Chapter 5:
Physical and Chemical Changes

Class 7 Science | CBSE Curriculum

Learn to distinguish between physical and chemical changes, with experiments on rusting, burning of magnesium, reaction of iron with copper sulphate, and crystallisation. Includes prevention of rusting.

Score:
0
Time:
05:00

Quick Revision: Physical and Chemical Changes

  • Physical Change: A change in which only the physical properties (shape, size, colour, state) of a substance change. No new substance is formed. It is generally reversible.
    Examples: Melting of ice, dissolving sugar in water, cutting paper, folding of clothes, stretching a rubber band.
  • Chemical Change (Chemical Reaction): A change in which one or more new substances are formed. It is generally irreversible.
    Examples: Rusting of iron, burning of magnesium ribbon, cooking of food, digestion of food, ripening of fruits, souring of milk.
  • Burning of Magnesium: Magnesium ribbon burns with a brilliant white light to form a white powdery ash (magnesium oxide). When dissolved in water, it forms magnesium hydroxide, which turns red litmus blue (basic).
  • Reaction of Iron with Copper Sulphate: When iron nails are dipped in blue copper sulphate solution, the solution turns green (iron sulphate) and a brown deposit of copper forms on the nails.
  • Reaction of Vinegar and Baking Soda: Acetic acid (vinegar) reacts with sodium hydrogencarbonate (baking soda) to produce carbon dioxide gas. CO₂ turns lime water milky (test for CO₂).
  • Rusting of Iron: The process of formation of a brown flaky substance (rust, iron oxide) on iron articles. Rusting requires both oxygen and water (or water vapour). It is faster in humid and coastal areas.
  • Prevention of Rusting: Painting, greasing, galvanisation (depositing a layer of zinc), and making stainless steel (alloy of iron with carbon, chromium, nickel).
  • Crystallisation: The process of obtaining large, pure crystals of a substance from its solution. It is a physical change. Example: Preparation of copper sulphate crystals.
  • Characteristics of Chemical Changes: Production of heat, light, sound; change in colour, smell; formation of a gas or precipitate.
  • Iron Pillar of Delhi: A 1600-year-old iron pillar near Qutub Minar that has not rusted, showcasing ancient Indian metal technology.

Chapter Summary: Physical and Chemical Changes

This chapter distinguishes between physical and chemical changes. Physical changes affect only the physical properties of a substance, such as its shape, size, colour, or state, without forming any new substance. These changes are often reversible, like melting ice or dissolving sugar. In contrast, chemical changes (or chemical reactions) result in the formation of one or more new substances with different properties. These changes are generally irreversible and may be accompanied by the release or absorption of heat, light, or sound, or by changes in colour or smell. Key experiments in the chapter include the burning of magnesium ribbon (forming magnesium oxide, which turns water basic), the reaction of iron nails with copper sulphate solution (displacement reaction forming iron sulphate and copper), and the reaction of vinegar with baking soda (producing carbon dioxide that turns lime water milky).

The chapter also discusses rusting of iron as a common and damaging chemical change, requiring both oxygen and moisture. Prevention methods include painting, greasing, and galvanisation (zinc coating). Finally, crystallisation is introduced as a physical change used to obtain pure substances from their solutions. Understanding these changes helps us appreciate the transformations occurring all around us, from cooking to metal corrosion.