CBSE Class 9 Science Advanced Chemistry Matter in Our Surrounding Long Answer Questions Set 01

Long Answer Type

Question 1. Water as ice has a cooling effect, whereas water as steam may cause severe burns. Explain these observations.
Answer:
Ice: Ice at 273 K has to absorb the latent heat of fusion (3.34 × 10⁵ J/kg) to melt. It takes this heat from the surroundings, so the surroundings cool. Water at 273 K does not have to melt, so it cools less than ice.
Steam: Steam at 373 K has absorbed latent heat of vaporisation (about 22.6 × 10⁵ J/kg), which is much more than the heat in boiling water at the same temperature.
When steam touches the skin it condenses and gives out this extra heat, in addition to the heat of the water formed. So steam causes more severe burns than boiling water.
In simple words: Ice absorbs heat while melting; steam releases a lot of heat while condensing.

Teacher's Note:
1. Mention the latent heats for both.
2. Compare ice with water at 0 °C, and steam with water at 100 °C.

Question 2. (a) Conversion of solid to vapour is called sublimation. Name the term used to denote the conversion of vapour to solid. (b) Conversion of solid state to liquid state is called fusion; what is meant by latent heat of fusion?
Answer:
(a) The change of vapour directly into solid is called deposition (the book also calls it sublimation in reverse, or frosting).
(b) The latent heat of fusion is the amount of heat energy required to change 1 kg of a solid into liquid at its melting point, at atmospheric pressure, without any change in temperature. For ice it is 3.34 × 10⁵ J/kg.
In simple words: Vapour to solid is deposition, and the heat to melt 1 kg of solid is the latent heat of fusion.

Teacher's Note:
1. Do not forget the unit J/kg.
2. Water vapour turning to frost on a cold night is deposition.

Question 3. Comment on the following statements: (a) Evaporation produces cooling. (b) Rate of evaporation of an aqueous solution decreases with increase in humidity. (c) Sponge though compressible is a solid.
Answer:
(a) During evaporation the fastest particles at the surface escape. The liquid supplies the latent heat of vaporisation, and so the liquid and its surroundings lose heat and cool. This is why sweat, acetone on the palm or sprinkled water cools us.
(b) Humidity is the amount of water vapour in the air. When it is high, the air can take up little more vapour, and as particles also return to the liquid, the net rate of evaporation falls. So evaporation decreases as humidity increases.
(c) A sponge has many tiny air-filled holes, and pressing it squeezes the air out of them, so it seems to be compressible. But the sponge material itself is a solid with a definite shape and its particles are closely packed. So it is a solid.
In simple words: Evaporation takes heat, humid air slows it, and a sponge compresses only because of its trapped air.

Teacher's Note:
1. Give the reason in one sentence for each part.
2. Compressibility of a solid should be explained by the holes in it.

Question 4. You are provided with a mixture of naphthalene and ammonium chloride by your teacher. Suggest an activity to separate them with well labelled diagram.
Answer:
Both naphthalene and ammonium chloride sublime, so sublimation cannot separate them. We use the fact that ammonium chloride dissolves in water, but naphthalene does not.
Method: (1) Stir the mixture with water in a beaker; ammonium chloride dissolves. (2) Filter the mixture through filter paper in a funnel. (3) The residue on the filter paper is naphthalene; wash it and dry it. (4) Heat the filtrate to evaporate the water; ammonium chloride crystals are left.

In simple words: Water dissolves ammonium chloride but not naphthalene, so filtering and evaporating separates them.

Teacher's Note:
1. Draw the funnel, filter paper, residue and filtrate with labels.
2. The book gives no answer; a sublimation answer would be wrong because both solids sublime.

Question 5. (a) The graph shows the heating curve for a pure substance. The temperature rises with time as the substance is heated:
(b) What is the physical state of the substance at the points A,B,C,D, E and F ?
(c) What is the melting point of the substance ? What is its boiling point ?
(d) What happens to the temperature during change of the state ?
(e) The substance is not water. How can you judge from the graph ?
Answer:
(b) A is solid; B is the start of melting (solid and liquid together); C is liquid, the end of melting; D is the start of boiling (liquid and gas together); E is gas at the end of boiling, and F is gas.
(c) The melting point is 15 °C (the flat part BC) and the boiling point is 110 °C (the flat part DE).
(d) During a change of state the temperature stays constant, as the heat supplied goes into the latent heat.
(e) The substance is not water, because water melts at 0 °C and boils at 100 °C, not at 15 °C and 110 °C.
In simple words: Flat parts of a heating curve are melting and boiling points, and they are different from those of water.

Teacher's Note:
1. Read the flat parts of the graph first.
2. Compare with 0 °C and 100 °C for water.

Question 6. Explain the factors affecting the rate of evaporation.
Answer:
1. Temperature: a higher temperature gives more particles enough kinetic energy to escape, so the rate increases.
2. Surface area: evaporation takes place at the surface, so a larger surface area gives a faster rate. That is why clothes are spread out to dry.
3. Humidity: humidity is the amount of water vapour in the air. If the humidity is low the rate is high, and if it is high the rate is low.
4. Wind speed: wind carries away the water vapour, so the rate increases with wind speed.
5. Nature of the substance: a liquid with a low boiling point evaporates quickly, and one with a high boiling point evaporates slowly.
In simple words: Evaporation is faster when it is hot, windy, dry and the surface is large.

Teacher's Note:
1. Give a daily-life example for each factor.
2. Note that depth of the liquid is not a factor.

Question 7. The heating curve of a pure substance at one atmosphere pressure is shown in the following figure.
(a) What is the physical state of the substance at points A, B, C, D and E.
(b) What is the melting point of the substance ?
(c) What the boiling point of the substance ?
(d) What happens to the temperature when the substance is changing its state ?
(e) Can the given substance be ice at point A ?
Answer:
(a) A: solid. B: solid and liquid together (melting). C: liquid. D: liquid and gas together (boiling). E: gas.
(b) Melting point = 273 K (0 °C).
(c) Boiling point = 373 K (100 °C).
(d) The temperature remains constant during a change of state, from solid to liquid and from liquid to gas.
(e) Yes. The melting point 273 K and the boiling point 373 K are those of water, so the substance can be water and the solid at A can be ice.
In simple words: The flat parts at 273 K and 373 K show that the substance is water.

Teacher's Note:
1. Read the temperatures on the axis, not the dots.
2. State the unit K as in the graph.