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NCERT Exemplar · Class 9 Science Matter in Our Surroundings

27 questions · 27 still being checked

Short Answer Questions 11–22 (part 2 of 3)

  1. Exercise 11

    A sample of water under study was found to boil at 102\displaystyle 102°C at normal temperature and pressure. Is the water pure? Will this water freeze at 0\displaystyle 0°C? Comment.

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    NCERT’s answer
    It’s freezing point will be below $\displaystyle 0$°C due to the presence of a non-volatile impurity in it.
    Pure water boils and freezes at fixed points at normal pressure: \[T_b^{\text{pure}} = 100\,^\circ\text{C}, \qquad T_f^{\text{pure}} = 0\,^\circ\text{C} \] The sample boils at \[T_b^{\text{obs}} = 102\,^\circ\text{C} \neq T_b^{\text{pure}} \] A dissolved solute shifts both points together: \(\displaystyle \text{solute added} \Rightarrow T_b\uparrow,\ T_f\downarrow\). Answer: Not pure water; it will freeze below \(\displaystyle 0\,^\circ\text{C}\), not at \(\displaystyle 0\,^\circ\text{C}\).

  2. Exercise 12

    A student heats a beaker containing ice and water. He measures the temperature of the content of the beaker as a function of time. Which of the following (Fig. 1.1\displaystyle 1.1) would correctly represent the result? Justify your choice. NCERT_Question_Class9_Science_Exemplar_Ch1_Q12

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    NCERT’s answer
    Since ice and water are in equilibrium, the temperature would be zero. When we heat the mixture, energy supplied is utilized in melting the ice and the temperature does not change till all the ice melts because of latent heat of fusion. On further heating, the temperature of the water would increase. Therefore the correct option is (d).
    The beaker holds ice and water, so it starts at \(\displaystyle 0\,^\circ\text{C}\). Heat supplied melts the ice while temperature stays constant — absorbed as latent heat of fusion, not as a rise. Once all ice melts, temperature climbs towards \(\displaystyle 100\,^\circ\text{C}\). Only graph (d) shows flat-then-rising; the rest rise immediately, fall, or plateau elsewhere.Answer: (d)

  3. Exercise 13

    Fill in the blanks:
    (a)
    Evaporation of a liquid at room temperature leads to a——— effect.
    (b)
    At room temperature the forces of attraction between the particles of
    solid substances are———than those which exist in the gaseous state.
    (c)
    The arrangement of particles is less ordered in the ——— state. However,
    there is no order in the ——— state.
    (d)
    ——— is the change of solid state directly to vapour state without going
    through the ———state.
    (e)
    The phenomenon of change of a liquid into the gaseous state at any
    temperature below its boiling point is called———.

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    NCERT’s answer
    (a)
    cooling (b) stronger (c) liquid, gaseous (d) sublimation, liquid (e) evaporation
    Answer: (a) cooling (b) much greater (c) liquid; gaseous (d) sublimation; liquid (e) evaporation.

  4. Exercise 14

    Match the physical quantities given in column A to their S I units given in
    column B :
    (A)
    (B)(a)
    Pressure
    (i)
    cubic metre
    (b)
    Temperature
    (ii)
    kilogram
    (c)
    Density
    (iii)
    pascal
    (d)
    Mass
    (iv)
    kelvin
    (e)
    Volume
    (v)
    kilogram per cubic metre

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    NCERT’s answer
    (a)
    — (iii) (b) — (iv) (c) — (v) (d) — (ii) (e) — (i)
    Pressure and density follow from their defining formulae: \[P = \frac{F}{A} \ \Rightarrow\ \text{SI unit: pascal} \] \[\rho = \frac{m}{V} \ \Rightarrow\ \text{SI unit: kg m}^{-3} \] Mass, volume and temperature take their SI base/derived units directly: kilogram, cubic metre, kelvin. Answer: (a)-(iii) pascal, (b)-(iv) kelvin, (c)-(v) \(\displaystyle \text{kg m}^{-3}\), (d)-(ii) kilogram, (e)-(i) cubic metre.

