Article about Anomalous behavior of water below 4°C
Each object expands (the volume of objects increases) when the temperature increases, and objects shrink (the volume of objects decreases) when the temperature decreases. Water also expands when the temperature increases and shrinks when the temperature decreases, but not at 0 oC – 4 oC. Between 0 °C and 4 °C, the volume of water decreases (water shrinks) as temperature increases. If we heat the water at 0 oC, the water gets hotter, the water volume decreases. The shrinkage process stops when the water reaches 4 oC. Above 4 oC, the volume of water increases as temperature increases. Instead, water expands (the volume of water increases) when the temperature decreases from 4 oC to 0 oC.
For example, we put water at 30 oC in the freezer. When in the freezer, the water temperature decreases. When the water temperature decreases, the volume of water also decreases (water shrinks). When it reaches 4 oC, the water begins to expand (the volume of water increases). Expansion of water stops when the temperature reaches 0 °C. Unlike other objects that continue to shrink (the volume of objects decreases) when the temperature of the object decreases to 0 oC. When objects shrink (volume of objects decreases), density increases. Conversely, when objects expand (the volume of objects increases), density decreases. Formula of density = mass / volume. The mass of objects is always fixed. While the volume can change, depending on the temperature. The water shrinks (the volume of water decreases) only to 4 oC, therefore, water has the highest density at 4 oC.
The benefits of water anomalies are felt in areas that have winter. When winter arrives, the air temperature decreases. Water surfaces in contact with air, so the water temperature on the surface of the river or lake surface is also reduced. Because the temperature of the water decreases, the volume of water also decreases. Because the volume of water decreases, the density of water increases. Water on the surface has a density that is greater than the water below. Because it has a greater density, the water on the surface moves down (sinks). The water below has a greater density, so it moves up (floating). This process occurs until the water temperature reaches 4 oC. Therefore, water has the highest density at 4 oC.
When the temperature of the water on the surface of the river or lake surface becomes smaller than 4 oC, then the water on the surface expands (the volume of water increases). Because as the volume of water increases, the density of water decreases. The density of water on the surface is smaller than the density of water in the bottom so that the water on the surface cannot fall. The water on the surface remained above and froze first as the temperature of the water decreases to 0 °C. Because of it in contact with the water on the surface, the water below the surface also begins to freeze as the water temperature decreases to 0 oC. So on. So, the freezing process starts from the water on the surface. Water at the bottom of the lake or river usually does not freeze because the air temperature is not freezing. Because the water at the bottom does not freeze, the fish and living things that are at the bottom of the lake or the river remain alive during the winter.
Water exhibits many anomalous behaviors due to its unique molecular structure and the nature of hydrogen bonding between its molecules. One of the most significant of these anomalies is the behavior of water’s density as it cools, especially when the temperature approaches the freezing point. Here’s a rundown of this phenomenon:
- Density and Temperature: Most substances become denser as they cool and less dense as they heat. This is because particles move closer together when cooled and farther apart when heated.
- Water’s Anomaly: Water, however, behaves differently near its freezing point. While it does become denser as it cools from room temperature, this trend reverses at about 4°C. Below 4°C, water becomes less dense as it gets colder, until it reaches 0°C and freezes.
- Ice Formation: Ice has a hexagonal lattice structure due to the orientation of hydrogen bonds. This structure makes ice less dense than liquid water. That’s why ice floats on water.
- Ecological Significance: This anomalous behavior is crucial for aquatic life. When surface water cools in winter, it becomes denser and sinks until the temperature reaches 4°C. Below this temperature, as the water continues to cool, it remains at the surface and eventually freezes. This results in ice forming on the surface while the water below remains liquid, allowing aquatic life to survive. If ice were denser than liquid water (like how most solids are denser than their liquid form), it would sink to the bottom, leading to the entire body of water freezing solid, which would be catastrophic for aquatic ecosystems.
- Cause of the Anomaly: The reason for this anomalous behavior lies in the hydrogen bonding between water molecules. As water cools towards 4°C, the hydrogen bonds adjust to hold water molecules in a more extended structure, making it denser. But below 4°C, the tendency for water to adopt the hexagonal structure that it has in ice becomes predominant. This structure is more open and less dense, leading to the observed decrease in density as water cools below 4°C.
Understanding this behavior of water is not just academically interesting—it’s vital for understanding many ecological and environmental processes.