Class 7 Science Notes Chapter 6 (Physical and Chemical Changes) – Science Book

Okay, here are the detailed notes for NCERT Class 7 Science, Chapter 6: Physical and Chemical Changes, structured for government exam preparation.

NCERT Class 7 Science
Chapter 6: Physical and Chemical Changes

Introduction:
Changes are constantly occurring around us. These changes can be broadly classified into two types: Physical Changes and Chemical Changes. Understanding the difference between these is crucial.

1. Physical Changes

• Definition: A change in which a substance undergoes a change in its physical properties such as shape, size, colour, or state (solid, liquid, gas), but no new substance is formed. The chemical composition of the substance remains the same.
• Key Characteristics:
  • Affects only physical properties.
  • No new substance is formed.
  • The change is generally reversible (though not always easily, e.g., tearing paper).
  • The chemical nature of the substance does not change.
  • Usually, temporary changes.
• Examples from NCERT:
  • Tearing a piece of paper: Changes shape and size, but it's still paper.
  • Melting of ice: Changes state (solid to liquid), but it's still water (H₂O). Reversible by freezing.
  • Boiling of water: Changes state (liquid to gas/vapour), but it's still water (H₂O). Reversible by condensation.
  • Stretching a rubber band: Changes shape and size temporarily. Reversible when released.
  • Cutting wood into small pieces: Changes size, but it's still wood.
  • Dissolving sugar or salt in water: Forms a solution, changes the state of the solute, but sugar remains sugar and salt remains salt. Reversible by evaporation.
  • Formation of clouds: Change of state (water vapour to tiny water droplets).
  • Glowing of an electric bulb: Filament heats up and glows (physical change due to heat/electricity), returns to normal when switched off. No new substance formed.

2. Chemical Changes

• Definition: A change in which one or more new substances are formed, with entirely different chemical compositions and properties from the original substance(s).
• Key Characteristics:
  • Results in the formation of new substance(s).
  • The chemical composition and properties of the substance(s) change.
  • The change is generally irreversible by simple physical means.
  • Often accompanied by observable events.
  • Usually, permanent changes.
• Indicators of a Chemical Change (One or more may occur):
  • Production of Heat and/or Light: Example: Burning of magnesium ribbon, burning wood. (Exothermic reaction)
  • Absorption of Heat: Some reactions get cold. (Endothermic reaction)
  • Production of Sound: Example: Burning firecrackers, reaction of acid with baking soda.
  • Change in Smell or Production of a New Smell: Example: Spoiling of food, cooking food.
  • Significant Colour Change: Example: Rusting of iron (silvery grey to reddish-brown), reaction of copper sulphate with iron (blue to green).
  • Evolution of a Gas: Example: Reaction of vinegar with baking soda (produces CO₂), reaction of acid with metal.
  • Formation of a Precipitate: An insoluble solid formed during a reaction in a liquid solution.
• Examples from NCERT:
  • Burning of Magnesium Ribbon:
    • Magnesium (Mg) burns in air (Oxygen, O₂) with a brilliant white light.
    • Forms a powdery ash of Magnesium Oxide (MgO), a new substance.
    • Reaction: Magnesium (Mg) + Oxygen (O₂) → Magnesium Oxide (MgO)
    • Magnesium Oxide dissolved in water forms Magnesium Hydroxide [Mg(OH)₂], which is basic (turns red litmus blue).
    • Reaction: Magnesium Oxide (MgO) + Water (H₂O) → Magnesium Hydroxide [Mg(OH)₂]
  • Reaction between Copper Sulphate and Iron:
    • Copper Sulphate (CuSO₄) solution is blue.
    • When an iron nail (Fe) is placed in it, the iron displaces copper.
    • The solution turns green due to the formation of Iron Sulphate (FeSO₄), a new substance.
    • A brown deposit of Copper (Cu), another new substance, forms on the iron nail.
    • Reaction: Copper Sulphate (CuSO₄) (Blue) + Iron (Fe) → Iron Sulphate (FeSO₄) (Green) + Copper (Cu) (Brown deposit)
  • Reaction between Vinegar and Baking Soda:
    • Vinegar contains Acetic Acid (CH₃COOH).
    • Baking Soda is Sodium Hydrogen Carbonate (NaHCO₃).
    • When mixed, they react to produce bubbles of Carbon Dioxide (CO₂) gas (a new substance), along with other substances like Sodium Acetate.
    • Reaction: Acetic Acid + Sodium Hydrogen Carbonate → Carbon Dioxide + Other substances
    • The Carbon Dioxide gas turns limewater (Calcium Hydroxide solution, Ca(OH)₂) milky due to the formation of Calcium Carbonate (CaCO₃), a white precipitate.
    • Reaction: Carbon Dioxide (CO₂) + Limewater [Ca(OH)₂] → Calcium Carbonate (CaCO₃) (White Precipitate) + Water (H₂O)
    • Note: Turning limewater milky is a standard test for Carbon Dioxide.
  • Rusting of Iron: (Discussed in detail below)
  • Digestion of Food: Complex food substances are broken down into simpler, new substances.
  • Cooking of Food: Changes the composition and properties of raw ingredients.
  • Curdling of Milk: Formation of paneer/curd from milk.
  • Fermentation: (e.g., grapes fermenting to wine)

