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NCERT Exemplar · Class 9 Science Is Matter Around Us Pure

42 questions · 42 still being checked

Short Answer Questions 20–32 (part 3 of 4)

  1. Exercise 20

    Name the process associated with the following
    (a)
    Dry ice is kept at room temperature and at one atmospheric pressure.
    (b)
    A drop of ink placed on the surface of water contained in a glass spreads
    throughout the water.
    (c)
    A potassium permanganate crystal is in a beaker and water is poured
    into the beaker with stirring.
    (d)
    A acetone bottle is left open and the bottle becomes empty.
    (e)
    Milk is churned to separate cream from it.
    (f)
    Settling of sand when a mixture of sand and water is left undisturbed
    for some time.
    (g)
    Fine beam of light entering through a small hole in a dark room,
    illuminates the particles in its paths.

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    NCERT’s answer
    (a)
    Sublimation (f) Sedimentation (b) Diffusion (g) Scattering of light (Tyndall effect) (c) Dissolution/diffusion (d) Evaporation, diffusion (e) Centrifugation
    (a) Sublimation. (b) Diffusion. (c) Dissolution — forms a solution. (d) Evaporation and diffusion. (e) Centrifugation. (f) Sedimentation. (g) Tyndall effect — scattering of light by suspended dust.Answer: (a) Sublimation (b) Diffusion (c) Dissolution (d) Evaporation, diffusion (e) Centrifugation (f) Sedimentation (g) Tyndall effect (scattering of light by dust).

  2. Exercise 21

    You are given two samples of water labelled as ‘A’ and ‘B’. Sample ‘A’ boils at 100\displaystyle 100°C and sample ‘B’ boils at 102\displaystyle 102°C. Which sample of water will not freeze at 0\displaystyle 0°C? Comment.

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    NCERT’s answer
    Sample ‘B’ will not freeze at $\displaystyle 0$°C because it is not pure water. At $\displaystyle 1$ atm, the boiling point of pure water is $\displaystyle 100$°C and the freezing point of pure water is $\displaystyle 0$°C.
    A dissolved impurity raises a liquid's boiling point and lowers its freezing point: \[\text{impurity present} \;\Rightarrow\; T_b\uparrow,\ T_f\downarrow \] Sample A boils at \(\displaystyle 100^\circ\text{C}\), the value for pure water, so it freezes normally at \(\displaystyle 0^\circ\text{C}\). Sample B boils at \(\displaystyle 102^\circ\text{C}\), so it carries a dissolved impurity, and its freezing point lies below \(\displaystyle 0^\circ\text{C}\).Answer: Sample B — it carries a dissolved impurity, so it freezes below \(\displaystyle 0^\circ\text{C}\).

  3. Exercise 22

    What are the favourable qualities given to gold when it is alloyed with copper or silver for the purpose of making ornaments?

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    NCERT’s answer
    Pure gold is very soft as compared to gold alloyed with silver or copper. Thus for providing strength to gold, it is alloyed.
    Pure ($\displaystyle 24$-carat) gold is too soft to hold its shape in fine ornaments. \[\text{Au (soft)} + \text{Cu or Ag} \;\Rightarrow\; \text{hardness}\uparrow,\ \text{durability}\uparrow \] The alloy resists wear and scratching while keeping gold's lustre, so a design holds its shape and shine far longer.Answer: Alloying with copper or silver makes gold harder and more durable, so ornaments keep their shape and resist wear while keeping gold's lustre.

  4. Exercise 23

    An element is sonorous and highly ductile. Under which category would you classify this element? What other characteristics do you expect the element to possess?

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    NCERT’s answer
    This element is a metal. Other characteristics expected to be possessed by the element are-lustre, malleability, heat and electrical conductivity.
    Sonorous and ductile are metallic properties: \[\text{sonorous + ductile} \;\Rightarrow\; \text{metal} \] It should also be malleable, lustrous, a good conductor of heat and electricity, and form basic oxides; most metals are solid with a high melting point, though exceptions exist (mercury is liquid).Answer: A metal; expect it to also be malleable, lustrous and a good conductor of heat and electricity.

  5. Exercise 24

    Give an example each for the mixture having the following characteristics.
    Suggest a suitable method to separate the components of these mixtures
    (a)
    A volatile and a non-volatile component.
    (b)
    Two volatile components with appreciable difference in boiling points.
    (c)
    Two immiscible liquids.
    (d)
    One of the components changes directly from solid to gaseous state.
    (e)
    Two or more coloured constituents soluble in some solvent.

