Physical and Chemical Changes MCQs

Q1. A blacksmith heats a straight iron rod until it glows and then hammers it into a curved horseshoe; afterwards he lets it cool and the metal is still iron. Why is the shaping of the rod regarded as a physical change?

• A.Heat always produces a chemical change in metals.
• B.No new substance forms; only the shape changes and the material remains iron.
• C.A new compound called horseshoe iron is created.
• D.The iron is converted into steel by the hammering.

Answer: B. No new substance forms; only the shape changes and the material remains iron.

During shaping, the iron is merely deformed and remains iron throughout. Since no new substance is produced and the composition is unchanged, only shape (a physical property) is altered, so it is a physical change.

Q2. In a kitchen a cook leaves a peeled apple slice on a plate and after some time its cut surface turns brown. What kind of change has the apple undergone and why?

• A.A chemical change, because the apple reacts with oxygen to form a new brown substance.
• B.No change, because the apple is still an apple.
• C.A physical change, because only the colour of the apple altered.
• D.A physical change, because the apple can be made white again easily.

Answer: A. A chemical change, because the apple reacts with oxygen to form a new brown substance.

Exposed apple tissue reacts with atmospheric oxygen (enzymatic browning), producing new brown coloured compounds. Formation of a new substance that is not easily reversed marks this as a chemical change.

Q3. During a birthday celebration a candle is lit and over an hour the wax shrinks while soot collects on a metal spoon held in the flame. Which statement best describes the changes occurring at the candle?

• A.Both the melting and the burning are purely physical changes.
• B.The wax melting is physical, but the burning of wax vapour producing soot and gases is chemical.
• C.Both the melting and the burning are purely chemical changes.
• D.Only the melting is chemical and the burning is physical.

Answer: B. The wax melting is physical, but the burning of wax vapour producing soot and gases is chemical.

Near the wick the solid wax melts (a reversible state change, physical). The molten wax then vaporises and burns in air, forming carbon dioxide, water vapour and soot, which are new substances, so burning is a chemical change.