Distillation is a common way of separating a mixture of chemicals having different boiling points. Increasing the temperature of the mixture causes each chemical to vaporize and move into a condenser where it is cooled and separated as a pure liquid. Unfortunately, some compounds will decompose before they boil. Citric acid decomposes at 347 degrees F, so standard pressure distillation would be useless in separating it. Heat is also unsuitable for separating biological molecules, such as enzymes. Enzymes have an optimum working temperature of around 97 degrees; anything above 104 degrees causes them to degrade. This is one of the reasons why a high body temperature can be very serious, as it results in the disruption of the body's biochemistry.
The boiling of a substance depends on the pressure of the atmosphere around it. Lowering the atmosphere inside the distillation apparatus would therefore lower the boiling point of fragile compounds such as citric acid, but this reduced pressure would not affect the decomposition temperature. This type of procedure is called reduced pressure distillation. Proteins can be separated using chromatography. There are many different types of chromatography, but in each kind different types of protein traveling through a chromatographic column will do so at a different rate, meaning that it will emerge at the bottom of the column as a pure substance.
Some compounds will decompose at room temperature and atmospheric pressure, sometimes with explosive consequences. Hydrogen peroxide is widely used in the laboratory as an oxidizing agent; however, the peroxide group features an oxygen-oxygen single bond. This type of bond is inherently unstable and readily breaks up to form more stable bonds like oxygen-hydrogen, as found in water, or oxygen-oxygen double bonds.?In storage, hydrogen peroxide will slowly decompose to form water and oxygen; however, this break down is accelerated by light, so it must be stored in darkness. Over time there is risk of a bottle cracking or even exploding due to oxygen build-up.
The decomposition of hydrogen peroxide is an example of an irreversible decomposition, as there is no way to get water and oxygen to react on their own to create hydrogen peroxide. The decomposition of dinitrogen tetraoxide is an example of a decomposition that can be reversed. Heating a sealed mixture of dinitrogen tetraoxide results in the complete decomposition to nitrogen dioxide, characterized by a distinct color change from colorless to brown. At 284 degrees F the equilibrium entirely favors the formation of nitrogen dioxide; however, increasing the pressure or lowering the temperature reverses this process.