For any substance, why does the temperature remain constant during the change of state Class 9

Text Solution

Solution : For any substance, temperature ramians constant during the change of its state because the heat supplied during this state is used up in over coming the forces of attraction between the particles. This heat is known as the latent heat.

• Answer:

  The temperature remains constant during the change of state because the heat supplied during the change is used up in overcoming the intermolecular forces between the particles of the state.
  

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Page 2

• Answer:

  In order to liquefy a gas, the constituent particles or molecules have to be brought closer. The atmospheric gases can be liquefied either by increasing pressure or by decreasing temperature.
  

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Page 3

• Answer:

  The cooling in a desert cooler is caused by the evaporation of water. A desert cooler cools better on a hot and dry day because the higher temperature on a hot day increases the rate of evaporation of water and the dryness of air (low humidity of air) also increases the rate of evaporation of water. And due to increased rate of evaporation of water, a desert cooler cools better on a hot and dry day.
  

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• Answer:

  An earthen pot (matka) has many small pores. Water seeps out through them and evaporates from the surface of the pot. The energy needed for evaporation is taken from the water kept in the earthen pot. As a result, water kept in earthen pot becomes cool.
  

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• Answer:

  Both acetone and perfume are low boiling liquids. When they are poured on the palm, they evaporate readily and for this change of state they take the energy from the palm and we get a cooling sensation.
  

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• Answer:

  We are able to sip hot tea faster from a saucer rather than a cup because a saucer has a greater surface area. As a result, rate of evaporation increases.
  

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• Answer:

  We should wear cotton clothes in summer. During summer, we perspire more because of the mechanism of our body which keep us cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it to the atmosphere for easy evaporation.   
  

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• Answer:

  (a) We know that, temperature in °C = K - 273, where, C = °Celsius, K = Kelvin                                                                = 293 - 273 = 20°C                            (b) °C = K - 273 = 470 - 273 = 197°C                                              
  

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Page 9

• Answer:

  (a) We know that temperature in K = °C + 273                                                                                          K = 25° + 273 = 298 K                                                           (b)   K = °C + 273 = 373 + 273 = 646 K                                               
  

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Page 10

• Answer:

  (a): Naphthalene balls disappear with time without leaving any solid because they undergo sublimation i.e., they directly change into vapour without passing through the liquid state.   (b): We can get the smell of perfume sitting several metres away due to diffusion. The perfumes contain solvent which carries pleasant smelling vapour. They diffuse quite fast and can reach a person sitting several metres away.  
  

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Page 11

• Answer:

  The forces of attraction are the strongest in solids, followed by liquids and the weakest in gases. Oxygen is a gas, water is a liquid and sugar is a crystalline solid. So the increasing order of forces of attraction is Oxygen < Water < Sugar.                                 
  

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Page 12

• Answer:

  (a) 25°C, the physical state of water is a liquid. (b) 0°C, the physical state of water can be either a solid or a liquid. (c) 100°C, the physical state of water can be either a liquid or a gas (steam).  
  

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• Answer:

  (a) Water is a liquid at room temperature due to following reasons :                   (i) Water has not fixed shape but fixed volume. It takes the shape of the container. (ii) Water can flow easily hence it is not rigid but a fluid. (b) An iron almirah is a solid at room temperature due to following reasons : (i) An almirah has a fixed shape and a fixed volume.                                 (ii) It cannot flow, hence it is rigid.                                                   
  

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• Answer:

  Cooling takes place when heat is removed from a system. In case of ice at 273 K, it will take latent heat from the medium to convert itself into water at 273 K and then into water at a higher temperature. Thus, in case of ice at 273 K there will be a change in physical state, whereas in case of water at 273 K there will be no change in state. Hence lesser energy will be taken from the medium.
  

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Page 15

• Answer:

  Steam contains more heat, in the form of latent heat, than boiling water. So when steam comes in contact with skin it gives out 22.5 X 105 joules per kilogram more heat than boiling water, so steam causes more severe burns.
  

