![density of water at different temperatures density of water at different temperatures](https://hi-static.z-dn.net/files/df2/3b8fdaee8c610227fc08a6d13e9f8c06.jpg)
In principle, the lifetime of the individual clusters in this unstable state is usually only a few trillionths of a second. However, these clusters are not yet stable and dissolve after a short time (the hydrogen bonds are only stable when the water movement of the molecules is literally frozen). The clusters therefore form crystal-like structures of solid ice. With further cooling near the solidification point, the clusters become larger and larger and the hydrogen bonds become more stable or less unstable. If the temperature drops further below 3.98 ☌, the effect of the growing clusters exceeds the effect of thermal contraction and the water volume increases again. At this point, the water has the smallest volume or the highest density.
![density of water at different temperatures density of water at different temperatures](https://1.bp.blogspot.com/-IbLTS9u6r0Q/XPE0zTelEJI/AAAAAAAAMXU/r0SsLXniJ6wgzXjMqYNqvR43E3n9rZf-wCLcBGAs/s320/density%2Bof%2Bwater%2B-%2Btemperature%2Btable.jpg)
At 3.98 ☌, the effect of the growing clusters is as great as the effect of thermal contraction. Animation: Formation of clusters as cause of negative thermal expansion of water (density anomaly)Īs the temperature drops further, the volume of the clusters increases disproportionately and gradually compensates for the effect of thermal contraction. The latter effect dominates at first, so that water contracts as expected during cooling. At the same time, however, the individual clusters move closer together due to the effect of thermal contraction.
![density of water at different temperatures density of water at different temperatures](https://i.stack.imgur.com/KAOP2.png)
The clusters thus become larger and larger as the temperature drops, and the space required by the clusters increases accordingly. Figure: Formation of clusters as cause of negative thermal expansion of water (density anomaly) Conversely, the intensity of particle motion of the water molecules decreases with decreasing temperature, so that more water molecules can accumulate on clusters without being torn away immediately. At too high temperatures, the Brownian motion is so intense that the clusters are torn apart again due to the weak hydrogen bonds. However, such clusters can only form stable at relatively low temperatures. These are accumulations of water molecules that are held together by hydrogen bonds (H-bonds). The reason for negative thermal expansion of water are so-called clusters. Thus, the water under the ice layer usually remains liquid, allowing the fish to survive in winter. The density anomaly is crucial for life on earth! Among other things, the anomaly causes ice to form on the surface of a lake. Negative thermal expansion (density anomaly) refers to the paradoxical behavior of a substance to contract when heated instead of expanding! At 4 ☌ (more precisely: 3.98 ☌) water has the smallest volume or its highest density of 0.99997 g/cm³! Such a negative thermal expansion does not only occur with water, but also with other substances such as silicon or germanium. That’s why this phenomenon is known as negative thermal expansion (NTE) or density anomaly. in the case of heating: When water is heated in the range between 0☌ and 4☌, the water contracts instead of expanding. Let’s look at the situation the other way round, i.e. The density decreases accordingly in this temperature range. The volume does not decrease as usual as it cools down further, but increases. Figure: Experiment on the density anomaly of water (negative thermal expansion)īelow 4 ☌, however, water behaves differently than most liquids. Note that volume and density are reciprocal to each other. As expected, the volume of the water decreases and the density increases. At first, water contracts more and more due to the decreasing Brownian motion. Now the water is cooled down more and more. In order to illustrate this, several test tubes are shown in the figure below, each filled identically with water.
![density of water at different temperatures density of water at different temperatures](http://patentimages.storage.googleapis.com/WO2011005919A2/imgf000015_0001.png)
Negative thermal expansion (density anomaly)ĭue to its special molecular structure, water behaves differently than most other substances during cooling.