Matter — Important Questions

Temperature remains constant at 100°C as steam releases latent heat to become water

Step 1: Condensation is the reverse process of vaporization/boiling — steam changes into water. Step 2: Just like melting/freezing, condensation occurs at a fixed, constant temperature (100°C for steam-to-water). Step 3: During condensation, the steam RELEASES (gives out) heat to the surroundings, not absorbs. Step 4: This released heat is latent heat — the temperature does not change during the process even though heat is being given out. Step 5: Option A is wrong as temperature does not rise. Option C is wrong because steam releases heat (does not absorb) and temperature stays at 100°C. Opti

Molecules are in constant random motion

Step 1: When perfume is sprayed, the perfume molecules are released into the air as a gas. Step 2: Gas molecules possess high kinetic energy and move in random directions at high speeds. Step 3: As the perfume molecules move randomly, they mix with air molecules and spread throughout the room — this is called diffusion. Step 4: If molecules were not in constant motion, the smell would only be detected near the corner where it was sprayed. Step 5: Option A (intermolecular spaces) explains salt dissolving in water without volume change. Option B (small size) explains why molecules can't be seen.

672 × 10³ J

Step 1: Identify the given values. Mass of ice (m) = 2 kg. Specific latent heat of fusion of ice (L) = 336 × 10³ J/kg. Step 2: The formula for heat absorbed during melting is: Q = m × L. Step 3: Substitute values: Q = 2 kg × 336 × 10³ J/kg. Step 4: Calculate: Q = 672 × 10³ J. Step 5: This heat is used only to break intermolecular bonds and change the state — the temperature remains at 0°C throughout. Option A (168 × 10³ J) would be the answer if mass were 0.5 kg. Option B is the latent heat for 1 kg only. Option D would be for 3 kg of ice.

Evaporation occurs at all temperatures from the surface; boiling occurs at a fixed temperature throughout the liquid

Step 1: Evaporation is the process of liquid changing to vapour from its surface at ANY temperature. Step 2: Boiling (vaporization) is a rapid process where the entire liquid converts to vapour at a specific fixed temperature called the boiling point. Step 3: For water, boiling occurs at 100°C, but evaporation occurs even at 25°C or 40°C etc. Step 4: Option A is exactly the opposite of the correct answer — evaporation is at the surface and boiling is throughout. Step 5: Option C is wrong because evaporation is slow and boiling is rapid. Option D is wrong because evaporation produces COOLING of