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    NCERT’s answer
    (a)
    Evaporation or distillation (b) Distillation (c) Separation by using separating funnel (d) Sublimation (e) Chromatography
    (a) Salt in water — evaporation. (b) Acetone and water, \(\displaystyle \Delta T_b>25\,\text{K}\) — simple distillation. (c) Oil and water — separating funnel. (d) A mixture of \(\displaystyle \text{NH}_4\text{Cl}\) and \(\displaystyle \text{NaCl}\): \[\text{NH}_4\text{Cl(s)} \xrightarrow{\Delta} \text{NH}_3(g) + \text{HCl}(g) \xrightarrow{\text{cool}} \text{NH}_4\text{Cl(s)} \] Sublimation separates out the \(\displaystyle \text{NH}_4\text{Cl}\), leaving \(\displaystyle \text{NaCl}\) behind. (e) Dyes in ink — paper chromatography.Answer: (a) evaporation (b) distillation (c) separating funnel (d) sublimation (e) chromatography.

  6. Exercise 25

    Fill in the blanks
    (a)
    A colloid is a ——— mixture and its components can be separated by
    the technique known as ———.
    (b)
    Ice, water and water vapour look different and display different ——
    properties but they are ——— the same.
    (c)
    A mixture of chloroform and water taken in a separating funnel is mixed
    and left undisturbed for some time. The upper layer in the separating
    funnel will be of——— and the lower layer will be that of ———.
    (d)
    A mixture of two or more miscible liquids, for which the difference in
    the boiling points is less than 25\displaystyle 25 K can be separated by the process
    called———.
    (e)
    When light is passed through water containing a few drops of milk, it
    shows a bluish tinge. This is due to the ——— of light by milk and the
    phenomenon is called ——— . This indicates that milk is a ———
    solution.

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    NCERT’s answer
    (a)
    heterogenous, centrifugation (b) physical, chemically (c) water, chloroform (hint- density of water is less than that of chloroform) (d) fractional distillation (e) scattering, Tyndall effect, colloidal
    (a) heterogeneous; centrifugation. (b) physical; chemically. (c) water — upper, less dense; chloroform — lower, denser (\(\displaystyle \rho\approx1.49\,\text{g cm}^{-3}\) against water's \(\displaystyle 1.00\,\text{g cm}^{-3}\)). (d) fractional distillation. (e) scattering; Tyndall effect; colloidal.Answer: (a) heterogeneous, centrifugation (b) physical, chemically (c) water above, chloroform below (d) fractional distillation (e) scattering, Tyndall effect, colloidal.

  7. Exercise 26

    Sucrose (sugar) crystals obtained from sugarcane and beetroot are mixed together. Will it be a pure substance or a mixture? Give reasons for the same.

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    NCERT’s answer
    It is a pure substance because chemical composition of sugar crystals is same irrespective of its source.
    Sucrose is one chemical compound, \(\displaystyle \text{C}_{12}\text{H}_{22}\text{O}_{11}\), whichever plant it comes from -- sugarcane and beetroot sucrose have the identical fixed composition. Mixing two samples of the same compound only gives more of that same compound, not a mixture.Answer: A pure substance -- both samples are the same compound, \(\displaystyle \text{C}_{12}\text{H}_{22}\text{O}_{11}\), with a fixed composition.

  8. Exercise 27

    Give some examples of Tyndall effect observed in your surroundings?

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    NCERT’s answer
    Hint—Tyndall effect can be seen when light passes through a heterogeneous mixture. Example, when sunlight passes through the canopy of a dense forest.
    Sunlight scattered by dust as it slants through a forest canopy.
    A torch or headlight beam scattered by fog or smoke.
    A beam of light entering a dark room through a small hole.
    Answer: Forest-canopy sunbeams, fog/smoke-scattered torch beams, and a light beam through a hole in a dark room.

  9. Exercise 28

    Can we separate alcohol dissolved in water by using a separating funnel? If yes, then describe the procedure. If not, explain.

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    NCERT’s answer
    Hint— Water and alcohol are miscible.
    No. Ethanol and water are completely miscible, so they never form two separate layers for a separating funnel to exploit -- that technique only works on two immiscible liquids. This pair is separated instead by fractional distillation.Answer: No -- alcohol and water are miscible and form no separate layers; use fractional distillation instead.

  10. Exercise 29

    On heating calcium carbonate gets converted into calcium oxide and carbon
    dioxide.
    (a)
    Is this a physical or a chemical change?
    (b)
    Can you prepare one acidic and one basic solution by using the
    products formed in the above process? If so, write the chemical equation
    involved.

