When pure water is heated, it does not increase in volume. In fact, water typically undergoes a decrease in volume when it is heated and transitions from its liquid state to its gaseous state (water vapor). This phenomenon is known as the expansion of water upon heating.
However, there is a particular temperature range where water does exhibit an increase in volume upon heating. This range is between 0 degrees Celsius (32 degrees Fahrenheit) and 4 degrees Celsius (39.2 degrees Fahrenheit). As water cools down from its liquid state at higher temperatures, it contracts and becomes denser. But as it approaches the freezing point at 0 degrees Celsius, a reversal occurs, and the water starts to expand.
The expansion of water near its freezing point is due to the unique behavior of its hydrogen bonds. Water molecules form a network of hydrogen bonds, which give water its many exceptional properties. As the temperature decreases, these hydrogen bonds rearrange themselves and become more structured, causing an increase in the space between water molecules and leading to expansion.
This expansion is the reason why ice (solid water) is less dense than liquid water. When water freezes, its volume increases significantly, which is why ice floats on water. This property is essential for aquatic ecosystems as it creates an insulating layer of ice on top of bodies of water, helping to protect the organisms living beneath it.
It's important to note that the expansion of water upon heating within the range of 0 to 4 degrees Celsius is an exception to the general behavior of water, which contracts when heated and expands when cooled.