As pure water is cooled from 4 degrees Celsius down to 0 degrees Celsius, how does its density change in this temperature range?

Introduction / Context:
Water exhibits an unusual behaviour known as anomalous expansion near 4 degrees Celsius. Unlike most substances, whose density increases steadily as they are cooled, water reaches a maximum density at around 4 degrees Celsius and then becomes less dense as it is cooled further toward the freezing point. This question tests whether you know how the density of water changes when it is cooled from 4 degrees Celsius to 0 degrees Celsius.

Given Data / Assumptions:

• We are dealing with pure water at normal atmospheric pressure.
• Water has its maximum density at approximately 4 degrees Celsius.
• The temperature range considered is from 4 degrees Celsius down to 0 degrees Celsius.
• We are asked about the trend in density, not exact numerical values.

Concept / Approach:
As water is cooled from higher temperatures, its density increases until it reaches a maximum at around 4 degrees Celsius. Below this temperature, the structure of water begins to form more open hydrogen bonded clusters that resemble the structure of ice. These clusters take up more space, causing water to expand slightly as it cools from 4 degrees Celsius to 0 degrees Celsius. Because density is mass divided by volume, an increase in volume for the same mass means a decrease in density. This anomalous behaviour explains why ice floats on water and why lakes freeze from the top downward, which is important for aquatic life in cold climates.

Step-by-Step Solution:
Step 1: Recall that density is given by rho = mass / volume.Step 2: Note that water reaches its maximum density at about 4 degrees Celsius.Step 3: As water is cooled below 4 degrees Celsius toward 0 degrees Celsius, the formation of more open structures causes it to expand slightly.Step 4: This expansion means that for the same mass of water, the volume increases.Step 5: With mass constant and volume increasing, density must decrease according to rho = mass / volume.Step 6: Therefore, between 4 degrees Celsius and 0 degrees Celsius, the density of water decreases from its maximum value.

Verification / Alternative check:
Observationally, this behaviour explains why ice floats and why cold water tends to sink until it reaches about 4 degrees Celsius. In a lake, as the surface water cools, the densest water at around 4 degrees Celsius sinks to the bottom, and once the whole body reaches this temperature, further cooling of the surface produces less dense water that remains on top and eventually freezes. This pattern would not happen if density continued to increase all the way down to 0 degrees Celsius. Data tables of water density versus temperature confirm that the maximum occurs near 4 degrees Celsius and that density decreases as temperature approaches 0 degrees Celsius.

Why Other Options Are Wrong:
The statement that density increases continuously from 4 degrees Celsius to 0 degrees Celsius contradicts the known anomaly of water. Saying that density remains the same ignores measured variations and the physical expansion due to structural changes. The option that none of the above is correct is also wrong because one of the options accurately describes the behaviour. Only the statement that density decreases below its maximum at 4 degrees Celsius matches the observed properties of water.

Common Pitfalls:
Many learners assume that all substances behave like ideal solids and liquids where cooling always increases density. Others may remember that water is special but forget the specific temperature at which density is maximum. To avoid confusion, remember the key fact: water has its maximum density at about 4 degrees Celsius, and as you cool it further to 0 degrees Celsius, it expands slightly and its density decreases.

Final Answer:
As water is cooled from 4 degrees Celsius to 0 degrees Celsius, its density decreases below the maximum value it has at 4 degrees Celsius.