As liquid water is cooled from 4 degrees Celsius down to 0 degrees Celsius, how does its density change?

Introduction / Context:
This physics and general science question tests your understanding of the unusual behaviour of water as it cools near its freezing point. Water has an anomaly where its density changes in a way that is different from many other substances, and this property has important environmental consequences, especially in lakes and ponds during winter.

Given Data / Assumptions:

• The temperature range considered is from 4 degrees Celsius down to 0 degrees Celsius.
• Density refers to mass per unit volume of liquid water.
• The question assumes standard atmospheric pressure and pure water conditions.

Concept / Approach:
Most substances become denser as they cool and less dense as they warm. Water behaves normally above about 4 degrees Celsius, but between 4 degrees Celsius and 0 degrees Celsius it behaves anomalously. At around 4 degrees Celsius, water reaches its maximum density. As it cools further from 4 degrees Celsius to 0 degrees Celsius, its density decreases instead of increasing. This is due to the formation of a more open hydrogen bonded structure as it approaches the crystalline arrangement of ice. As a result, water near the freezing point expands slightly and becomes less dense, which is why ice floats on liquid water.

Step-by-Step Solution:

Verification / Alternative check:
To verify, you can consult a table or graph of water density versus temperature in a physics or chemistry reference. Such tables show that at 4 degrees Celsius the density of water is at a maximum. As the temperature falls to 0 degrees Celsius, the density values become slightly smaller, confirming the decrease. Observations from nature also support this: in a lake during winter, water at 4 degrees Celsius tends to sink to the bottom, while colder, slightly less dense water and ice remain nearer the surface, allowing aquatic life to survive underneath.

Why Other Options Are Wrong:

Increases is incorrect because water has already reached its maximum density at around 4 degrees Celsius; further cooling toward 0 degrees Celsius does not make it denser.

Remains the same is incorrect because careful measurements show a small but real change in density in this temperature interval.

Becomes zero is incorrect because density cannot realistically become zero for a liquid in this context; such a value would imply no mass in a given volume, which is not physically meaningful for water.

Common Pitfalls:
A common mistake is to assume that water behaves like most other substances and becomes steadily denser as it cools. Students may remember that cooling usually reduces volume and increases density, and apply this rule without considering exceptions. Water is a classic example of an anomalous substance that requires special attention. Remember that this anomaly is why ice floats and why lakes freeze from the top downward, a fact often highlighted in school level science courses.

Final Answer:
As liquid water is cooled from 4 degrees Celsius down to 0 degrees Celsius, its density Decreases slightly due to the formation of a more open molecular structure.