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Page 16

• Answer:

   A → Fusion or melting, B → Vaporization, C → Condensation, D →  Solidification, E → Sublimation, F → Sublimation
  

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Page 17

• Answer:

  (c) Evaporation, diffusion and expansion of gases increase on raising the temperature. Evaporation rate increases because on increasing temperature, kinetic energy of molecules increases, so the molecules present at the surface of the liquid leave the surface quickly and go into the vapour state. Diffusion and expansion of gases also increase as the molecules move more rapidly and try to occupy more space.
  

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Page 18

• Answer:

    (c) Low temperature and high pressure are required to liquefy gases to liquids. There is a lot of space between the particles of a gas. On applying high pressure, the particles of gas move get so close that they start attracting each other sufficiently forming a liquid. When gas is compressed too much, heat is produced, so it is necessary to cool it. Cooling lowers the temperature of compressed gas and helps in liquefying it, Hence, a gas can be liquefied by applying high pressure and lowering the temperature (cooling).
  

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Page 19

• Answer:

    (c) Gases and liquids behave like fluids. Both gases and liquids tend to flow due to less force of attraction between their particles. Also, they require vessel to keep them. Solids do not flow.
  

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• Answer:

    (d) During summer, water kept in an earthen pot becomes cool because of the phenomenon of evaporation. Earthen pot has a large number of tiny pores in its walls and some of the water molecules continuously keep seeping through these pores to outside the pot. This water evaporates continuously and take the latent heat required for vaporization from the remaining water. In this way, the remaining water loses heat and gets cooled.
  

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• Answer:

    (c) The correct order of increasing 'force of attraction' between their particles is Oxygen < Water < Sugar It is because the force of attraction increases in the order i.e., Gas < Liquid < Solid.
  

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• Answer:

    (d) Therefore, the correct sequence of temperature will be 298 K, 311 K and 339 K. On converting 25°C, 38°C and 66°C, to kelvin scale, we get the following temperatures 25°C+ 273 = 298 K

  
  38°C + 273 = 311 K
  
   66°C + 273 = 339 K Therefore, the correct sequence of temperature will be 298 K, 311 K and 339 K.
  

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Page 23

• Answer:

    (b) Conversion of solid into vapours on heating or vapours into solid on cooling without undergoing in liquid state is called sublimation. The conversion of liquid into gas (vapour) is called vaporization. The conversion of liquid into solid is called freezing. The conversion of solid into liquid is called melting.
  

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Page 24

• Answer:

    (c) The correct order of boiling points of diethyl ether, acetone and n-butyl alcohol in kelvin scale is 308 K, 329 K and 391 K, which can be explained as (

  
  PC + 273 = TK). Boiling point of diethyl ether=35°C+273=308 K Boiling point of acetone = 56°C + 273 = 329 K Boiling point of n-butyl alcohol = 118°C + 273 = 391 K
  

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Page 25

• Answer:

    (a) Increase in temperature of water will increase the evaporation of water. It is because, on increasing the temperature, kinetic energy of water molecules increases and more particles get enough kinetic energy to go into the vapour state. This increases the rate of evaporation. Evaporation is the surface phenomenon so it depends upon the exposed surface area. Higher the exposed surface area of water, higher will be the evaporation. When common salt is added to water then surface is occupied by the solvent as well as non-volatile solute particles. So, the escaping tendency of solvent particles decreases and thus the evaporation of water decreases. Therefore, other options (b), (c) and (d) will decrease the evaporation of water.
  

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Page 26

• Answer:

    (c) (ii) and (iii) are the correct options because In option (ii) hydrogen gas leaking from the container leaves some vacant space inside the container. So, hydrogen gas molecules inside the container occupy all the space available and the distance between the molecules of hydrogen gas will be increased. In option (iii) on increasing the volume of the container of hydrogen gas, more space will be available inside the container and hydrogen gas molecules will occupy all the space available and hence distance between the molecules will be increased. In option (i) on increasing pressure, hydrogen molecules will come closer and the distance between them will be decreased, In option (iv) more hydrogen gas molecules are available in less volume, so the distance between them will be decreased.
  

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