Questions & Answers

At what temperature is the density of water at its greatest?
(A) ${{0}^{\circ }}C$
(B) ${{4}^{\circ }}C$
(C) ${{100}^{\circ }}C$
(D) ${{37}^{\circ }}C$

Answer Verified Verified
Hint: The highest density of water is observed at a temperature at which there is an equilibrium between the process of the collapsing of the lattice structure and that of the formation of it.

Complete answer:
- Let us analyse this phenomenon step-by-step to understand it a bit better so as to give the correct answer to this question.
- In ice, the water molecules are in a crystal lattice that has a lot of empty space.
- When the ice melts to its liquid state-water, its structure collapses and the density of the liquid increases.
- At temperatures well above freezing, the molecules move faster and get further apart. The density decreases as temperature increases.
- At temperatures near ${{0}^{\circ }}C$ , the water still contains many ice-like clusters. These clusters are free to move with respect to each other, so water is still in the liquid phase. The clusters still have empty space in its structure, so they decrease the density of the liquid.
- The molecules of the water are closer together, and this increases the density of the liquid.
-Upon cooling the water, the temperature of warm water decreases, the water molecules slow down and the density increases.
- At ${{4}^{\circ }}C$ , the clusters start forming, the molecules are still slowing down and coming closer together, but the formation of clusters makes the molecules be further apart.
- As cluster formation is the bigger effect, so the density starts to decrease. Thus, we can conclude that the density of water is a maximum at ${{4}^{\circ }}C$ .
- Therefore, the answer to this question is (B) ${{4}^{\circ }}C$ .

So, the correct answer is “Option B”.

Note: Do not blindly believe that as water is in solid state at ${{0}^{\circ }}C$, it will have the highest density at that temperature. Actually this solid state is less dense than the equilibrium state which involves formation and breakdown of clusters.