Physical and Chemical Changes: Transformations in Matter

Every day we witness many changes in our surroundings. These changes may involve one or more substances. Physical changes involve alterations in the state or form of matter without changing its chemical composition. While chemical changes result in the formation of new substances with different properties due to the rearrangement of atoms.

Understanding Changes: Physical and Chemical Perspectives

• Broadly these changes are of two kinds: Physical and Chemical.

Physical Changes

• Meaning: Properties such as shape, size, colour and state of a substance are called its physical properties. 
  • A change in which a substance undergoes a change in its physical properties is called a physical change. 
• Reversible Nature: A physical change is generally reversible and no new substance is formed. 
  • Example: Melting ice, cutting a piece of paper.

Chemical Changes

• Meaning: A change in which one or more new substances are formed is called a chemical change. 
  • It is also called a chemical reaction.
• Example: 

• • Burning of thin strip or ribbon of magnesium leaves a powdery ash of magnesium oxide

Magnesium (Mg) + Oxygen (O₂) → Magnesium Oxide (MgO)

1. On dissolving the ash of MgO in water it forms a new substance called Magnesium hydroxide which is a base.

Magnesium oxide (MgO) + Water (H₂O) → Magnesium hydroxide [Mg(OH)₂]

• Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit).
• Vinegar (Acetic acid) + Baking soda (Sodium hydrogen carbonate) → Carbon dioxide + other substances.
• Milky Water: When carbon dioxide is passed through lime water, calcium carbonate is formed, which makes lime water milky. 
  • The turning of lime water into milky is a standard test of carbon dioxide.

Carbon dioxide (CO₂) + Lime water [Ca(OH)₂] → Calcium Carbonate (CaCO₃) + Water (H₂O)

• Chemical changes are very important in our lives because all new substances are formed as a result of chemical changes. 
• For example, extraction of metal from an ore, a medicine, plastics and detergents, are produced by chemical reactions.
• In addition to new products, the following may accompany a chemical change.
  • Heat, light or any other radiation (ultraviolet, for example) may be given off or absorbed.
  • Sound may be produced.
  • A change in smell may take place or a new smell may be given off.
  • A colour change
  • A gas may be formed. 

Rusting of Iron: Changes in Presence of Oxygen and Water

• A piece of iron in the open for some time, acquires a film of brownish substance. 
• This substance is called rust and the process is called rusting. 
  • Rust is not iron. 
• It is different from iron on which it gets deposited.
• Since iron is used in making bridges, ships, cars, truck bodies and many other articles, the monetary loss due to rusting is huge. 
• For rusting, the presence of both oxygen and water/water vapour is essential. 
• Rusting becomes faster if the content of moisture in air is high (humid).
• The salt water of the sea makes the process of rust formation faster. 
• Therefore, ships suffer a lot of damage from rusting in spite of being painted.
• Representation: The process of rusting can be represented by:

Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → rust (iron oxide Fe₂O₃)

Preventing Rust: Protective Changes

• By applying a coat of paint or grease regularly.
• Galvanisation: Depositing a layer of a metal like chromium or zinc on iron is called “galvanisation”. 
  • The iron pipes used in homes to carry water are galvanised to prevent rusting.

Crystallisation: Transformative Changes

• The salt obtained by the evaporation of seawater is not pure and the shape of its crystals cannot be seen clearly. 
• However, large crystals of pure substances can be formed from their solutions. 
• Thus, the process of deriving large crystals of pure substances from their solution is called “crystallisation”. 
• It is an example of a physical change.