Physical and Chemical Changes — Important Questions

The colour of the mixture changes from yellow-grey to black, and the product (iron sulphide) is not attracted to a magnet and does not dissolve in carbon disulphide.

Step 1: The reaction is: Fe(s) + S(s) → FeS(s). Iron sulphide, a new compound, is formed with completely different properties. Step 2: Iron is greyish-black and magnetic; sulphur is yellow and dissolves in carbon disulphide. But iron sulphide (FeS) is black, non-magnetic, and does not dissolve in carbon disulphide. Step 3: These completely different properties confirm a chemical change has occurred. Step 4: Option B is wrong — heat being released can also occur in physical changes (like dissolving quick lime); it alone does not prove a chemical change. Step 5: Option C is wrong — this would be

It is a chemical change because CaO reacts with water to form a new substance, calcium hydroxide Ca(OH)₂, with a large release of heat energy.

Step 1: The reaction is: CaO(s) + H₂O(l) → Ca(OH)₂(s/aq) + Heat. This is called the slaking of lime. Step 2: Calcium hydroxide (slaked lime) is a NEW substance with different properties from calcium oxide. Step 3: The large amount of heat released is a strong indicator of a chemical change (exothermic reaction). Step 4: Option A is wrong — dissolving salt is physical because no new substance forms; here Ca(OH)₂ is a new compound. Option C is wrong — the heat is produced due to a chemical reaction, not friction. Step 5: Option D is wrong — the key indicator of a chemical change is the formation

Ammonium chloride changes directly from solid to gas on heating, but on cooling the vapour changes back into solid ammonium chloride — no new substance is formed, so it is a physical change.

Step 1: Sublimation is the process where a solid changes directly into a gas (vapour) on heating without passing through the liquid state. Step 2: For ammonium chloride: NH₄Cl(s) ⇌ NH₄Cl(g). The same compound exists in both states — no new substance is formed. Step 3: When the vapour is cooled, it deposits back as solid ammonium chloride (called the sublimate) — this proves the change is reversible and temporary, both characteristics of a physical change. Step 4: Option B describes thermal decomposition of ammonium chloride (which can happen at very high temperatures), but simple sublimation a

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Step 1: Aerobic respiration uses glucose (C₆H₁₂O₆) and oxygen (O₂) as reactants. Step 2: The products are carbon dioxide (CO₂), water (H₂O), and energy. Step 3: Balancing: 1 glucose + 6 O₂ → 6 CO₂ + 6 H₂O. Check: Left side — 6C, 12H, 18O. Right side — 6C (in CO₂), 12H (in H₂O), 12+6=18O. Balanced! Step 4: This is a chemical change because entirely new substances (CO₂ and H₂O) are formed from glucose, and the change is irreversible — you cannot get glucose back from CO₂ and H₂O in our body cells. Step 5: Option B is wrong — water is a product, not a reactant. Option C is wrong — hydrogen gas (H