We learned at school that there are three states of matter, solid, liquid and gas. Actually, there are four if we include plasma. This is found where there is intense heat, such as the atmosphere of the Sun. But for this article, we will consider the first three. We know that solid substances have a melting point. Taking the most common example, when ice is heated to its melting point (as it rises above zero degrees Celsius), it changes to water. If we then heat the water to 100 degrees Celsius, we will see the water turn into its gaseous state, which is steam.
We should also remember that the boiling point of water, for example, is dependent on the ambient pressure. An airliner passenger cabin at cruising height is pressurised to what we would experience at around 7,000 feet. When you order your tea or coffee, the water boils at a lower temperature. In this example, it is about 90 degrees Celsius. Since tea really needs to be as hot as possible, perhaps your coffee will taste better.
There are exceptions to this rule of changes of state. It is possible to heat water above its boiling point and for it to remain a liquid. This is called superheating. It is an experiment which is not easy to carry out and is definitely not to be attempted unless in a properly equipped lab with professional technicians present. For the experiment to work, several criteria must be met. The container must be perfectly smooth, and the liquid must remain undisturbed. If either of these criteria is not met and bubbles form, then the liquid will explode into its gaseous state. This is called a catastrophe.
Recently, a research team decided to raise the temperature of a thin film of gold above its melting point. It is very difficult to measure the internal temperature of materials undergoing this process, and the team wanted to find a way to measure it accurately. To heat the gold, they used a powerful pulsed laser. The heating process was unbelievably quick, around 45 quadrillionths of a second. A second pulse followed, just as quick. Inside the gold film, the atoms were racing around very fast due to the heat. The second laser pulse scattered them. At this point, the team were able to measure the speed of these atoms and therefore calculate their temperature. Clever, eh?
However, what the team were expecting and the result they got surprised them. They thought the gold would reach a temperature of x3 times its melting point. In fact, it reached x14 times that value. The temperature measured was 18,700 degrees Celsius. This needed some explaining. If not, then it would indicate that they had broken the Second Law of Thermodynamics, which, in physics, let us say, we believe is impossible. The reason they came up with was that the temperature increase only occurred because the heating process was so quick. If it had happened more slowly, the result would have been different with a lower temperature. I have not explained the Second Law of Thermodynamics, as that is a subject in itself.
John Pullen 