  5. Exercise 15

    The non S I and S I units of some physical quantities are given in column
    A and column B respectively. Match the units belonging to the same
    physical quantity:
    (A)
    (B)(a)
    degree celsius
    (i)
    kilogram
    (b)
    centimetre
    (ii)
    pascal
    (c)
    gram per centimetre cube
    (iii)
    metre
    (d)
    bar
    (iv)
    kelvin
    (e)
    milligram
    (v)
    kilogram per metre cube

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    NCERT’s answer
    (a)
    — (iv) (b) — (iii) (c) — (v) (d) — (ii) (e) — (i)
    Each non-SI unit is matched by the physical quantity it measures.
    (a)
    –(iv): degree celsius and kelvin — temperature.
    (b)
    –(iii): centimetre and metre — length.
    (c)
    –(v): gram per centimetre cube and kilogram per metre cube — density.
    (d)
    –(ii): bar and pascal — pressure.
    (e)
    –(i): milligram and kilogram — mass.
    Answer: (a)-(iv), (b)-(iii), (c)-(v), (d)-(ii), (e)-(i)

  6. Exercise 16

    ‘Osmosis is a special kind of diffusion’. Comment.

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    NCERT’s answer
    Yes, this is true. In both the phenomena, there is movement of particles from region of higher concentration to that of lower concentration. However, in the case of osmosis the movement of solvent is through a semi permeable membrane which is permeable only to water molecules. (b) Filtration
    Diffusion is the net movement of particles from higher to lower concentration, through any medium. Osmosis narrows this to solvent (water) molecules only, moving only across a semi-permeable membrane, from a dilute solution toward a concentrated one.\[[\text{H}_2\text{O}]_{\text{dilute side}} > [\text{H}_2\text{O}]_{\text{concentrated side}} \ \Rightarrow\ \text{net water flow: dilute} \to \text{concentrated} \] Answer: Osmosis is diffusion of water alone, across a semi-permeable membrane — a special, restricted case of diffusion, not a separate process.

  7. Exercise 17

    Classify the following into osmosis/diffusion
    (a)
    Swelling up of a raisin on keeping in water.
    (b)
    Spreading of virus on sneezing.
    (c)
    Earthworm dying on coming in contact with common salt.
    (d)
    Shrinking of grapes kept in thick sugar syrup.
    (e)
    Preserving pickles in salt.
    (f)
    Spreading of smell of cake being baked through out the house.
    (g)
    Aquatic animals using oxygen dissolved in water during respiration.

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    NCERT’s answer
    (a)
    Osmosis (b) Diffusion (c) Osmosis (d) Osmosis (e) Osmosis (f) Diffusion (g) Diffusion
    Osmosis: water only, across a membrane, along a concentration gradient. Diffusion: any particles, through any medium, membrane or not.
    (a)
    Osmosis — raisin absorbs water.
    (b)
    Diffusion — virus spreads in air.
    (c)
    Osmosis — salt draws water out of the earthworm.
    (d)
    Osmosis — syrup draws water out of the grape.
    (e)
    Osmosis — salt draws water out of microbes.
    (f)
    Diffusion — aroma spreads in air.
    (g)
    Diffusion — dissolved \(\displaystyle \text{O}_2\) moves into the body.
    Answer: Osmosis: (a), (c), (d), (e). Diffusion: (b), (f), (g).

  8. Exercise 18

    Water as ice has a cooling effect, whereas water as steam may cause severe burns. Explain these observations.

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    NCERT’s answer
    In case of ice the water molecules have low energy while in the case of steam the water molecules have high energy. The high energy of water molecules in steam is transformed as heat and may cause burns. On the other hand, in case of ice, the water molecules take energy from the body and thus give a cooling effect.
    Ice absorbs latent heat of fusion from the skin as it melts (endothermic): \[Q_{\text{ice}} = mL_f, \qquad L_f = 334\ \text{J g}^{-1} \] this heat is taken from the skin — a cooling effect. Steam releases latent heat of vaporisation as it condenses on the skin: \[Q_{\text{steam}} = mL_v, \qquad L_v = 2260\ \text{J g}^{-1} \] this heat is given to the skin, and \(\displaystyle L_v \gg L_f\) — hence a severe burn. Answer: Ice draws heat out of the skin on melting (cooling); steam dumps far more heat onto the skin on condensing (burns).

