Physical and Chemical Changes

In a chemical change, a new substance with different composition and properties is formed permanently

Step 1: The key characteristic of a chemical change is the formation of one or more new substances with completely different chemical composition and properties. Step 2: This change is permanent and irreversible — the original substance cannot be recovered by simple physical methods. Step 3: Option A is wrong — colour change alone can occur in physical changes too (e.g., painting). Option C is wrong — being reversible by cooling is a feature of physical changes, not chemical changes. Option D is wrong — size and shape changes describe physical changes. Step 4: Examples of chemical changes: bur

The vapours, when cooled, change back to the original solid camphor — no new substance is formed

Step 1: In the sublimation activity, solid camphor is heated and directly converts to vapour (gas state). Step 2: When these vapours reach the cooler part of the inverted funnel, they deposit back as solid camphor on the inner walls. Step 3: The substance that forms is identical to the original camphor — no new substance is formed. The chemical composition remains unchanged throughout. Step 4: This proves that sublimation is a physical change — only the physical state changes (solid → gas → solid). Step 5: Option C is wrong — camphor does not react with oxygen during simple sublimation. Option

An endothermic reaction because energy (sunlight) is absorbed

Step 1: Photosynthesis is a chemical change — glucose (a new substance) is formed from carbon dioxide and water with the help of sunlight. Step 2: In terms of energy, reactions where energy is absorbed are called endothermic reactions. Step 3: In photosynthesis, sunlight (light energy) is absorbed by plants to drive the chemical reaction: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Step 4: Option A is wrong — exothermic reactions RELEASE energy. The presence of sunlight as an input means energy is being taken in, not released. Step 5: Option C is wrong — photosynthesis clearly forms new substa

Rusting of iron

Step 1: Slow changes are those that take hours, days, months, or even years to complete. Step 2: Rusting of iron is a slow chemical change. Iron reacts with oxygen and moisture in the air to form iron oxide (rust): 4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O. This process happens gradually over many days. Step 3: Bursting of a cracker, blinking of eyes, and switching on a fan all happen in fractions of a second — these are fast changes. Step 4: It is important to note that rusting is also a chemical change (irreversible, new substance formed), but the question specifically asks about the speed of the chang