CBSE Class 9 Chemistry Matter in Our Surroundings MCQs Set 14

Answer: (b) 298 K

Explanation:
1. Standard room temperature is 25 degrees C, which is 298 K.
2. 273 K is 0 degrees C.

Teacher's Note:
1. Kelvin = Celsius + 273.
2. 25 + 273 = 298.

Answer: (b) 10 g of water at 0 degrees C

Explanation:
1. Ice must absorb latent heat of fusion to become water at the same temperature.
2. So water at 0 degrees C has more heat content.

Teacher's Note:
1. Latent heat is hidden energy stored in the liquid.
2. The temperatures are the same.

Answer: (a) joules per kilogram

Explanation:
1. Latent heat is heat per unit mass for a change of state.
2. Its SI unit is joule per kilogram.

Teacher's Note:
1. Latent heat has no temperature change in its unit.
2. Calories per gram per degree is the unit of specific heat.

Answer: (b) Change of solid to liquid

Explanation:
1. Fusion is melting, the change of a solid to a liquid.

Teacher's Note:
1. Liquid to solid is freezing or solidification.
2. Liquid to vapour is vaporisation.

Answer: (d) both (a) and (b)

Explanation:
1. A dissolved impurity lowers the freezing point and raises the boiling point.
2. So both (a) and (b) are correct.

Teacher's Note:
1. Salt on icy roads melts ice by lowering the freezing point.
2. Salted water boils slightly above 100 degrees C.

Answer: (b) boiling point of a liquid decreases

Explanation:
1. Atmospheric pressure is lower at high altitude.
2. A liquid boils when its vapour pressure equals the outside pressure, so it boils at a lower temperature.

Teacher's Note:
1. Food takes longer to cook on mountains.
2. That is why pressure cookers are used there.

Answer: (b) adhesion

Explanation:
1. Adhesion is the attraction between unlike molecules, here water and glass.
2. Cohesion is attraction between molecules of the same kind.

Teacher's Note:
1. Water drop sticking to glass shows adhesion.
2. Cohesion makes drops round.

Answer: (b) the higher pressure increases the boiling point of water

Explanation:
1. Inside a pressure cooker the pressure is higher, so water boils at more than 100 degrees C.
2. Food cooks faster at the higher temperature.

Teacher's Note:
1. Boiling point rises with pressure.
2. This is the opposite of what happens on mountains.

Answer: (c) overcome the forces of attraction between molecules in liquid state

Explanation:
1. During boiling the temperature stays constant.
2. The heat supplied is used to overcome the forces between liquid molecules so that they escape as vapour.

Teacher's Note:
1. Latent means hidden.
2. No rise in temperature occurs during the change of state.

Answer: (a) sublimation

Explanation:
1. Air freshener blocks and naphthalene balls change directly from solid to vapour without becoming liquid.
2. This is sublimation.

Teacher's Note:
1. Camphor, naphthalene and ammonium chloride also sublime.
2. Dry ice sublimes as well.

Answer: (d) naphthalene balls and camphor

Explanation:
1. Naphthalene balls and camphor change from solid to vapour directly, which is sublimation.
2. Incense sticks burn and perfume evaporates from a liquid.

Teacher's Note:
1. Sublimation is solid to gas directly.
2. Burning is a chemical change.

Answer: (b) decrease in interparticle separation

Explanation:
1. A gas turns to liquid when its particles are brought closer by lowering the temperature and raising the pressure.
2. That decreases the separation between particles.

Teacher's Note:
1. Cool and compress to liquefy a gas.
2. More kinetic energy would keep the particles apart.

Answer: (b) Endothermic

Explanation:
1. Evaporation takes heat from the liquid and its surroundings, so it is endothermic.
2. This causes cooling.

Teacher's Note:
1. Sweating cools the body by evaporation.
2. Evaporation is a surface phenomenon.

Answer: (c) Rate of evaporation is inversely proportional to surface area of liquid

Explanation:
1. A larger surface area gives faster evaporation, so the rate is directly proportional to surface area.
2. Statement (c) is therefore false.

Teacher's Note:
1. Larger area means more particles at the surface.
2. That is why clothes dry faster when spread out.

Answer: (b) Y

Explanation:
1. Evaporation causes cooling and the rate of evaporation increases with surface area.
2. Container Y has the largest exposed surface, so more petrol evaporates and its temperature falls most.

Teacher's Note:
1. Larger area, faster evaporation, more cooling.
2. The figure shows Y as the widest container.

Answer: (a) evaporation leads to cooling

Explanation:
1. Dogs do not sweat.
2. Saliva evaporates from the tongue and cools the body.

Teacher's Note:
1. Evaporation takes heat from the surface.
2. This is how dogs lower their body temperature.

Answer: (b) is more in non-coastal areas than in coastal areas

Explanation:
1. Coastal air is humid, so evaporation is slow there.
2. Non-coastal air is drier, so evaporation is faster.

Teacher's Note:
1. Higher humidity lowers the rate.
2. That is why sweat dries slowly near the sea.

Answer: (d) Plasma state

Explanation:
1. Plasma consists of ionised gas with very energetic particles.
2. It is found in stars and in neon signs.

Teacher's Note:
1. The Sun is mostly plasma.
2. Bose-Einstein condensate forms at extremely low temperatures.

Answer: (d) Liquid mercury

Explanation:
1. Aurora, fluorescent tubes and neon signs contain ionised gas, which is plasma.
2. Liquid mercury is an ordinary liquid metal.

Teacher's Note:
1. Plasma is ionised gas.
2. Lightning is also plasma.

Answer: (a) at which all molecular motion ceases

Explanation:
1. Absolute zero is 0 K, or -273.15 degrees C, the lowest possible temperature.
2. At this temperature particle motion is at its minimum.

Teacher's Note:
1. It is the lower limit of the Kelvin scale.
2. Helium boils near 4 K, not at 0 K.