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Science: Class 7 — Chapter 4: Acids, Bases and Salts

4.1 Acids and Bases

Core Concepts
  • Acids: Substances that taste sour and have an acidic chemical nature. The word originates from the Latin word acere (meaning sour).
  • Bases: Substances that taste bitter and feel soapy to the touch. Their chemical nature is basic.
  • Indicators: Special substances used to test whether a solution is acidic or basic by changing their color.
  • Neutral Solutions: Solutions that do not change the color of either red or blue litmus paper; they are neither acidic nor basic.

Common Acids and Their Sources
  • Acetic acid: Found in Vinegar
  • Formic acid: Found in Ant's sting
  • Citric acid: Found in Citrus fruits (oranges, lemons)
  • Lactic acid: Found in Curd
  • Oxalic acid: Found in Spinach
  • Ascorbic acid (Vitamin C): Found in Amla, Citrus fruits
  • Tartaric acid: Found in Tamarind, grapes, unripe mangoes

Common Bases and Their Sources
  • Calcium hydroxide: Found in Lime water
  • Ammonium hydroxide: Found in Window cleaner
  • Sodium hydroxide / Potassium hydroxide: Found in Soap
  • Magnesium hydroxide: Found in Milk of magnesia
4.2 Natural Indicators Around Us

Litmus: A Natural Dye
  • Extracted from lichens. It has a mauve (purple) color in distilled water.
  • Turns red in acidic solutions.
  • Turns blue in basic solutions.
  • Commonly available as red and blue litmus paper strips.

Turmeric
  • Remains yellow in acidic and neutral solutions.
  • Turns red when exposed to basic solutions (such as soap solution).

China Rose (Gudhal)
  • Turns acidic solutions to dark pink (magenta).
  • Turns basic solutions to green.
4.3 Neutralisation

The Neutralisation Reaction
  • Neutralisation: The reaction between an acid and a base. It produces salt, water, and evolves heat.
  • General Equation: Acid + Base → Salt + Water (Heat is evolved)
  • Example: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H₂O)
  • Phenolphthalein: A synthetic indicator used in neutralisation. It turns pink in basic solutions and remains colorless in acidic solutions.

Acid Rain
  • Rain becomes acidic when carbon dioxide, sulphur dioxide, and nitrogen dioxide pollutants dissolve in raindrops to form carbonic acid, sulphuric acid, and nitric acid.
  • Acid rain causes severe damage to buildings, historical monuments, plants, and aquatic life.
4.4 Neutralisation in Everyday Life

Real-World Applications
  • Indigestion: Excess hydrochloric acid in the stomach is neutralised by taking antacids like milk of magnesia (containing magnesium hydroxide).
  • Ant Bite: Ant stings inject formic acid. This is neutralised by rubbing moist baking soda (sodium hydrogencarbonate) or calamine solution (zinc carbonate).
  • Soil Treatment: Acidic soil is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). Basic soil is treated with organic matter (compost) which releases acids.
  • Factory Wastes: Acidic industrial wastes are neutralised with basic substances before being discharged into water bodies to protect aquatic life.
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