9th Class Science Sample Paper Sample Paper - 1 Term - 1

  • question_answer 31) Give reason for the following observations. (i) Water at room temperature is a liquid. (ii) Naphthalene balls disappear after sometime without leaving any solid. (iii) Sponge through compressible is a solid. (iv) We can get smell of agarbatti sitting several metres away. (v) Rate of evaporation of an aqueous solution decreases with increase in humidity. Or Distinguish between solids, liquids and gases  in a tabular form under the following characteristics. (i) Rigidity                   (ii) Force of attraction (iii) Compressibility  (iv) Intermolecular spaces (v) Kinetic energy of particles    

    Answer:

    (i) Water is liquid at room temperature because (a)Intermolecular spaces are very large. (b)Intermolecular forces of attraction between its molecules are less and kinetic energy is more. Hence, water molecules can interchange their spaces easily. So, it is liquid.                                                                          (1)           (ii) Naphthalene balls absorb heat energy from the surroundings and slowly sublime i.e., directly get converted into vapors from solid phase and disappear with time, without leaving any solid.                                                                       (1)        (iii) Sponge has minute holer in which air is trapped. On pressing it, the air is expelled out and we are able to compress it.                     (1)                  (iv) In air the molecules of agarbatti diffuse in all directions therefore, we can smell it from a large distance apart.(1)                      (v) Air around us cannot hold more than a definite amount of water vapour at a given temperature. So, with increased humidity air is already rich in water vapours, it cannot take up more water hence, rate of evaporation of water will decrease.                                   (1) OR Difference between solids, liquids and gases

    Characteristics Solids Liquids Gases
    Rigidity These are rigid and maintain their hardness and shape. These are fluids and flow easily. These are also fluid and capable of infinite of infinite expansion.
    Force of attraction Strongest. Which keep the particles together. Comparatively weaker but keep the particles within the bulk of liquid Extremely weak. Here particles are free to move in all direction.
    Compressibility Almost incompressible Relatively incompressible. Highly compressible
    Intermolecular spaces Approximately negligible. Lntermediate  between solid and gas. Very large.
    Kinetic energy of particles Very low Low High
     


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