Wednesday , May 18 2022
NCERT 6th Class (CBSE) Science: Sorting Materials into Groups

9th Class (CBSE) Science: Matter in Our Surroundings

Question: Explain giving examples the various factors on which rate of evaporation depends.

Answer: The rate of evaporation depends on the following factors:

  1. Surface area: If the surface area is increased the rate of evaporation also increases.
    (a) To dry the clothes we spread them to dry faster.
    (b) Tea in saucer cools faster than in a cup.
  2. Temperature: If the temperature is increased the rate of evaporation also increases. Due to increase in temperature the particles gain more kinetic energy and change their phase from liquid to gaseous. Water will evaporate faster in sun than in shade.
  3. Humidity: It is the amount of water vapor present in air. The air can hold definite amount of water vapor, at a given temperature. If the amount of water vapor is high in the air then the rate of evaporation decreases. On hot and humid day, desert coolers are not effective as the air cannot hold any more moisture to get the cooling effect.
  4. Wind speed: With the increase in wind speed, the rate of evaporation increases. The particles of water vapor move away with the wind, decreasing the amount of water vapor in the surrounding.

Question: Adil parked his bicycle on a sunny day in a parking stand of his school campus. When the school got over Adil saw his burst cycle type. Thereafter he kept less air in his cycle types and did not inflate them fully.

  1. Why did the type burst?
  2. Why is air compressible?
  3. What value of Adil is reflected in the above act?


  1. The Tyre burst because the air inside the Tyre got heated and therefore exerted pressure on the walls of the Tyre.
  2.  Air is compressible because it has large inter molecular space.
  3. Adil showed the value of intelligence, awareness and self responsibility.

Question: Akshay’s friend visited his house in Mumbai and he was surprised to see air conditioners installed in all of his rooms. His friend advised Akshay to use water-coolers and save electricity. On this Akshay told, him that the water-cooler is not at all effective in coastal areas.

  1. Why are water-cooler not effective in coastal areas?
  2. What are the other two factors on which evaporation of water depends?
  3. What value of Akshay’s friend is seen in this act?


  1. Water coolers are not effective in coastal areas due to high rate of humidity.
  2. The other two factors on which evaporation of water depends are temperature and surface area.
  3. Akshay’s friend showed the value of concerned citizen, morally responsible and friendly in nature.

Question: Sita lived in a village and could, not afford refrigerator in her house. She knew how to keep water cold and preserve all perishable items in her house. She kept ivet cloth surrounding the earthen pot to keep water cool, she also kept vegetables fresh by keeping them in wet gunny bag and timely sprinkled water over it.

  1. Why did Sita keep wet cloth surrounding the earthen pot?
  2. Suggest one more method of keeping the house cool in summer.
  3. What value of Sita is reflected in the above case?


  1.  The wet cloth gave the cooling effect to the pot, as the water in the cloth evaporated and evaporation causes cooling effect.
  2. By sprinkling some water on the lawn/veranda of the house can keep the house cool.
  3. Sita showed the value of responsible behavior.

Question: Shreya commutes in a CNG fitted van to school every day along with many other students. She told the van driver to get the CNG connection certified and timely checked it for any leakage or loose connection of pipes. She told the driver to be more careful during summers.

  1. What is CNG?
  2. Why should one be more careful with CNG cylinders during summer?
  3. What value of Shreya is seen in the above act?


  1. CNG is Compressed Natural Gas used as fuel.
  2. During summers, the CNG connections and cylinder need to be checked because the gas expands due to heat and if there would be any leakage then it would cause fire in the vehicle.
  3. Shreya showed the value of concerned citizen and morally responsible behavior.

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