  9. Exercise 19

    Alka was making tea in a kettle. Suddenly she felt intense heat from the puff of steam gushing out of the spout of the kettle. She wondered whether the temperature of the steam was higher than that of the water boiling in the kettle. Comment.

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    NCERT’s answer
    The temperature of both boiling water and steam is $\displaystyle 100$°C, but steam has more energy because of latent heat of vapourisation.
    During boiling, all the heat supplied changes state, not temperature: \[T_{\text{water(boiling)}} = T_{\text{steam}} = 100\,^\circ\text{C} \quad (\text{at normal pressure}) \] The extra energy in steam is latent heat, released only on condensing on the skin: \[Q_{\text{steam}} = mL_v, \qquad L_v = 2260\ \text{J g}^{-1} \] Answer: No — steam and boiling water are both at \(\displaystyle 100\,^\circ\text{C}\); steam only feels hotter because it releases extra latent heat when it condenses on the skin.

  10. Exercise 20

    A glass tumbler containing hot water is kept in the freezer compartment of a refrigerator (temperature < 0\displaystyle 0°C). If you could measure the temperature of the content of the tumbler, which of the following graphs (Fig.1.2) would correctly represent the change in its temperature as a function of time. NCERT_Question_Class9_Science_Exemplar_Ch1_Q20

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    NCERT’s answer
    (a)
    The water will cool initially till it reaches $\displaystyle 0$°C, the freezing point. At this stage the temperature will remain constant till all the water will freeze. After this temperature would fall again.
    Placed in the freezer, the hot water's temperature falls steadily to \(\displaystyle 0\,^\circ\text{C}\), then holds there while it gives up its latent heat of fusion at constant temperature and freezes; once fully frozen, the ice cools further, below \(\displaystyle 0\,^\circ\text{C}\): \(\displaystyle \text{fall}\Rightarrow\text{plateau at }0^\circ\text{C}\Rightarrow\text{fall below }0^\circ\text{C}\). Only (a) shows this whole sequence: (b) has no plateau, (c) plateaus at \(\displaystyle 0\,^\circ\text{C}\) but never falls further, (d) falls to \(\displaystyle 0\,^\circ\text{C}\) and then rises again.Answer: (a)

  11. Exercise 21

    Look at Fig. 1.3\displaystyle 1.3 and suggest in which of the vessels A,B, C or D the rate of evaporation will be the highest? Explain. NCERT_Question_Class9_Science_Exemplar_Ch1_Q21

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    NCERT’s answer
    (c)
    The rate of evaporation increases with an increase of surface area because evaporation is a surface phenomenon. Also, with the increase in air speed, the particles of water vapour will move away with the air, which will increase the rate of evaporation.
    Evaporation rate rises with surface area and wind speed, and falls when the surface is covered: \(\displaystyle \text{area}\uparrow,\text{wind}\uparrow \Rightarrow \text{evaporation}\uparrow\), \(\displaystyle \text{covered}\Rightarrow\text{evaporation}\downarrow\). A and C share the same wide mouth, but only C sits under the moving fan, so it alone gets faster air flow that sweeps vapour away, keeping the air above it unsaturated. B has a narrower mouth than A; D, as wide as A and C, is lidded, trapping vapour.Answer: vessel C

  12. Exercise 22

    (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?

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    (a)
    Vapour turning directly to solid is called deposition (many books use sublimation for this reverse change too).
    (b)
    Latent heat of fusion is the heat needed to melt \(\displaystyle 1\ \text{kg}\) of a solid at its melting point, at atmospheric pressure, without a change in temperature.
    Answer: (a) deposition (b) heat to melt $\displaystyle 1$ kg of solid at its melting point, without a change in temperature.
    NCERT prints: (a) Sublimation — but the question's own opening line defines sublimation as solid turning to vapour, so naming the reverse change the same way contradicts that definition; the correct term is deposition.