In school science classes, we are taught that water can exist in three different states, as either vapor gas, liquid water, or solid ice. An international team of scientists has however recently found signs that liquid water might actually exist in two different states.
The International Journal of Nanotechnology recently published an experimental paper where researchers expressed their surprise when finding that a number of the physical properties of water change between 50℃ and 60℃. A heated discussion could be sparked in the scientific community regarding the potential change to a second liquid state of water. If the finding is confirmed, it could have repercussions in a wide range of fields, including biology and nanotechnology.
“Phases” or states of matter, is a key concept in the study of systems made from molecules and atoms. Speaking broadly, a system molded from many molecules can be organized in a finite number of configurations depending on its total energy. At low temperatures, the molecules arrange themselves in a very specific configuration, producing a solid. At higher temperatures, the molecules have more configurations that are possible and so form a more ordered phase (a liquid). At even higher temperatures, the molecules are disorganized and move about relatively freely (the gas phase).
This illustration is accurate for relatively simple molecules such as methane or carbon dioxide, which have three clear, different states (liquid, solid and gas). For molecules that are more complex however, there is a larger number of possible configurations, giving rise to more phases. A good example of this is the rich behavior of liquid crystals. They are formed by complex organic molecules and flow like liquids, while still having a solid-like crystalline structure.
Many physical properties of a substance will change abruptly when it goes from one state to another, as the configuration of its molecules determines the phase of a substance. In the recent research, scientist measured several key physical properties of water in its liquid state at temperatures between 0℃ and 100℃ under normal atmospheric conditions. They were surprised to find that properties such as the water’s refractive index (a measure of how light travels through it) and its surface tension changed at around 50℃.
The structure of a water molecule, H₂O can be pictured as an arrow tip, with the two hydrogen atoms flanking the oxygen atom at the top. The electrons in the molecule are normally distributed asymmetrically, resulting in the oxygen side being negatively charged relative to the hydrogen side. This leads to hydrogen bonding between water molecules, in which the opposite charges attract each other. Because of the hydrogen bonding, water has properties that break the trends observed for other simple liquids in many cases. Unlike most other substances, a fixed mass of water takes up more room as a solid (ice) than as a (liquid) because of the way it molecules form a specific regular structure. The surface tension of liquid water is also roughly twice that of other non-polar, simpler, liquids.
Water is simple, but not overly so. One possible explanation for the apparent extra phase of water is that it behaves somewhat like a liquid crystal. Although the hydrogen bonds between molecules keep some order at low temperatures, it could eventually take a second, less ordered liquid phase at higher temperatures. This would explain the anomalies the researchers observed in their data.
The authors’ findings could have many applications if these are validated. One example would be that if a change in temperature causes changes in a substance’s physical properties, it could potentially be used for sensing applications.
More fundamentally, water mostly makes up biological systems. How biological molecules (e.g. proteins) interact with each other probably depends on the specific way in which water molecules arrange to form a liquid phase. Understanding how water molecules arrange themselves differently at different temperatures could shed light on the workings of how the molecules interact in biological systems.
The discovery is an exciting opportunity for experimentalists and theorists, and a classic example of how even the most familiar of substance still hides secrets from us.