Matter

Sublimation – Solid → Gas (directly, without passing through liquid state)

Step 1: Most substances change from solid to liquid (melting) and then from liquid to gas (vaporisation) when heated. Step 2: However, certain substances like naphthalene, camphor, iodine, and dry ice (solid CO₂) directly convert from solid to gaseous (vapour) state on heating without forming a liquid. This is called sublimation. Step 3: Since naphthalene undergoes sublimation at room temperature slowly, the balls gradually turn into vapour and diffuse into the air, which is why no liquid is ever seen. Step 4: Option A is wrong – evaporation requires a liquid stage. Option C is wrong – condens

Diffusion – because gas molecules have large intermolecular spaces and move freely at high speed

Step 1: Diffusion is the process by which particles of one substance mix with particles of another substance due to their random motion. Step 2: Gas molecules have very large intermolecular spaces and very weak intermolecular forces, allowing them to move freely in all directions at high speed. Step 3: In liquids, molecules are more closely packed and intermolecular forces are stronger, so particles cannot move as freely – diffusion is slower. Step 4: This is why the fragrance from an incense stick (a gas/vapour) spreads across the whole room rapidly. Step 5: Option A is wrong – cohesion refer

Liquids expand on heating and contract on cooling; the constriction traps mercury at the reading

Step 1: A thermometer is based on the principle that liquids expand on heating and contract on cooling. Step 2: When the thermometer is placed under the tongue, body heat causes mercury to expand and rise up the narrow capillary tube. Step 3: The thermometer has a tiny constriction (narrow point) near the bulb that prevents mercury from flowing back immediately when removed. Step 4: This lets us read the temperature even after removal. The mercury is later pushed back by shaking. Step 5: Option B is wrong – mercury does not change its state (freeze or boil) at body temperature. Option C is wro

Melting of wax = physical change; burning of wax vapour to produce CO₂ and water = chemical change

Step 1: A physical change is temporary, reversible, and does not produce any new substance. Step 2: When a candle is lit, wax near the flame melts (solid → liquid) and then vaporises (liquid → gas). This is a physical change – no new substance is formed and the wax can in theory be recovered. Step 3: The wax vapour then reacts with oxygen in air and burns. This produces two completely new substances – carbon dioxide (CO₂) and water (H₂O). This is a chemical change. Step 4: Chemical changes are permanent and irreversible – the burnt wax cannot be turned back into candle wax. Step 5: Option A in