Question

Give reason density of the water is maximum at 4 degree celsius

Answer 1

due to the anomalous behaviour of water the density is maximum at 4 degree Celsius.... hope this helps

Answer 2

When Water is in its Solid Form (Ice) below 0° C, it is forced into a regular Cage like Structure with small voids due to which the Volume of Ice is larger.  

figure 1

At 4° C, the Cage like Structure of Ice starts to break and the molecules can slide along each other, closing the empty voids due to which momentarily the Volume of Water decreases. Due to decrease in Volume, Density increases because Density is inversely proportional to Volume.  

figure 2

As temperature rises to over 4° C, the extra space needed by increased motion of water molecules starts being larger than the space gained due to structural changes and the molecules start to move away from each other due to which the Volume again increases and Density decreases.

Thus, Density maximum is reached at 4°C.

Hope this helps