Chapter 4:
Acids, Bases and Salts

Class 7 Science | CBSE Curriculum

Learn about the properties of acids and bases, natural indicators like litmus and turmeric, the neutralisation reaction, and its applications in everyday life such as antacids and soil treatment.

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0
Time:
05:00

Quick Revision: Acids, Bases and Salts

  • Acids: Substances that taste sour. They turn blue litmus red. Examples: Natural acids like citric acid (lemon), acetic acid (vinegar), lactic acid (curd), oxalic acid (spinach), tartaric acid (tamarind).
  • Bases: Substances that taste bitter and feel soapy to touch. They turn red litmus blue. Examples: Calcium hydroxide (lime water), ammonium hydroxide (window cleaner), magnesium hydroxide (milk of magnesia), sodium hydroxide (soap).
  • Indicators: Special substances used to test whether a material is acidic or basic. They change colour in acidic or basic media.
  • Litmus: A natural indicator extracted from lichens. It turns red in acids and blue in bases. Available as red and blue litmus paper.
  • Turmeric: A natural indicator. Turmeric paper turns reddish-brown in basic solutions. It remains yellow in acids.
  • China Rose (Gudhal): A natural indicator. Its solution turns dark pink (magenta) in acids and green in bases.
  • Neutral Substances: Substances that do not change the colour of either red or blue litmus. They are neither acidic nor basic (e.g., distilled water, sugar solution, common salt solution).
  • Neutralisation Reaction: The reaction between an acid and a base to form salt and water. Heat is evolved during this process.
    Acid + Base → Salt + Water + Heat
    Example: HCl + NaOH → NaCl + H₂O
  • Phenolphthalein: A synthetic indicator. It is colourless in acids and pink in bases.
  • Acid Rain: Rain containing excess of acids (carbonic, sulphuric, nitric acid) due to pollutants like CO₂, SO₂, NO₂ dissolving in rainwater. It damages buildings, monuments, and plants.
  • Applications of Neutralisation:
    • Indigestion: Excess stomach acid (HCl) neutralised by antacids like milk of magnesia (Mg(OH)₂).
    • Ant Bite: Formic acid injected is neutralised by rubbing moist baking soda (NaHCO₃) or calamine solution (ZnCO₃).
    • Soil Treatment: Acidic soil treated with quick lime (CaO) or slaked lime (Ca(OH)₂). Basic soil treated with organic matter (compost).
    • Factory Wastes: Acidic wastes neutralised by adding basic substances before releasing into water bodies.

Chapter Summary: Acids, Bases and Salts

This chapter introduces the concepts of acidity and basicity. We learn that substances can be classified as acidic (sour taste, e.g., lemon juice, vinegar), basic (bitter taste, soapy feel, e.g., baking soda, lime water), or neutral (e.g., sugar solution, common salt). Since tasting or touching unknown substances is dangerous, we use indicators to test their nature. Common natural indicators include litmus (from lichens, turns red in acids, blue in bases), turmeric (turns reddish-brown in bases), and China rose (turns magenta in acids, green in bases). Synthetic indicators like phenolphthalein (colourless in acid, pink in base) are also used.

The core of the chapter is the neutralisation reaction, where an acid and a base react to form salt and water, with the evolution of heat. This reaction has many practical applications in everyday life: taking antacids for indigestion (neutralising excess stomach acid), treating ant bites (neutralising formic acid with baking soda), treating soil acidity with lime, and neutralising acidic factory wastes before they are released into the environment. The chapter also touches upon the harmful effects of acid rain caused by environmental pollutants. Understanding the properties of acids and bases helps us appreciate their role in various natural and industrial processes.