iGCSE Chemistry (0620) Core:6.1 Physical and chemical changes: Exam Style Questions Paper 1- New Syllabus
▶️ Answer/Explanation
Detailed solution:
A physical change is one where no new substance is formed. The metal melting (statement 1) is simply a change of state from solid to liquid caused by the heat of the reaction – the metal itself does not change into a new substance. Steam being produced (statement 3) is water evaporating due to the heat released, again just a change of state. Hydrogen being produced (statement 2) and the pH increasing due to alkali formation (statement 4) both involve the creation of new chemical substances, so these are chemical changes.
▶️ Answer/Explanation
Detailed solution:
A physical change is one where no new substance is formed—the change is easily reversible and involves changes in state or dissolving. Rusting of iron (1) is a chemical change because a new substance, hydrated iron(III) oxide, is formed. Cracking of alkanes (4) is also a chemical change because large hydrocarbon molecules are broken down into smaller, different molecules. However, condensing of steam (2) only involves a change of state from gas to liquid, and dissolving sugar in water (3) does not produce a new substance—both are physical changes. So changes 2 and 3 are physical, making option C correct.
▶️ Answer/Explanation
Detailed solution:
A chemical change produces a brand-new substance, while a physical change just alters the form or state. Dissolving copper(II) sulfate, distilling ethanol, and freezing water are all reversible physical processes where the chemical identity of the substance stays the same. Neutralising copper(II) oxide, however, means reacting it with an acid to form a copper salt and water. This creates entirely new chemical compounds, making it the only chemical change in the list.