Water has its maximum density and minimum volume at __________?

Most substances contract as they are cooled and expand when heated. However, water behaves differently between 0 °C and 4 °C. This behavior is known as the anomalous expansion of water.

When water cools from higher temperatures, it follows the normal rule and contracts. But once its temperature drops below 4 °C, water starts expanding again instead of contracting. As a result:

• 
  

  At 4 °C, water has its maximum density.

  
  


• 
  

  Below 4 °C, the density decreases again as volume increases.

  
  


• 
  

  At 0 °C, water freezes into ice, which is even less dense than liquid water, which is why ice floats.

  
  

This property is extremely important for nature and life:

1. 
   

   Survival of Aquatic Life
   In cold climates, when air temperature drops below freezing, lakes and ponds freeze from the top down. Since water is densest at 4 °C, heavier water sinks to the bottom while lighter ice floats on top, forming an insulating layer. This allows fish and other organisms to survive in liquid water underneath during winter.

   
   


2. 
   

   Climate and Environment
   The anomaly helps regulate the Earth’s climate. Oceans and large bodies of water absorb heat during summer and release it slowly during winter, moderating temperature variations.

   
   


3. 
   

   Engineering and Physics Importance
   Engineers and scientists must consider this property in designing water-based systems in colder regions. It explains phenomena like water pipes bursting in freezing temperatures due to expansion when water turns to ice.

   
   

To analyze the given options:

• 
  

  0 °C → water is about to freeze, not maximum density.

  
  


• 
  

  4 °C → correct, maximum density and minimum volume.

  
  


• 
  

  100 °C → water boils, not relevant.

  
  


• 
  

  None of these → incorrect.

  
  

Thus, water has its maximum density at 4 °C (Option B), making this a fundamental principle in physics and environmental science.