Which of the following sets comprises chemical changes?
2021
Which of the following sets comprises chemical changes?
- A.
Boiling of water, breaking of glass, rusting of iron
- B.
Dissolving salt in water, digestion of food, shredding of paper.
- C.
Cooking of food, boiling of water, dissolving sugar in water.
- D.
Rusting of iron, burning of paper, digestion of food
Show answer & explanation
Correct answer: D
Concept: A chemical change (a chemical reaction) turns a substance into one or more NEW substances with a different chemical composition -- new bonds form, and signs such as a colour change, gas evolution, or release of heat/light typically appear; it generally cannot be reversed by simple physical means. A physical change only alters the form, size, or state of a substance while its chemical composition stays exactly the same, and it can usually be reversed by physical means alone.
Application: Checking every listed change against this test -- boiling water only turns liquid water into water vapour (same substance, physical); breaking glass, shredding paper, and dissolving salt or sugar in water only change shape, size, or dispersion while the original substance can be recovered unchanged (all physical). Rusting of iron, however, forms a new compound (hydrated iron oxide) from iron, oxygen, and moisture; burning of paper converts cellulose into carbon dioxide, water vapour, and ash; and digestion of food breaks complex molecules into entirely new, simpler substances through enzyme-driven reactions -- all three are genuine chemical changes.
Boiling of water, breaking of glass, rusting of iron: boiling and breaking are physical changes; rusting is a chemical change.
Dissolving salt in water, digestion of food, shredding of paper: dissolving and shredding are physical changes; digestion is a chemical change.
Cooking of food, boiling of water, dissolving sugar in water: boiling and dissolving are physical changes; cooking is a chemical change.
Rusting of iron, burning of paper, digestion of food: rusting, burning, and digestion are each chemical changes.
Cross-check: In every other set, at least one listed process leaves the substance's chemical identity unchanged (a state, shape, or dispersion change only). Rusting of iron, burning of paper, and digestion of food are each chemical changes, which is why this set is the answer the question is asking for.