MCQ Detailed View

Water is a unique substance because its behavior when cooled is unlike most other liquids. Normally, liquids become denser as their temperature decreases, meaning their molecules come closer together. However, water has maximum density at 4°C and not at its freezing point (0°C). This unusual property is essential for life on Earth and has important implications in physics, chemistry, and environmental science.

As water cools from room temperature, it becomes denser until it reaches 4°C. Below this point, something unusual happens: water molecules start forming a more open, hexagonal crystalline structure as they prepare to freeze. This structure takes up more space, making ice less dense than liquid water, which is why ice floats on water.

The density of water at 4°C is approximately 1 g/cm³, which is considered its maximum density. At 0°C, as water turns to ice, the density drops to about 0.917 g/cm³, causing frozen water to occupy more volume than liquid water.

This behavior has a critical role in nature. In lakes and rivers during winter, the surface water cools and sinks until it reaches 4°C. Below this temperature, the water becomes lighter and stays on top, eventually freezing and forming a layer of ice. This layer insulates the water below, allowing aquatic life to survive even in freezing conditions.

The other options are scientifically incorrect:

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  0°C is the freezing point, not the point of maximum density.

  
  


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  4°K (Kelvin) is extremely cold (−269°C) and unrelated to water density.

  
  


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  4.8°C is not an accurate physical value for maximum density.

  
  

Thus, 4°C is the correct temperature at which water has its maximum density, an essential property for understanding thermodynamics and environmental balance in ecosystems.