100 Important Scientific Reasons: Equatorial Forests Appear Evergreen and Petroleum Often Called Black Gold

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51) Why do equatorial forests appear evergreen?

Ans: In equatorial forests trees often shed a few leaves or shed their leaves seasonally, but most of the trees retain their leaves for most of the time so that the forests appear evergreen.

52) Why is petroleum often called black gold?

Ans:

  • Petroleum is often called black gold because of its great demand in the modern industry and for domestic use.

  • It provides fuel for heat and lighting, lubricant for machinery and raw material for a number of industries.

53) Why are the kangaroos called marsupials?

Ans: Marsupial means broad-pouch. Since kangaroos have a pouch-like fold of skin near the stomach in which they carry their young ones, they are called marsupials.

54) Why is platypus considered a strange animal?

Ans:

  • Platypus is a strange animal because it is an animal-bird that survives under water, walks on the ground and digs tunnel under the ground.

  • It is a four-legged animal that lays eggs like a bird. It is found in Australia.

55) Why is a person in moving vehicle thrown forward when the vehicle stops suddenly?

Ans: A person in a moving vehicle is in a state of motion. When the vehicle suddenly stops his body tends to remain in a state of motion due to inertia and he is thrown forward.

56) Earth is continuously pulling moon towards its centre. Why does not the moon fall on to the earth?

Ans: It is so because the gravitational attraction of the earth provides the necessary centripetal force to the moon for its orbital motion around the earth due to which the moon is revolving around the earth.

57) Which of the two-glass or rubber is more elastic and why?

Ans: Glass is more elastic than rubber because for a given applied force per unit area, the strain produced in glass is much more than that produced in rubber.

58) Animal like camel can easily walk in the desert sand while other animals like donkeys, dogs and horses cannot. Explain.

Ans:

  • The camel has very broad and large feet. As a result of the large surface area in contact with the ground, it exerts less pressure on the sand and sinks only slightly in it.

  • Other animals have smaller feet which exert more pressure on the sand. As a result, they sink more in the sand and cannot walk easily in desert.

59) Why does a small quantity of liquid assume spherical form?

Ans:

  • A small quantity of liquid assumes a spherical form due to surface tension which tends to reduce the surface area.

  • A given mass will acquire minimum surface area if it assumes a spherical shape.

60) Why does an iron needle float on clean water but sink when some detergent is added to this water?

Ans:

  • Due to surface tension, the free surface of liquid at rest behaves like a stretched membrane. When an iron needle floats on the surface of clean water, its weight is supported by the stretched membrane.

  • When some detergent is added to this water, its surface tension decreases. As a result of it, the stretched membrane on the surface of water is weakened and is not able to support the weight of needle. Hence the needle sinks in such water.

61) Why is cooking quicker in a pressure cooker?

Ans: The boiling point of water depends upon the pressure on its surface. Steam produced inside the cooker builds up pressure thereby raising the boiling point of water, which results in quick cooking.

62) Why does steam cause more severe burns than boiling water?

Ans:

  • The amount of heat (latent heat) possessed by steam is much greater than the amount of heat possessed by water at the same temperature.

  • Therefore, steam causes more severe burns than boiling water.

63) Why does ice not melt readily when salt is sprinkled over it?

Ans:

  • When salt is sprinkled over ice, some of it dissolves. As dissolution of the salt is accompanied by absorption oh heat.

  • The temperature of the system falls below 0 degree Celsius. Hence, ice does not melt readily.

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