Question

Water has maximum density at:
(A) Room temperature
(B) ${4^ \circ }C$
(C) ${0^ \circ }C$
(D) $ - {4^ \circ }C$

Answer 1

Hint: In ice the water molecules are present in crystal structure which has large empty space to increase its volume. Also remember that density is inversely proportional to volume (with increase in volume, density decreases and vice-versa).

Complete step by step solution:
-When water is present in its ice form the water molecules are present in its crystal structure, which has a lot of empty space (voids). Due to these spaces the volume of ice is larger.
-When the ice starts to melt into liquid water the crystal structure starts to collapse, causing the volume to decrease and so the density of the liquid increases.
-At a temperature of ${4^ \circ }C$ water still contains many ice-like crystal structures. These crystals are free to slide along each other and move relative to each other. This movement of crystals helps in closing the spaces or voids present and so the volume of water decreases.

    $density = \dfrac{{mass}}{{volume}}$ And so $density \alpha \dfrac{1}{{volume}}$

Since volume is inversely proportional to density: with decrease in volume of water, its density increases.
-But when temperature rises to more than ${4^ \circ }C$ the motion of the water molecules starts increasing and thus starts requiring more extra space for movement than was required by the crystal structure of ice. Due to increased movement the molecules start to move away from each other, thus increasing the volume. As volume increases, the density decreases.
-So, we conclude that the density of water is maximum at ${4^ \circ }C$.

So, the correct option is: (B) ${4^ \circ }C$

Note: When the temperature is ${0^ \circ }C$ the volume is large so density is low but as temperature increases the density also increases (volume decreases). This occurs only upto ${4^ \circ }C$. After this temperature volume again starts to increase (due to increased molecular movement) so density again decreases.