Which one of the following is a chemical change?
- (a)Dissolving sugar in water
- (b)Melting of ice
- (c)Crystallization
- (d)Milk turning sour
Correct — D, Milk turning sour. A chemical change forms a new substance with a different chemical composition and is usually hard to reverse. Milk turns sour because bacteria ferment its lactose into lactic acid — a new substance is produced — so it is a chemical change. Dissolving sugar in water, melting of ice and crystallization only change the physical state or arrangement; the substances keep their chemical identity, so those are physical changes.
- (a)Dissolving sugar in water — The sugar can be recovered unchanged by evaporating the water — no new substance forms, so this is a physical change.
- (b)Melting of ice — Ice melting to water is only a change of state; the substance is still H₂O, so it is a physical change.
- (c)Crystallization — Crystallization rearranges particles into a solid crystal without changing their chemical identity, so it is a physical change.
A chemical change produces one or more new substances with new properties and is generally not easily reversed, often showing signs such as a colour change, a smell, a gas, a precipitate or the release of heat or light. A physical change alters only the state, shape or appearance while the chemical identity stays the same.
Three options — dissolving, melting and crystallization — merely change the physical state, so the substances are still the same. Only the souring of milk creates a new substance (lactic acid), which marks it as the chemical change.
- A chemical change produces a new substance and is generally not easily reversed.
- Souring of milk is lactic-acid fermentation — a chemical change.
- Dissolving, melting and crystallization are physical changes; the chemical identity is unchanged.
- Signs of a chemical change include a new colour, smell, gas, precipitate or heat/light.
- Assuming anything involving milk or food is automatically 'physical'.
- Treating dissolving or melting as chemical just because the appearance changes.
Pick the single chemical change among physical changes — look for the option that produces a new substance (souring of milk).
UPSC_2014_GS1_Q702014Which 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
Same concept and same example — souring of milk as a chemical change. That UPSC item marks only souring of milk (3) as chemical while crystallization and melting are physical; this NDA item asks the same, with milk turning sour again the answer.
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
Same concept — pick the chemical change among physical changes. That NDA item marks an acid–base reaction (a new substance) as chemical; this one marks souring of milk.
Which one of the following is NOT a chemical change?
- (a) Burning of coal
- (b) Rusting of a metal
- (c) Melting of ice
- (d) Reaction of an acid with a base
Answer(c) Melting of ice
Same concept, inverted — that NDA item asks which is NOT chemical (melting of ice, a physical change); this one asks which IS chemical (souring of milk).
- practice — not a real PYQ
Which one 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 — only the state changes, no new substance forms.
- practice — not a real PYQ
Rusting of iron is a chemical change because
- (a)the iron changes shape
- (b)a new substance (iron oxide) is formed
- (c)it can be easily reversed
- (d)no mass changes occur
Answer(b) a new substance (iron oxide) is formed.