ICSE Class 7 Answered

A special kind of a bottle that is used to keep hot liquids hot and cold liquids cold for a longer time is called a thermos flask, vacuum bottle or Dewarflask 

The heat  loss by radiation is  prevented by the  silvered surface of the walls  and reflects back any radiant energy that tends to escape into the bottle. The  silvering  of  the  outer  surface  of  the  inner  wall  makes  it  a  poor  radiator  of  heat  while  the silvering of the inner surface of the outer wall makes it a good reflector of heat.