Question

why water has maximum density at 4 degree?

Answer 1

because 4degree celcius is it's freezing point

Answer 2

Freezing point of water is zero.
But the thing happens here is.....
the water molecules in ice are loosely packed which doesn't have that much of density
where as water which is above 4 degree also have water molecules which are loosely packed,
But exactly at 4 degree, it is neither to cool nor normal... hence molecules get attached closely which increases its mass and thus density would be also high