At what temperature does pure water have its maximum density?

Introduction / Context:
Water has an unusual behaviour compared to most other liquids. Instead of continuously contracting as it cools, water reaches a maximum density at a temperature slightly above its freezing point. This anomaly is important for aquatic life and environmental science because it influences how lakes freeze in winter. This question asks you to recall the specific temperature at which pure water has its highest density.

Given Data / Assumptions:
- The substance is pure water under normal atmospheric pressure.
- The question concerns the temperature of maximum density in degrees Celsius.
- Options include 0 C, minus 15 C, 4 C and 15 C.

Concept / Approach:
Most liquids become denser as they cool, but water behaves differently between 0 C and 4 C. As water cools from room temperature, its density increases until it reaches a maximum at 4 C. Below 4 C down to 0 C, water actually becomes less dense. This is why ice, which forms at 0 C, floats on liquid water and why colder water stays on top of slightly warmer, denser water in lakes. The well-known special value is 4 C as the temperature of maximum density for pure water.

Step-by-Step Solution:
Step 1: Recall the anomalous expansion of water between 0 C and 4 C. Step 2: Note that as water cools from higher temperatures to 4 C, its density increases. Step 3: At 4 C, water reaches its maximum density. Step 4: As the temperature decreases further from 4 C to 0 C, water expands slightly and its density decreases. Step 5: Therefore, the temperature of maximum density is 4 C.

Verification / Alternative check:
Laboratory measurements of water density versus temperature show a peak at approximately 4 C. Environmental observations also support this: in cold climates, water at around 4 C sinks to the bottom of lakes, while colder water and ice remain at the surface. This allows aquatic organisms to survive beneath the ice layer, a direct consequence of water having maximum density at 4 C rather than at the freezing point itself.

Why Other Options Are Wrong:
0 C: This is the freezing point of water, but not the temperature of maximum density; ice is less dense than liquid water at 4 C.
-15 C: At this temperature, water would be solid ice and less dense than liquid water at 4 C.
15 C: At this higher temperature, water is less dense than at 4 C, because density has already started to decrease as temperature rises above 4 C.

Common Pitfalls:
Many learners assume that maximum density must occur right at the freezing point, which is true for many substances but not for water. It is easy to memorise 0 C for several properties of water and mistakenly apply it here. To avoid confusion, remember the special role of 4 C in freshwater lakes and rivers and associate this temperature specifically with maximum density of water.

Final Answer:
Pure water has its maximum density at 4 C.