
  • Remain colourless with phenolphthalein.
  • Can be corrosive (especially strong acids).

• Types:

  • Natural Acids (Organic Acids): Found in plants and animals. Generally weak acids.
  • Mineral Acids: Prepared from minerals. Usually strong acids and highly corrosive (e.g., Hydrochloric acid, Sulphuric acid, Nitric acid). Handle with extreme care.

• Common Natural Acids and their Sources:

  Acid Name	Found In
  Acetic Acid	Vinegar
  Formic Acid	Ant's sting
  Citric Acid	Citrus fruits (Oranges, Lemons)
  Lactic Acid	Curd
  Oxalic Acid	Spinach
  Ascorbic Acid (Vit C)	Amla, Citrus fruits
  Tartaric Acid	Tamarind, Grapes, Unripe Mangoes

3. Bases

• Definition: Substances that are bitter in taste, feel soapy to touch, and turn red litmus paper blue.

• Properties:

  • Bitter taste.
  • Soapy feel (Caution: Strong bases can be corrosive).
  • Turn red litmus blue.
  • Do not change the colour of blue litmus.
  • Turn turmeric indicator to reddish-brown.
  • Turn China rose indicator to green.
  • Turn phenolphthalein pink.

• Alkalis: Bases that are soluble in water (e.g., Sodium Hydroxide, Potassium Hydroxide). All alkalis are bases, but not all bases are alkalis.

• Common Bases and their Uses/Sources:

  Base Name	Chemical Formula (Optional for Class 7)	Found In / Used As
  Calcium Hydroxide	Ca(OH)₂	Lime water
  Ammonium Hydroxide	NH₄OH	Window cleaner
  Sodium Hydroxide	NaOH	Soap manufacturing
  Potassium Hydroxide	KOH	Soap manufacturing
  Magnesium Hydroxide	Mg(OH)₂	Milk of Magnesia (Antacid)
  Sodium Bicarbonate / Sodium Hydrogen Carbonate	NaHCO₃	Baking Soda (Mild Base)

4. Indicators

• Definition: Special substances used to test whether a substance is acidic or basic. They change their colour when added to a solution containing an acidic or a basic substance.

• Types:

  • Natural Indicators: Obtained from natural sources.
    • Litmus:
      • Most common natural indicator.
      • Extracted from Lichens.
      • Available as a solution or strips of paper (red and blue litmus paper).
      • Acidic Solution: Turns blue litmus red.
      • Basic Solution: Turns red litmus blue.
      • Neutral Solution: No change in colour (Litmus solution is purple in distilled water/neutral solution).
    • Turmeric (Haldi):
      • Yellow in colour.
      • Acidic/Neutral Solution: Remains yellow.
      • Basic Solution: Turns reddish-brown (seen when soap solution falls on turmeric stain).
    • China Rose Petals (Gudhal):
      • Indicator prepared by soaking petals in warm water. Solution is light pink.
      • Acidic Solution: Turns dark pink (magenta).
      • Basic Solution: Turns green.
      • Neutral Solution: No change.
  • Synthetic Indicators: Chemically synthesized indicators.
    • Phenolphthalein:
      • Acidic/Neutral Solution: Remains colourless.
      • Basic Solution: Turns pink.

• Summary of Indicator Colour Changes:

  Indicator	Original Colour	Colour in Acid	Colour in Base	Colour in Neutral
  Blue Litmus	Blue	Red	No Change	No Change
  Red Litmus	Red	No Change	Blue	No Change
  Turmeric	Yellow	Yellow	Reddish-Brown	Yellow
  China Rose	Light Pink	Dark Pink (Magenta)	Green	No Change
  Phenolphthalein	Colourless	Colourless	Pink	Colourless

5. Neutralisation

• Definition: The reaction between an acid and a base is known as neutralisation.
• Products: Salt and Water are produced in this process.
• Heat Evolution: Heat is always evolved (released) during a neutralisation reaction, making it an exothermic reaction. The reaction mixture becomes warm.
• General Equation:
  Acid + Base → Salt + Water + Heat
• Example:
  Hydrochloric Acid (HCl) + Sodium Hydroxide (NaOH) → Sodium Chloride (NaCl) (Common Salt) + Water (H₂O) + Heat
• Salt:
  • The substance formed by the neutralisation reaction.
  • Salts can be acidic, basic, or neutral in nature (depends on the strength of the acid and base reacted - concept detailed in higher classes). Sodium Chloride (common salt) is a neutral salt.

6. Neutralisation in Everyday Life

• Indigestion:
  • Our stomach produces Hydrochloric Acid (HCl) for digestion.
  • Excess acid causes indigestion (acidity).
  • Remedy: Taking an antacid like Milk of Magnesia (contains Magnesium Hydroxide, a base) neutralises the excess acid. Baking soda solution can also be used.
• Ant Bite:
  • When an ant bites, it injects Formic Acid into the skin, causing irritation.
  • Remedy: Rubbing moist Baking Soda (Sodium Hydrogen Carbonate, basic) or Calamine solution (contains Zinc Carbonate, basic) neutralises the formic acid.
• Soil Treatment:
  • Plants require a relatively neutral soil pH for optimal growth.
  • Acidic Soil: Treated by adding bases like Quicklime (Calcium Oxide) or Slaked Lime (Calcium Hydroxide) to neutralise the excess acid.
  • Basic Soil: Organic matter (compost) is added. Decomposition of organic matter releases acids, which neutralise the excess base in the soil.
• Factory Wastes:
  • Wastes from many factories contain acids.
  • If discharged directly into water bodies, they harm aquatic life.
  • Remedy: Factory wastes are treated with basic substances to neutralise the acids before discharge.

7. Acid Rain

• Rain containing excess acids is called acid rain.
• Cause: Air pollutants like Carbon Dioxide, Sulphur Dioxide, and Nitrogen Dioxide dissolve in raindrops to form Carbonic Acid, Sulphuric Acid, and Nitric Acid, respectively.
• Harmful Effects: Damages buildings, historical monuments (like Taj Mahal - marble corrosion), plants, and animals; makes water bodies acidic, harming aquatic life.

Key Takeaways for Exams:

• Memorize definitions of acids, bases, indicators, neutralisation, and salts.
• Know the properties (taste, feel, effect on indicators) of acids and bases.
• Learn the common examples of acids and bases and their sources/uses (Tables are important).
• Understand how different indicators (Litmus, Turmeric, China Rose, Phenolphthalein) work and their colour changes in acidic, basic, and neutral solutions.
• Understand the neutralisation reaction (reactants, products, heat evolution).
• Be familiar with the applications of neutralisation in everyday life (Indigestion, Ant bite, Soil treatment, Factory waste).
• Understand the concept and causes/effects of acid rain.