Temperature can affect both the structure of a molecule and how it interacts with surrounding molecules. Heating some molecules, particularly large, biological molecules such as proteins or nucleic acids, will cause them to change shape or even to break up. Heating a substance will also cause its molecules to become excited, which can cause them to change physical state. Heating a solid to certain temperature will change it into a liquid; further heating will change it into a gas.
Proteins consist of a long chain of different amino acids. At body temperature, a protein will have a set, three dimensional shape, however an increase in temperature will alter this shape, sometimes irreversibly. This change of shape is called denaturation. Denaturation often leads to the loss of solubility and biological activity of the molecule. This explains why an increase in body temperature of just a few degrees can be fatal, as an increase in temperature causes enzymes to denature, leading to the disruption of crucial biochemical processes.
Exposing molecules to very high temperatures will cause them to break up. Above about 150 degrees F biological molecules such as proteins and nucleic acids will start to decompose into their constituents. However, high temperatures are also used in the production of useful organic chemical intermediates. A large proportion of crude oil comprises long chain saturated hydrocarbons that are of little use other than as tar or bitumen. Heating alkanes in the presence of a catalyst yields short-chained, unsaturated hydrocarbons called alkenes or alkylenes. These alkylenes are used to produce a range of organic compounds including polymers, alcohols, aldehydes, ketones and carboxylic acids.
Change in Physical State
Heat also affects how molecules interact with each other. Heating a solid above a certain temperature will cause it to melt to form a liquid; further heating causes this liquid to boil to form a vapor. Very high temperatures will cause the vaporized molecules to break down into constituent atoms and still higher temperatures will ionize these atoms to form a plasma.
Heat and Entropy
These examples all demonstrate a link between temperature and disorder. At high temperatures molecules exist in a highly disordered, gaseous state. Decreasing the temperature makes them condense into a more ordered, liquid state; decreasing still further freezes them into a highly ordered solid state. The thermodynamic measurement of disorder is called entropy; it follows that, at absolute zero (-460 degrees F), entropy will also be zero. This principle is called the Third Law of Thermodynamics.
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