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    NCERT’s answer
    (a)
    Chemical change (b) Acidic and basic solutions can be prepared by dissolving the products of the above process in water CaO+\(\displaystyle H_{2}\)O →Ca(OH)$\displaystyle 2$ (basic solution) \(\displaystyle CO_{2}\) + \(\displaystyle H_{2}\)O→\(\displaystyle H_{2}\)CO $\displaystyle 3$ (acidic solution)
    (a)
    Chemical change: new substances, calcium oxide and carbon dioxide, form and the reaction cannot be reversed by physical means.
    \[\text{CaCO}_3 \xrightarrow{\Delta} \text{CaO} + \text{CO}_2 \]
    (b)
    Yes.
    \[\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \quad \text{(basic solution)} \]
    \[\text{CO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{CO}_3 \quad \text{(acidic solution)} \]
    Answer: (a) Chemical change. (b) Yes -- \(\displaystyle \text{Ca(OH)}_2\) (basic) and \(\displaystyle \text{H}_2\text{CO}_3\) (acidic).

  11. Exercise 30

    Non metals are usually poor conductors of heat and electricity. They are
    non-lustrous, non-sonorous, non-malleable and are coloured.
    (a)
    Name a lustrous non-metal.
    (b)
    Name a non-metal which exists as a liquid at room temperature.
    (c)
    The allotropic form of a non-metal is a good conductor of electricity.
    Name the allotrope.
    (d)
    Name a non-metal which is known to form the largest number of
    compounds.
    (e)
    Name a non-metal other than carbon which shows allotropy.
    (f)
    Name a non-metal which is required for combustion.

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    NCERT’s answer
    (a)
    Iodine (b) Bromine (c) Graphite (d) Carbon (e) Sulphur, phosphorus (f) Oxygen
    (a)
    Iodine.
    (b)
    Bromine -- the only liquid non-metal.
    (c)
    Graphite.
    (d)
    Carbon.
    (e)
    Sulphur (also phosphorus).
    (f)
    Oxygen.
    Answer: (a) Iodine (b) Bromine (c) Graphite (d) Carbon (e) Sulphur (f) Oxygen.

  12. Exercise 31

    Classify the substances given in Fig. 2.2\displaystyle 2.2 into elements and compounds NCERT_Question_Class9_Science_Exemplar_Ch2_Q31

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    NCERT’s answer
    Elements Compounds Cu \(\displaystyle CaCO_{3}\) Zn \(\displaystyle H_{2}\)O \(\displaystyle F_{2}\) \(\displaystyle O_{2}\) Diamond (carbon) Hg
    Elements: \(\displaystyle \text{Cu}\), \(\displaystyle \text{Zn}\), \(\displaystyle \text{O}_2\), \(\displaystyle \text{F}_2\), \(\displaystyle \text{Hg}\), and diamond (a form of the element carbon). Compounds: \(\displaystyle \text{H}_2\text{O}\) and \(\displaystyle \text{CaCO}_3\). Sand, \(\displaystyle \text{NaCl(aq)}\) and wood are mixtures, not pure substances, so neither list takes them. Answer: Elements: \(\displaystyle \text{Cu}\), \(\displaystyle \text{Zn}\), \(\displaystyle \text{O}_2\), \(\displaystyle \text{F}_2\), \(\displaystyle \text{Hg}\), diamond; Compounds: \(\displaystyle \text{H}_2\text{O}\), \(\displaystyle \text{CaCO}_3\).

  13. Exercise 32

    Which of the following are not compounds?
    (a)
    Chlorine gas
    (b)
    Potassium chloride
    (c)
    Iron
    (d)
    Iron sulphide
    (e)
    Aluminium
    (f)
    Iodine
    (g)
    Carbon
    (h)
    Carbon monoxide
    (i)
    Sulphur powder

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    NCERT’s answer
    Chlorine gas, Iron, Aluminium, Iodine, Carbon, Sulphur powder.
    Elements: (a) \(\displaystyle \text{Cl}_2\), (c) \(\displaystyle \text{Fe}\), (e) \(\displaystyle \text{Al}\), (f) \(\displaystyle \text{I}_2\), (g) \(\displaystyle \text{C}\), (i) \(\displaystyle \text{S}\). Compounds: (b) \(\displaystyle \text{KCl}\), (d) \(\displaystyle \text{FeS}\), (h) \(\displaystyle \text{CO}\).Answer: (a), (c), (e), (f), (g), (i) are not compounds -- each contains only one kind of atom.