Grade 7 Chapter 13: Changes - Physical and Chemical

Introduction to Changes

Every day, we observe various changes happening around us. From the ripening of a fruit to the melting of ice, changes are a constant part of our lives and the environment. These changes can be categorized based on different criteria. Understanding these changes helps us to better comprehend the world around us and the processes involved in it.

Classification of Changes

Changes can be classified in several ways:

1. Fast and Slow Changes

• Fast Changes: Changes that occur quickly, often within seconds or minutes.
  • Examples: Bursting of a balloon, burning of paper, bursting of crackers, breaking of a glass object.
• Slow Changes: Changes that take a long time to occur, sometimes hours, days, months, or even years.
  • Examples: Ripening of fruit, rusting of iron, growth of a plant or animal, germination of a seed.

2. Reversible and Irreversible Changes

• Reversible Changes: Changes in which the original substance can be recovered. The change can be undone.
  • Examples: Melting of ice (water can be refrozen), folding of paper (paper can be unfolded), stretching of a rubber band.
• Irreversible Changes: Changes in which the original substance cannot be recovered. The change is permanent and cannot be undone.
  • Examples: Burning of wood (ash cannot become wood again), cooking of food, ripening of a fruit, germination of a seed.

3. Natural and Man-made Changes

• Natural Changes: Changes that occur on their own in nature, without human intervention.
  • Examples: Ripening of fruits, falling of leaves, changes in seasons, tides in the sea, earthquakes, floods.
• Man-made Changes: Changes brought about by human activities or intervention.
  • Examples: Cooking of food, building a house, cutting wood, burning of fuel, making plastic articles.

4. Beneficial and Harmful Changes

• Beneficial Changes: Changes that are useful or desirable for humans.
  • Examples: Ripening of fruits, cooking food, making curd from milk, growth of crops, combustion of fuel to generate energy.
• Harmful Changes: Changes that are undesirable or cause damage.
  • Examples: Rusting of iron, spoiling of food, volcanic eruption, earthquake, floods, forest fires, diseases.

Physical Changes

A physical change is a temporary change in which only the physical properties of a substance are altered, such as its state, shape, size, or appearance. No new substance is formed in a physical change, and the chemical composition of the substance remains the same.

Characteristics of Physical Changes:

• Temporary Change: The change is not permanent and can often be reversed.
• No New Substance Formed: The chemical identity of the substance remains the same. Only its physical form changes.
• Chemical Composition Remains Unchanged: The molecules of the substance are not altered; only their arrangement might change.
• Reversible: In most physical changes, the original substance can be recovered by reversing the conditions.
• Change in Physical Properties: Only properties like shape, size, color, state (solid, liquid, gas), and texture may change.

Examples of Physical Changes:

• Melting of ice: Ice (solid water) changes to liquid water. Both are still H₂O.
• Boiling of water: Liquid water changes to steam (gaseous water). Still H₂O.
• Dissolving sugar in water: Sugar is still sugar, and water is still water; they can be separated.
• Breaking of glass: The glass changes shape but is still glass.
• Stretching a rubber band: Its shape changes but it remains rubber.
• Cutting of paper: Paper remains paper, just in smaller pieces.
• Sublimation of camphor: Camphor changes from solid to gas, but its chemical identity doesn't change.
• Bending of a metal wire: The wire changes shape but is still the same metal.
• Magnetization of iron: Iron becomes a magnet but can be demagnetized.

Chemical Changes (Chemical Reactions)

A chemical change, also known as a chemical reaction, is a permanent change in which one or more new substances with entirely different chemical properties are formed. The original substance loses its identity, and its chemical composition changes.

Characteristics of Chemical Changes:

• Permanent Change: The change is irreversible; the new substances cannot be easily converted back to the original ones.
• New Substance(s) Formed: The most defining characteristic is the formation of entirely new substances with different chemical compositions and properties.
• Change in Chemical Composition: The molecules of the original substance are broken down and rearranged to form new molecules.
• Irreversible: Generally, chemical changes cannot be undone by simple physical methods.
• Energy Changes: Chemical changes often involve the absorption (endothermic) or release (exothermic) of energy (heat, light, etc.).

Indicators (Signs) of Chemical Changes:

The occurrence of a chemical change is often indicated by one or more of the following observations:

• Evolution of Heat or Light (or both):
  • Examples: Burning of a candle, burning of magnesium ribbon, mixing quicklime with water (releases heat).
• Absorption of Heat:
  • Examples: Dissolving ammonium chloride in water (absorbs heat, feels cold).
• Evolution of Gas:
  • Examples: Adding lemon juice to baking soda (produces carbon dioxide gas), reaction of zinc with dilute acid.
• Change in Color:
  • Examples: Ripening of fruit, rusting of iron (reddish-brown coating), tarnishing of silver, reaction of iodine with starch (blue-black color).
• Formation of a Precipitate (insoluble solid):
  • Examples: Mixing barium chloride solution with sodium sulphate solution (forms white precipitate of barium sulphate).
• Change in Smell:
  • Examples: Spoiling of food, fermentation.
• Production of Sound:
  • Examples: Bursting of crackers, reaction of sodium with water.

Examples of Chemical Changes:

• Burning of wood/paper: Ash and gases are formed, which are new substances.
• Rusting of iron: Iron reacts with oxygen and moisture to form iron oxide (rust).
• Cooking of food: Raw ingredients change chemically to form new substances with different tastes and textures.
• Ripening of fruit: Complex chemical reactions change the composition, color, and taste.
• Digestion of food: Food is broken down into simpler substances by enzymes.
• Photosynthesis: Plants convert carbon dioxide and water into glucose and oxygen.
• Respiration: Glucose reacts with oxygen to release energy, carbon dioxide, and water.
• Souring of milk: Lactose is converted to lactic acid by bacteria.
• Burning of a candle: Wax burns to produce carbon dioxide and water vapor (though melting wax is physical).

Distinction between Physical and Chemical Changes

The key difference lies in the formation of new substances. If no new substance is formed, it's a physical change. If one or more new substances are formed with different properties, it's a chemical change.

Characteristic	Physical Change	Chemical Change
New substance formed?	No	Yes, one or more new substances
Nature of change	Temporary	Permanent
Reversibility	Generally reversible	Generally irreversible
Chemical composition	Remains the same	Changes
Energy change	Minimal or none (e.g., phase changes)	Often significant (heat/light absorbed or released)