Which one of the following statements is NOT correct?
- (a)The interconversion of states of matter is a physical change
- (b)During burning of a candle, both chemical and physical changes take place
- (c)During a physical change, the chemical composition of molecules changes
- (d)Rusting of iron is a chemical change
Correct — C, 'During a physical change, the chemical composition of molecules changes.' This is the statement that is NOT correct: in a physical change only the form or state alters while the chemical composition and identity remain exactly the same — ice, water and steam are all H2O. Because the question asks for the false statement, this one is the answer.
- (a)The interconversion of states of matter is a physical change — This is a correct statement, so it cannot be the answer. Melting, freezing, boiling and condensation only change how the molecules are packed; no new substance forms, which is the definition of a physical change.
- (b)During burning of a candle, both chemical and physical changes take place — This is correct too. The wax first melts and vaporises (physical), then the wax vapour burns in air to form carbon dioxide and water (chemical), so a candle genuinely shows both kinds of change.
- (d)Rusting of iron is a chemical change — Correct as stated. Iron reacts with oxygen and moisture to form a brand-new substance, hydrated iron(III) oxide (rust), so it is a chemical change and not the answer.
A physical change alters only the physical form of a substance — its state, shape or size — while its chemical composition and identity remain unchanged, and it is usually easy to reverse. A chemical change produces one or more new substances with new properties and is generally hard to reverse. The single test that separates them is whether a new substance has been formed.
The question inverts the usual task by asking for the statement that is NOT correct, so the trap is to pick a statement that merely sounds like a chemical fact. Changes of state and rusting are correctly classified, and a burning candle really does show both types; only the claim that a physical change alters chemical composition contradicts the definition, making it the false — and therefore correct — choice.
- In a physical change the chemical composition is unchanged; in a chemical change a new substance with a different composition is formed.
- Changes of state such as melting, boiling, freezing and sublimation are physical changes and are usually reversible.
- Rusting, burning, cooking and souring of milk are chemical changes because new substances are produced.
- A burning candle is a classic 'both' example — melting of the wax is physical, combustion of the vapour is chemical.
- A 'NOT correct' question rewards the false statement — read the stem twice so you do not pick a true one.
- A burning candle is not purely chemical; the melting and vaporising of wax are physical steps.
Physical-versus-chemical change is asked as 'which is an example of a chemical change' or, as here, 'which statement is NOT correct' — always test whether a new substance is formed.
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.
- (a) 1 and 2 only
- (b) 3 only
- (c) 1, 2 and 3
- (d) None
Answer(b) 3 only
Exactly the same concept — sorting changes into physical or chemical. UPSC asked students to see that crystallisation and melting are physical (composition unchanged) while only the souring of milk is chemical; this NDA item tests the same rule from the other direction, by flagging the false definition of a physical change.
Which one among the following is an example of chemical change?
- (a) Sublimation of camphor
- (b) Melting of ice
- (c) Heating of an iron rod
- (d) Mixing of NaOH and HCl
Answer(d) Mixing of NaOH and HCl
The same physical-versus-chemical distinction. There the three wrong options (sublimation, melting, heating) are physical changes and only the acid-base reaction is chemical; here the false statement is the one that misdescribes a physical change.
- practice — not a real PYQ
Which one of the following is a purely physical change?
- (a)Burning of paper
- (b)Melting of wax
- (c)Digestion of food
- (d)Rusting of iron
Answer(b) Melting of wax — only the state changes from solid to liquid and no new substance forms.
- practice — not a real PYQ
Which one of the following always accompanies a chemical change?
- (a)A change of state only
- (b)Formation of a new substance
- (c)A change of shape only
- (d)A change of size only
Answer(b) Formation of a new substance — that is the defining feature of a chemical change.