‘Fixed air’ (Carbon Dioxide)
Black’s research into the nature of alkalinity led him into discovering a gas, which he named ‘fixed gas’ due to its properties: it was heavier than common air; it could extinguish a flame and suffocate an animal. He found that Magnesia alba behaved similarly to Calcium carbonate in that it gives off a gas when reacted with acids, however upon measuring Black noticed that Magnesia alba weighed significantly less after being heated and therefore something must have been ‘lost’. Black came to the conclusion that this was a form of air but that it was different from common air and therefore demonstrated that gases could be chemical substances themselves, thus disproving the idea that they were atmospheric air in different states of purity. This gas was carbon dioxide and Black conducted many experiments to discover the properties of this abundant gas, essentially he was the first person to bubble gas through limewater and see the cloudy solution with a white precipitate.
It must be noted that chemist van Helmont discovered a gas which is now known to be carbon dioxide about a century before Black, however, the rise of the Phlogiston Theory resulted in the theory being lost; so perhaps it would be ideal to state that Joseph Black rediscovered Carbon dioxide.
Heat and temperature
These two terms, although related, are quite distinct; while temperature can be measured easily using a thermometer, heat as a form of energy is more complicated.
Black discovered through experiments that different amounts of heat were required to produce the same temperature rise in equal masses of different substances. For example, 1.7 times more heat is required to raise the temperature of water by 10 degrees than the same increase for ethanol.
Latent and Specific Heat
Latent heat is the heat energy that is taken in or given out by a substance when it breaks bonds and thus changes state; latent heat of fusion is from solid to liquid, latent heat of vaporisation is from liquid to vapour.
Prior to Black’s discovery scientists were unable to understand why melting ice absorbs a large amount of heat without any increase in temperature, and similarly evaporating water absorbs even more heat without a change in temperature. Black questioned why water does not boil away suddenly when the temperature reaches boiling point, and ice does not suddenly melt when the temperature is over the freezing point. To prove this phenomenon he conducted an experiment in which he took two similar glass flasks and poured the same volume of water into both before placing in a freezing mixture; one froze into ice while the other had a little alcohol added to prevent freezing and was thus still a liquid. These flasks were removed from the bath and allowed to warm up naturally, the results showed that although the flasks had to be absorbing heat at the same rate the temperature of the water and alcohol solution increased while the ice remained at its freezing point despite absorbing enough heat to raise the temperature of the same quantity of water by 78 degrees Celsius. This showed that ice while in the process of melting does not warm up until it is completely melted, this became known as the latent heat of fusion of water.
This concept was applied in industrial practice; in particular, it was used to improve steam engines under the work of James Watt. It was also used to design heat insulation so that ice could be stored for months without melting. Today, this principle is frequently used in operating and designing air conditioning, refrigeration, nuclear power plants and many other areas.
From latent heat, Black developed the idea of ‘specific heat’ by measuring the amount of heat required to raise the temperature of a substance to be a specified number of degrees.
Joseph Black discovered and proved theories that are now well established in scientific knowledge today; his discoveries formed one of the first foundation blocks in the science of thermodynamics. His contribution to science can be demonstrated by the fact that both universities that he worked at (Edinburgh and Glasgow) have named their chemistry buildings after him.
By Ishani Shah