For pure water at standard atmospheric pressure, at which temperature in degree Celsius is its density maximum?

Introduction / Context:
This question is from basic physics and deals with an unusual but very important property of water. Most substances become denser as they cool, but water behaves differently near its freezing point. Its density reaches a maximum at a temperature slightly above the freezing point. This anomaly has significant implications for lakes, rivers and aquatic life in cold climates.

Given Data / Assumptions:

• The substance is pure water.
• Pressure is assumed to be approximately one atmosphere.
• The temperature range of interest is near the freezing point: from 0 degree Celsius up to about 12 degree Celsius.
• We want the temperature at which the density of water reaches its maximum value.

Concept / Approach:
Water exhibits anomalous expansion near 4 degree Celsius. As water is cooled from higher temperatures, its density increases up to about 4 degree Celsius. Below 4 degree Celsius, water actually expands as it approaches the freezing point, so its density decreases. This means the maximum density occurs at around 4 degree Celsius, not at 0 degree Celsius where ice begins to form. This anomaly is why cold lakes have warmer, denser water at the bottom during winter.

Step-by-Step Solution:
Step 1: Recall the general rule that cooling a liquid usually increases its density because the molecules move less and come closer. Step 2: Recognise that water follows this pattern down to around 4 degree Celsius. Step 3: Below about 4 degree Celsius, water behaves anomalously and begins to expand on further cooling. Step 4: Expansion means that the same mass occupies a larger volume, so the density decreases. Step 5: Therefore, the density reaches a maximum at the turning point of this behaviour, which is around 4 degree Celsius. Step 6: Check the options and see that 4 degree Celsius is one of them. Step 7: Conclude that the density of water is maximum at 4 degree Celsius.

Verification / Alternative check:
Temperature density tables for water show that at 0 degree Celsius, water and ice coexist and the density of liquid water is slightly less than at 4 degree Celsius. As temperature rises from 0 to 4 degree Celsius, density increases; above 4 degree Celsius, density decreases gradually again with further heating. This symmetric behaviour confirms that 4 degree Celsius is the point of maximum density. Observations of lakes in winter, where water at 4 degree Celsius sinks to the bottom, also support this property.

Why Other Options Are Wrong:
12 degree Celsius is warmer and water at this temperature is less dense than at 4 degree Celsius. 8 degree Celsius is still above the temperature of maximum density; density at 8 degree Celsius is lower than at 4 degree Celsius. 0 degree Celsius is the freezing point of water, but due to anomalous expansion, water at this temperature is less dense than at 4 degree Celsius.

Common Pitfalls:
Many students assume that the maximum density must occur at the freezing point or at the lowest temperature before freezing, but water is an exception. It is important to memorise that water has its maximum density at about 4 degree Celsius. This single fact explains why ice floats and why ponds freeze from the surface downwards, allowing fish to survive in liquid water below the surface ice during winter.

Final Answer:
The density of pure water is maximum at 4 degree Celsius.