S6-WHAT IS “LATENT HEAT”( I.E. HIDDEN HEAT)?

By PragyanSharma Polavarapu

An outstanding experimental scientist Dr Joseph Black (1728-1799) of Scotland noted surprising facts about heat energy utilization when ice changes to water and hot water changes to steam.

(1)He noted during several controlled experiments that when water/ice mixture in a vessel was heated, the temperature of the mixture does not rise until entire ice was melted into water! One can only see the ice melting and finally disappearing and then only the reading in thermometer starts to move upwards showing that temperature of water has started to increase. We now in modern times know the reason. Much heat energy is required to complete the “change of state” i.e. to lessen the attraction between all the H2O molecules and to make them change to liquid type bonding. This utilization of heat energy which is not noticeable by a thermometer is called “latent heat of fusion” of ice i.e. a kind of “hidden heat”.

(2) Dr.Black also noted that when heating of already boiling water (boiling in a vessel) was further continued, then also the temperature did not rise. The thermometer showed only 100 degrees till entire water disappeared and was converted to steam i.e. till “change of state” from water to steam is completed. After this “change of state” is completed the temperature starts to rise if the steam is controlled and kept circulating in the vessel.We can now understand in modern times that the heat energy supplied to boiling water is used for all H2O molecules to change to free state of a gas.

Dr Black was a very famous professor of chemistry and medicine for a total 40 years at universities of Glasgow and Edinburg in Scotland. However his discovery of properties “specific heat” and “latent heat” ( in the science of heat energy in physics) is considered his greatest contribution to science. He was also discoverer of carbon dioxide gas (which he named as “fixed air”). At that time even the difference between elements and chemical compounds was not known and substances like air and water were treated as elements. Existence of important gases like oxygen, nitrogen, chlorine etc was not known.European scientists treated only water, salt, earth, fire and metal as elements. Dr Black,after discovery of carbon dioxide gas, felt that air is a most important substance in science and treated it as sixth element.

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