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
Correct — D, Mixing of NaOH and HCl. When sodium hydroxide and hydrochloric acid are mixed they react (neutralisation): NaOH + HCl gives NaCl + H2O. New substances — common salt and water — are formed and the change is not easily reversed, so this is a chemical change.
- (a)Sublimation of camphor — Sublimation (solid to vapour) is a physical change — camphor's chemical identity is unchanged and it can re-solidify.
- (b)Melting of ice — Melting is a change of state (solid to liquid); the water is chemically the same, so it is physical.
- (c)Heating of an iron rod — Simply heating an iron rod makes it expand and glow but does not by itself form a new substance — it is a physical change, reversible on cooling.
A chemical change forms one or more new substances with new properties and is usually not easily reversed (e.g. neutralisation, rusting, burning). A physical change alters only state, shape or size — no new substance forms and it is often reversible (melting, sublimation, dissolving, heating/expansion).
Test each option for a new substance: subliming, melting and simply heating change only physical state — no new material. Only mixing NaOH with HCl produces new substances (salt and water), so it is the chemical change.
- NaOH + HCl gives NaCl + H2O is a neutralisation (acid + base to salt + water) — a chemical change.
- Chemical change = new substance formed, generally not easily reversed.
- Sublimation of camphor and melting of ice are physical changes (state change only).
- Heating an iron rod is physical (thermal expansion); it becomes chemical only if the iron oxidises/reacts.
Only the acid-base reaction makes a new substance — hence (d).
- Calling melting or sublimation a chemical change — they are physical (state change).
- Thinking heating always causes a chemical change — it may be only expansion.
- Overlooking that neutralisation, though it makes a solution, does form new substances.
Exams list everyday processes and ask which is a chemical (or physical) change — look for the formation of a new substance.
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
UPSC tested the same physical-versus-chemical distinction: crystallisation and melting are physical (no new substance), only souring of milk is chemical — mirroring why mixing NaOH and HCl (a new-substance reaction) is the chemical change here.
- practice — not a real PYQ
Which of the following is a physical change?
- (a)Burning of paper
- (b)Rusting of iron
- (c)Melting of wax
- (d)Curdling of milk
Answer(c) Melting of wax — a change of state only; no new substance forms.
- practice — not a real PYQ
The reaction of an acid with a base to form salt and water is called:
- (a)Oxidation
- (b)Neutralisation
- (c)Sublimation
- (d)Reduction
Answer(b) Neutralisation — e.g. NaOH + HCl gives NaCl + H2O.