55) A vapour absorption refrigerator uses __________ as a refrigerant.

56) Thermal conductivity of air increases with rise in temperature.

57) The material of pipe lines for a system using ammonia as a refrigerant should be copper.

58) The wave length corresponding to the maximum energy is inversely proportional to the absolute temperature. This statement is called Planck's law.

59) Which of the following refrigerant has the highest freezing point.