Understanding Water Density from 0°C to 4°C

The question asks how the density of water changes when it is heated specifically from 0°C to 4°C. Water exhibits an unusual property in this temperature range, which is different from the behavior of most other substances.

Anomalous Expansion of Water

Most substances expand when heated and contract when cooled. This means their density usually decreases upon heating and increases upon cooling. However, water behaves differently between 0°C and 4°C.

• When water is heated from 0°C to 4°C, it actually contracts instead of expanding.
• Contraction means that the same mass occupies a smaller volume.
• Since density is mass divided by volume (\(\text{density} = \frac{\text{mass}}{\text{volume}}\)), if the mass remains constant and the volume decreases, the density must increase.

Water Density Behavior in Detail

Let's look at the density behavior of water around this critical temperature range:

• At 0°C, water exists in liquid form (just above freezing).
• As water is heated from 0°C to 4°C, its density increases.
• Water reaches its maximum density at approximately 4°C.
• Above 4°C, water behaves like most other substances; its density decreases as the temperature increases due to thermal expansion.

This unusual behavior between 0°C and 4°C is known as the anomalous expansion of water. It is crucial for aquatic life in cold climates, as it causes ice to form on the surface of lakes and rivers while denser, warmer water (at 4°C) sinks to the bottom, preventing complete freezing.

Analyzing the Options

Considering the behavior of water density from 0°C to 4°C:

• remains constant: This is incorrect, as density changes with temperature.
• increases: This aligns with the anomalous behavior of water between 0°C and 4°C, where it contracts and density increases.
• decreases: This is incorrect for the range 0°C to 4°C; density decreases when water is heated *above* 4°C or cooled *below* 4°C.
• first increases then decreases to its original value: This describes a different process or temperature range, not the specific range of 0°C to 4°C. In the range 0°C to 4°C, density only increases.

Therefore, when water is heated from 0°C to 4°C, its density increases.

Conclusion on Water Density Change

The unique property of water means its density rises as it is heated from 0°C up to 4°C. This is counter-intuitive compared to most materials but is a fundamental characteristic of water.

Revision Table: Water Properties

Additional Information: Importance of Water's Density Anomaly

The anomalous expansion of water is incredibly important, especially for aquatic ecosystems in cold environments:

• When the air gets cold, the surface water cools down.
• As surface water cools towards 4°C, its density increases, causing it to sink. Warmer, less dense water from below rises. This process continues, mixing the water until the entire body reaches 4°C.
• As the surface water cools from 4°C towards 0°C, its density decreases again. This colder, less dense water remains at the surface.
• When the surface water freezes at 0°C, ice forms. Ice is even less dense than liquid water at 0°C (approximately 917 kg/m\(^3\) vs. 999.8 kg/m\(^3\)). This is why ice floats.
• The floating layer of ice acts as an insulator, preventing the water below from freezing completely. This allows fish and other aquatic organisms to survive the winter in the unfrozen water at the bottom (which remains around 4°C).

Without this unique property, lakes and rivers would freeze from the bottom up, making survival much more difficult for aquatic life.