Which of the following is/are the example/examples of chemical change? 1. Crystallization of sodium chloride 2. Melting of ice 3. Souring of milk Select the correct answer using the code given below.
Contents10
- A1 and 2 only
- B3 only
- C1, 2 and 3
- DNone
Show answer
Answer: (B) 3 only
Chemical change = a new substance is formed (irreversible, change in composition).
Physical change = only form/state changes (reversible, same substance).
(1) Crystallisation of NaCl — PHYSICAL change.
Salt molecules simply arrange into crystal structure; no new substance forms.
(2) Melting of ice — PHYSICAL change.
H2O stays H2O; only state changes from solid to liquid.
(3) Souring of milk — CHEMICAL change.
Bacteria convert lactose (milk sugar) into lactic acid through fermentation.
A completely new substance (lactic acid) is formed, and you can't reverse soured milk back to fresh milk.
Only statement 3 is a chemical change.
(Ref: NCERT Class 7 Science, Ch.6)
Chemical changes create new substances that cannot be reversed (like milk souring into lactic acid), while physical changes only alter form or state of the same substance (like ice melting to water).
UPSC tests this fundamental concept because many students confuse processes that look dramatic (like crystallization) with actual chemical changes, when crystallization is just physical rearrangement of existing molecules.
Chemical vs Physical Changes
Science And Technology chemical change crystallization melting souring
Chemical vs Physical Changes: Definition & Key Differences
Chemical change = new substance forms, irreversible, composition changes
Physical change = same substance, only form/state changes, reversible
Souring of milk is chemical (lactose → lactic acid)
Crystallization and melting are physical changes
Core Concept
The key distinction lies in molecular composition. Chemical changes create entirely new substances with different properties, while physical changes only alter the appearance or state of the same substance.
Key Differences
Aspect | Chemical Change | Physical Change |
|---|---|---|
New substance | Yes - different composition | No - same substance |
Reversibility | Irreversible (difficult/impossible) | Reversible (easily undone) |
Energy | Usually involves heat/light | May require energy but no new bonds |
Examples | Burning, rusting, souring of milk | Melting, freezing, crystallization |
Question Analysis
Crystallization of NaCl: Salt molecules just rearrange into crystal structure - same NaCl, different arrangement
Melting of ice: H₂O remains H₂O, only changes from solid to liquid state
Souring of milk: Bacteria convert lactose into lactic acid through fermentation - completely new substance
Trap: Crystallization sounds chemical but is physical - no new substance, just molecular arrangement
Trap: Any state change (solid↔liquid↔gas) is always physical, never chemical
Trap: Fermentation processes (souring, brewing) are always chemical changes
UPSC often tests irreversibility - ask 'can I easily get the original substance back?'
Common Chemical Changes
Science And Technology souring of milk
Common Chemical Changes: Processes & Examples
Fermentation: Bacteria convert sugars to acids (milk souring, alcohol production)
Oxidation: Substance combines with oxygen (rusting, burning)
Decomposition: Complex substance breaks into simpler ones
Types of Chemical Changes
Type | Process | Example | New Substance Formed |
|---|---|---|---|
Fermentation | Microorganisms convert sugars | Milk souring | Lactose → Lactic acid |
Combustion | Substance burns in oxygen | Wood burning | Wood → CO₂ + H₂O + ash |
Oxidation | Combines with oxygen | Iron rusting | Iron → Iron oxide (rust) |
Decomposition | Complex breaks to simple | Food spoiling | Proteins → Ammonia + other compounds |
Milk Souring Mechanism
Lactobacillus bacteria naturally present in milk convert lactose sugar
Lactose (milk sugar) breaks down into lactic acid through bacterial enzymes
Lactic acid gives sour taste and changes milk's pH from neutral to acidic
Process is irreversible - cannot convert soured milk back to fresh milk
Trap: Curdling vs souring - both are chemical but different processes
Trap: Pasteurization is physical (heating) but fermentation is chemical
Remember: Any biological process (digestion, respiration, fermentation) is usually chemical
Common Physical Changes
Science And Technology crystallization melting of ice
Common Physical Changes: State Changes & Molecular Rearrangement
State changes: Solid ↔ Liquid ↔ Gas (melting, freezing, boiling, condensation)
Crystallization: Dissolved substance forms crystals - same substance, new arrangement
All physical changes are easily reversible
Types of Physical Changes
Type | Process | Example | Reversibility |
|---|---|---|---|
State Change | Solid ↔ Liquid ↔ Gas | Ice melting to water | Heat/cool to reverse |
Crystallization | Dissolved particles form crystals | Salt crystallization | Dissolve in water again |
Dissolution | Solid dissolves in liquid | Sugar in water | Evaporate water to get sugar |
Sublimation | Solid directly to gas | Dry ice subliming | Cool gas back to solid |
Crystallization Process
Salt molecules arrange themselves into regular, repeating 3D pattern
Same NaCl molecules - only their spatial arrangement changes
Can be reversed by dissolving crystals back in water
Energy required only to overcome intermolecular forces, not break chemical bonds
Trap: Crystallization involves energy but is still physical - no new bonds formed
Trap: Sublimation (solid→gas) is physical, decomposition (compound breaks) is chemical
Remember: If you can easily reverse the change, it's likely physical