question_answer

  Give reason for the following observations. (a) Naphthalene balls disappear with time without leaving any solid. (b) We can get the smell of perfume sitting several metres away. (c) Water at room temperature is a liquid. (d) An iron almirah is a solid at room temperature. (e) Ice at 273 K is more effective in cooling than water at the same temperature. Or (a) Dry ice is compressed under high pressure. What happens to it when pressure is released? (b) Define (i) Melting point (ii) Fusion (c) Osmosis is a special kind of diffusion, Comment.

Answer:

  (a) Naphthalene undergoes sublimation easily i.e., the change of state of naphthalene from solid togas takes place easily. Thus, naphthalene balls disappear with time without leaving any solid.                                     (1) (b) Gaseous particles possess high speed and large spaces between them. Particles of perfume diffuse into these gaseous particles at a very fast rate and reach our nostrils. This enables us to smell the perfume from a distance.                                                                                                                                                                                                         (1) (c) At room temperature (25°C), water is a liquid because it has the following characteristic properties of liquid (i) At room temperature, water has no shape but has a fixed volume that is, it occupies the shape of the container in which it is kept. (ii) At room temperature, water flows.                                                                                                                                  (1) (d) An iron almirah is a solid at room temperature (25°C) because (i) it has a definite shape and volume like a solid at room temperature. (ii) it is rigid as solid at room temperature.                                                                                                                            (1) (e) At 273 K, when ice melts, it absorbs the energy equal to the latent heat of fusion from the surroundings, therefore causes cooling effect.                                                                                                                                                               (1) Or (a) On releasing the pressure, dry ice sublimes to vapour state without undergoing to liquid state.           (1) (b) (i) Melting point The definite temperature at which a solid starts melting is called the melting point of that solid, e.g., melting point of ice is 0°C or 273.16 K.                                                                                                           (1) (ii) Fusion The process of conversion of a solid into liquid state on heating is called fusion or melting.       (1) (c) Diffusion is the process in which molecules of a substance move from the place of (their) higher concentration to the place of lower concentration.                                                                                                       (1) But during osmosis, the water (or solvent) molecules   move   from   (their)   higher concentration to the place of their lower concentration through a semipermeable membrane. Thus, osmosis is termed as a special kind of diffusion.                                                                                                                                                                             (1)