NDA (II) 2017 General Ability Test · Question 118 of 150

Correct — B, Graying of hair naturally. A chemical change alters the composition of the substance, produces something that was not there before and cannot be undone by simply reversing the conditions. Hair goes grey because the pigment-making cells in the follicle stop producing melanin, so the new hair grows out without its pigment; grey and white hair is not the result of a grey or white pigment but of the absence of melanin, and the shaft looks pale because of the way light is reflected from it. That is a change in what the hair is made of, it happens gradually, and no amount of cooling, warming or washing brings the colour back. The other three change only the size, shape or state of something and leave its composition untouched.

• (a)Cutting of hair — Cutting shortens the strand. Every cut piece is still keratin of exactly the same composition, so only the size has changed, which makes it a physical change.
• (c)Swelling of resin in water — The resin takes up water and expands, a process of absorption. Nothing new is formed, and drying it returns it to its earlier state, so the change is physical.
• (d)Cutting of fruit — Slicing changes shape and size only. The browning that may follow on the cut surface is a genuine chemical change, but the act of cutting named in the option is not.

The test for a chemical change is whether a new substance with new properties has appeared. Rusting, burning, cooking, digestion, the souring of milk and the reaction of an acid with a base are all chemical, and all are difficult or impossible to reverse. Melting, boiling, dissolving, crystallising, cutting, breaking and magnetising are physical, because the substance survives the process unchanged and can usually be recovered. Energy is often absorbed or given out in both kinds of change, so heat alone does not settle the question.

Two of the standard examples deserve care because they look the other way round at first. Dissolving sugar in water seems to make the sugar vanish, yet evaporating the water brings it back, so it is physical. Burning a candle is a mixed case: the wax first melts, which is physical, and then burns, which is chemical. The question here uses everyday human biology rather than a laboratory example, and the reasoning is the same — ask whether the material itself has become something different. Where a card must state the underlying cause, it is enough and accurate to say the follicle's melanocytes cease supplying melanin; the finer biochemistry of why they stop is still an active research question and should not be asserted.

• A chemical change produces a new substance and is usually not reversible by simple physical means.
• Grey and white hair is not caused by a grey or white pigment but by a lack of melanin in the growing shaft.
• Cutting, breaking, melting, boiling and dissolving are physical changes because the composition is unaffected.
• Souring of milk, rusting of iron, burning of coal and the reaction of an acid with a base are standard chemical changes.
• Burning a candle involves both — the wax melts physically and then burns chemically.

• Assuming any change involving a living body is automatically chemical; a haircut is not.
• Calling dissolving a chemical change because the solid disappears from view.
• Missing the mixed cases such as a burning candle, where both kinds of change occur together.

NDA gives four everyday processes and asks which is chemical, or inverts it and asks which is not.

• practice — not a real PYQ

  Which one of the following is NOT a chemical change ?

  • (a)Burning of coal
  • (b)Rusting of iron
  • (c)Melting of wax
  • (d)Souring of milk

  Answer(c) Melting of wax — a change of state only, and reversible on cooling.

• practice — not a real PYQ

  Dissolving common salt in water is a physical change because

  • (a)the salt disappears completely
  • (b)the salt can be recovered unchanged by evaporating the water
  • (c)heat is absorbed during the process
  • (d)the solution conducts electricity

  Answer(b) the salt can be recovered unchanged by evaporating the water — no new substance has been formed.