Almost everything we use today - plastics, medicines, synthetic fabrics - is made by some chemical process or the other. Many of these require organic solvents like benzene or acetone, which are environmental pollutants. How nice would it be if there was a way to make these useful things without needing harmful solvents?
Enter Supercritical Fluids
A supercritical fluid is a special state of a substance that exists above its critical point. For water, that's 374°C. At this temperature, water loses many peculiar properties it has in its liquid state, such as hydrogen bonding and repelling non-polar substances.
But wait. We're going too fast. Let's rewind, and try and understand critical point first.
Imagine an unbreakable glass ball, half-full of water, and the rest a vacuum above it. Some of the empty space will be filled by water vapour. Now let's heat this ball. As water boils, it forms steam. The density of liquid water decreases, while that of steam increases. As you keep heating it, at one point, the density of water will be equal to the density of steam. This temperature is called the critical point. At this point both liquid and gas states merge into a state called supercritical water (SCW).
Here's a video* showing you how this happens:
Reactions in supercritical water
Now let's understand why supercritical water is different. In its liquid state, water forms many 'hydrogen bonds' between its molecules. These are what make water expand when freezing, making ice lighter than water. However, as water is heated, the molecules move more and more about and the hydrogen bonds break. At the critical point, they disappear completely.
Many chemicals used to make plastics, medicines etc. are not soluble in water because its hydrogen bonds do not allow water molecules to mix with those molecules. That's why they need to be dissolved in solvents like benzene or acetone. However, when hydrogen bonds are broken, water molecules can dissolve chemicals that were previously insoluble. Now you'll be thinking, why not use SCW as a solvent instead of benzene or acetone? Well, many scientists have thought that too!
Putting supercritical water to work
Some factories have started making acetophenone, using SCW as the solvent. Acetophenone is a precursor molecule used to make many drugs and perfumes. Another important reaction carried out with SCW as solvent is the breakdown of triglycerides (commonly found in animal & vegetable fats) to glycerine and fatty acids. Fatty acids are used in the making of soap and biodiesel. It is also being thought of as a substitute for steam in thermal power plants.
The supercritical form of carbon dioxide is also useful. It is used nowadays in manufacturing decaffeinated coffee powder, and for creating nano-materials. When you become a scientist yourself, we're sure you'll find an exciting use for supercritical materials!
*Video titled "nottinghamscience - April 28, 2008 - Is it a liquid? Is it a gas? Professor Martyn Poliakoff demonstrates supercritical fluids in his office at the University of Nottingham." sourced from www.YouTube.com.