CBSE Class 9 Science Advanced Chemistry Matter in Our Surrounding MCQs Set 02

Answer: (b) Iodine

Explanation:
1. Among the four, only iodine changes directly from solid to vapour on gentle heating.
2. Sand and common salt do not sublime.
3. Sulphur melts on heating instead of subliming readily.
4. The answer is iodine.

Teacher's Note:
1. A sublimate is the solid obtained on cooling the vapour.
2. Iodine, camphor, naphthalene and ammonium chloride are the usual examples.

Answer: (a) boiling point

Explanation:
1. A pure liquid boils at a fixed, characteristic temperature.
2. Impurities change the boiling point.
3. The answer is boiling point.

Teacher's Note:
1. Melting point does the same job for solids.
2. Volume does not identify a substance.

Answer: (d) Naphthalene balls and camphor

Explanation:
1. Naphthalene balls and camphor both turn from solid to vapour directly, so sublimation is involved.
2. An incense stick burns and a perfume evaporates, so these are not sublimation.
3. The answer is (D).

Teacher's Note:
1. Match both substances in each option before choosing.
2. Only option D has two sublimating solids.

Answer: (c) B.P. of a liquid decreases

Explanation:
1. The atmospheric pressure is lower at a higher altitude.
2. A liquid needs a lower temperature to make its vapour pressure equal to the air pressure.
3. So the boiling point decreases, option (C).

Teacher's Note:
1. Food takes longer to cook on mountains for the same reason.
2. Lower pressure means lower boiling point.

Answer: (a) High pressure, low temperature

Explanation:
1. Compressing a gas pushes the particles closer.
2. Cooling slows them down, so attraction can hold them together.
3. High pressure and low temperature together liquefy a gas, option (A).

Teacher's Note:
1. Remember: squeeze and cool.
2. Carbon dioxide is liquefied at about 70 times atmospheric pressure.

Answer: (c) Melting point and freezing point of a substance are same numerically.

Explanation:
1. A substance melts and freezes at the same temperature.
2. For water both are 0 °C.
3. The answer is (C).

Teacher's Note:
1. The two change places only in direction of heat flow.
2. Check against ice and water if in doubt.

Answer: (c) 373K

Explanation:
1. Water boils at 100 °C at normal pressure.
2. In kelvin, 100 + 273 = 373 K.

Teacher's Note:
1. 273 K is the freezing point of water.
2. 100 is the value in °C, not in K.

Answer: (d) Fluidity

Explanation:
1. Liquids and gases can flow, solids cannot.
2. This property is fluidity, so the answer is (D).

Teacher's Note:
1. That is why liquids and gases are called fluids.
2. Mass is a property of all matter.

Answer: (c) remains the same

Explanation:
1. Floating ice displaces water equal to its own weight.
2. When it melts, the water formed has the same weight and fills exactly the space the ice displaced.
3. So the water level remains the same.

Teacher's Note:
1. Equal weight of water has equal volume to the part of ice under water.
2. If the ice rested on the bottom, the answer would be different.

Answer: (b) 193 K

Explanation:
1. K = °C + 273 = −80 + 273.
2. T = 193 K.
3. A temperature in kelvin cannot be negative, so (A) and (D) are impossible.

Teacher's Note:
1. Kelvin scale has no negative values.
2. Adding, not subtracting, 273 gives the answer.

Answer: (c) remain constant

Explanation:
1. During melting the heat is used up in changing the state.
2. So the temperature remains constant till all the solid has melted.

Teacher's Note:
1. That constant temperature is the melting point.
2. The same is true during boiling.

Answer: (d) Solid

Explanation:
1. Only solids have a definite shape of their own.
2. Liquids and gases take the shape of the container.
3. The answer is solid.

Teacher's Note:
1. Shape and volume are both definite for solids.
2. Liquids have definite volume only.

Answer: (c) Liquid

Explanation:
1. 298 K = 298 − 273 = 25 °C, ordinary room temperature.
2. Water is between 0 °C and 100 °C, so it is a liquid.

Teacher's Note:
1. Water freezes at 273 K and boils at 373 K.
2. Plasma exists only at extremely high temperatures.

Answer: (c) Vessel C

Explanation:
1. Vessel D is covered, so vapour cannot escape freely.
2. Vessel B has a small open surface and vessel A has a large open surface but no moving air.
3. Vessel C has a large open surface and the moving fan carries the vapour away.
4. So the rate is highest in vessel C.

Teacher's Note:
1. Large surface area and wind both speed up evaporation.
2. A lid cuts evaporation down.

Answer: (c) only (iii) is correct

Explanation:
1. The temperature must rise from −10 °C to 0 °C, stay at 0 °C while ice melts, then rise to 100 °C and stay there while water boils.
2. Graph (iii) shows exactly this.
3. Graph (i) stops at 10 °C, (ii) starts at 0 °C and (iv) shows flat parts at wrong temperatures.
4. The answer is only (iii) is correct.

Teacher's Note:
1. Check three things: the starting temperature, the flat part at 0 °C and the flat part at 100 °C.
2. Ice melts at 0 °C, not at 10 °C or 30 °C.