Question

in winter, water tanks start freezing from the surface and not from the bottom explain

Answer 1

In winters the atmosperic temperature gets as low as 4 degree Celcius thus the atmospher is directly in contact with the surface of the water and the surface freezes and ice's volume being less than that of water remains on the surface and as ice is a bad conductor of heat it does not freezes at the bottom

Answer 2

Since ice is lighter than water ice is at the surface...