3. Rusting of Iron

• Definition: A specific chemical change where iron objects react with oxygen and water (or water vapour) present in the atmosphere to form a reddish-brown flaky substance called rust.
• Nature of Rust: Rust is Iron Oxide (Fe₂O₃). It is a new substance, different from iron.
• Conditions Necessary for Rusting:
  1. Presence of Iron (Fe).
  2. Presence of Oxygen (O₂) (from air).
  3. Presence of Water (H₂O) or Water Vapour (moisture).
  • Note: Rusting is faster in humid conditions and near coastal areas (due to salt in the air/water acting as a catalyst).
• Chemical Equation (Simplified):
  Iron (Fe) + Oxygen (O₂) (from air) + Water (H₂O) → Rust (Iron Oxide, Fe₂O₃)
• Harmful Effects: Rusting damages iron objects, making them weak (e.g., bridges, ships, vehicles, railings). It is a slow destructive process.
• Prevention of Rusting: Methods involve preventing iron from coming into contact with oxygen and water/moisture.
  • Painting: Creates a barrier layer.
  • Greasing/Oiling: Creates a barrier layer, often used for tools and machine parts.
  • Galvanization: Coating iron with a layer of a more reactive metal like Zinc (Zn). Zinc reacts preferentially with oxygen and water, protecting the iron underneath (sacrificial protection). Used for pipes, buckets, etc.
  • Chrome Plating/Tin Plating: Coating with chromium or tin creates a barrier.
  • Alloying: Mixing iron with other metals like chromium and nickel to make Stainless Steel, which does not rust easily.

4. Crystallization

• Definition: The process of obtaining large, pure crystals of a substance from its saturated solution.
• Nature of Change: Crystallization itself is generally considered a physical change. Although it involves dissolving (physical) and then solidifying, the chemical substance remains the same (e.g., CuSO₄ remains CuSO₄). However, it's often used to purify substances obtained after chemical reactions.
• Process Example (Copper Sulphate Crystals):
  1. Dissolve impure Copper Sulphate powder in water (with a few drops of dilute sulphuric acid to prevent hydrolysis).
  2. Heat the solution to dissolve more solute until it becomes saturated (no more solute dissolves).
  3. Filter the hot solution to remove insoluble impurities.
  4. Allow the clear solution to cool down slowly and undisturbed.
  5. As the solution cools, the solubility decreases, and pure Copper Sulphate separates out in the form of large, blue, geometric crystals.
• Significance: Used for purification of solids (e.g., salt from seawater, sugar from sugarcane juice).

Comparison: Physical vs. Chemical Changes

Key Takeaways for Exams:

• Be able to clearly define and differentiate between physical and chemical changes.
• Memorize key examples for both types, especially those explicitly mentioned in the NCERT book.
• Understand the indicators that suggest a chemical change has occurred.
• Know the specific details of the reactions discussed: Magnesium burning, Copper Sulphate + Iron, Vinegar + Baking Soda (including the limewater test).
• Understand the process of rusting: what it is, conditions required, chemical nature of rust, and methods of prevention (especially galvanization).
• Understand the process and purpose of crystallization.
• Be prepared for questions asking you to classify given changes as physical or chemical, often with reasoning.