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NCERT Exemplar · Class 9 Science Matter in Our Surroundings

27 questions · 27 still being checked

Long Answer Questions 23–27 (part 3 of 3)

  1. Exercise 23

    You are provided with a mixture of naphthalene and ammonium chloride by your teacher. Suggest an activity to separate them with well labelled diagram.

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    NCERT’s answer
    Hint— Naphthalene is insoluble in water but soluble in ether an organic solvent. It is volatile at room temperature. Ammonium chloride is soluble in water and volatile at higher temperature. It decomposes on heating to dryness.
    Add water to the mixture and stir; \(\displaystyle \text{NH}_4\text{Cl}\) is water-soluble, naphthalene is not. NCERT_Solution_Class9_Science_Exemplar_Ch1_Q23 Filter the mixture: naphthalene stays on the filter paper as residue, the \(\displaystyle \text{NH}_4\text{Cl}\) solution passes through as filtrate. Warm the filtrate gently in a china dish and stop short of dryness — strong heating carries \(\displaystyle \text{NH}_4\text{Cl}\) away as sublimate too. Cool the concentrated solution to crystallise out solid \(\displaystyle \text{NH}_4\text{Cl}\). Answer: Naphthalene is recovered as the filter residue, ammonium chloride as the crystallised filtrate.

  2. Exercise 24

    It is a hot summer day, Priyanshi and Ali are wearing cotton and nylon clothes respectively. Who do you think would be more comfortable and why?

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    NCERT’s answer
    Cotton being a better absorber of water than nylon helps in absorption of sweat followed by evaporation which leads to cooling. So Priyanshi is more comfortable, whereas Ali is not so comfortable.
    Cotton is a natural, absorbent fibre; nylon is synthetic and far less absorbent. Sweat soaks into cotton and spreads over a larger area of the cloth \(\displaystyle \Rightarrow\) more surface for evaporation \(\displaystyle \Rightarrow\) faster evaporation. Evaporation draws its latent heat from the body \(\displaystyle \Rightarrow\) the skin cools. Nylon barely absorbs sweat, so it stays pooled on the skin with little spreading, and evaporation \(\displaystyle \Rightarrow\) cooling is slower. Answer: Priyanshi (cotton) is more comfortable — cotton's absorbency speeds up sweat evaporation and body cooling.

  3. Exercise 25

    You want to wear your favourite shirt to a party, but the problem is that it is still wet after a wash. What steps would you take to dry it faster?

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    NCERT’s answer
    Conditions that can increase the rate of evaporation of water are (a) an increase of surface area by spreading the shirt (b) an increase in temperature by putting the shirt under the sun (c) increase the wind speed by spreading it under the fan.
    Spread the shirt out fully on a hanger, not bunched \(\displaystyle \Rightarrow\) more surface area \(\displaystyle \Rightarrow\) faster evaporation. Hang it in the sun or a warm spot \(\displaystyle \Rightarrow\) higher temperature \(\displaystyle \Rightarrow\) particles gain kinetic energy faster \(\displaystyle \Rightarrow\) faster evaporation. Hang it where air moves freely, or use a fan \(\displaystyle \Rightarrow\) moving air carries vapour away \(\displaystyle \Rightarrow\) faster evaporation. Avoid a closed, humid room \(\displaystyle \Rightarrow\) lower humidity \(\displaystyle \Rightarrow\) the air can take up more vapour \(\displaystyle \Rightarrow\) faster evaporation. Answer: Spread it out flat in a warm, windy, low-humidity spot — each factor raises the rate of evaporation.

  4. Exercise 26

    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.

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    NCERT’s answer
    (a)
    Evaporation produces cooling as the particles at the surface of the liquid gain energy from the surroundings and change into vapour thereby producing a cooling effect. (b) Air around us cannot hold more than a definite amount of water vapour at a given temperature which is known as humidity. So, if the air is already rich in water vapour, it will not take up more water therefore, rate of evaporation of water will decrease. (c) A sponge has minute holes in which air is trapped. Also the material is not rigid. When we press it, the air is expelled out and we are able to compress it.
    (a)
    True. Evaporating molecules carry away their latent heat from the remaining liquid/surroundings \(\displaystyle \Rightarrow\) the surroundings cool.
    (b)
    True. Humid air already holds more water vapour and is nearer saturation \(\displaystyle \Rightarrow\) it takes up further vapour more slowly \(\displaystyle \Rightarrow\) the evaporation rate falls.
    (c)
    True. A sponge's holes trap air that escapes on pressing, but the sponge material itself keeps a definite shape and volume and springs back — the hallmark of a solid, not a fluid.
    Answer: All three statements are correct, for the reasons above.

  5. Exercise 27

    Why does the temperature of a substance remain constant during its melting point or boiling point?

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    NCERT’s answer
    The temperature of a substance remains constant at its melting and boiling points untill all the substance melts or boils because, the heat supplied is continuously used up in changing the state of the substance by overcoming the forces of attraction between the particles. This heat energy absorbed without showing any rise in temperature is given the name latent heat of fusion/latent heat of vapourisation.
    At the melting or boiling point, the heat supplied overcomes inter-particle forces of attraction and drives the change of state (solid\(\displaystyle \to\)liquid or liquid\(\displaystyle \to\)vapour), instead of raising kinetic energy. \[Q = mL \] \(\displaystyle Q\) = heat absorbed, \(\displaystyle m\) = mass changing state, \(\displaystyle L\) = latent heat (of fusion or of vaporisation) — the heat goes entirely into \(\displaystyle L\), none into kinetic energy. Temperature depends only on average kinetic energy, so it cannot change until the state-change is complete. NCERT_Solution_Class9_Science_Exemplar_Ch1_Q27 Answer: Temperature stays constant because the heat supplied is used as latent heat to change the state, not to raise the particles' kinetic energy.