Question

mercury is very hot during the day and very cold at night

Answer 1

It is hot during the day (over 400°C ) because it is very close to the sun.At night mercury loses all its heat since there is almost no atmosphere to keep the warmth there the temperature can fall to almost -175°C

Answer 2

Mercury is very close to the sun comparatively to the earth, hence the Mercury gets very hot during the day and
As Mercury lacks an atmosphere so all the heat absorbed by the planet is radiated out...
Hope it helps you...