At what temperature (in Celsius) does water have its maximum density?

Appeared in: DSSSB 6 September 2024

Explanation

Water has a unique property called anomalous expansion, where it does not behave like most other liquids upon cooling.
As liquid water cools, its density increases, but only until it reaches a temperature of 4°C.
At 4°C, water molecules are packed the closest together, resulting in its maximum density.
Below 4°C, as water approaches its freezing point, the molecules start forming an open hexagonal crystalline structure, which causes it to expand and become less dense.

Option A: At 0°C, water is at its freezing point. As it turns into ice, it becomes less dense than its liquid state, which is why ice floats.
Option C: At 100°C, water is at its boiling point and turns into steam (water vapor). Gases have a much lower density than liquids.
Option D: At -4°C, water is in its solid form (ice). Ice is less dense than liquid water at 4°C.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Anomalous expansion of water and its maximum density as background academic context rather than a clinical decision trigger.
This physical property has profound ecological significance. Because water at 4°C is densest, it sinks to the bottom of lakes and ponds in cold weather.
As the surface water cools further towards 0°C, it becomes less dense and eventually freezes, forming a layer of ice on top.
This surface ice insulates the water below, preventing the entire body of water from freezing solid and allowing aquatic life to survive the winter.

How to Approach the Question

Identify the core concept of the question: the relationship between water's temperature and its density.
This is a factual recall question based on a fundamental principle of physics/chemistry.
Recall the unique property of water known as 'anomalous expansion'.
Remember that unlike most substances that get denser as they cool, water reaches its maximum density at a temperature slightly above its freezing point.
Identify this specific temperature as 4°C.
Compare this fact with the given options and select the correct one.

Concept Tested & Keywords

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Practise the full DSSSB 6 September 2024

Attempt every question from this paper in a timed mock, then review the full solution for each one.