Thermal decomposition

For the biological process, see decomposition.

Thermal decomposition, or thermolysis, is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes.

The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. If decomposition is sufficiently exothermic, a positive feedback loop is created producing thermal runaway and possibly an explosion.

Examples

• Calcium carbonate (Limestone or chalk) decomposes into calcium oxide and carbon dioxide when heated:

CaCO₃ → CaO + CO₂

The reaction is used to make quick lime, which when hydrated becomes slaked lime and is used a building material.

Equipment used by Priestley in his experiments on gases

• Many oxides decompose at high enough temperatures, an example being the decomposition of mercuric oxide to give oxygen and mercury. The reaction was used by Joseph Priestley to make the gas for the first time.

• Some foods will decompose exothermically at cooking temperatures; anyone who has overheated sugar or syrupy foods will know how long they take to cool. Mild versions of the process will produce caramelised dishes that are pleasant, but cannot be tasted safely before they have cooled to a comfortable temperature. Once they start to char, such dishes commonly will continue in a positive feedback loop; they become dangerously hot and continue to blacken from the inside out, and smoke even well after being removed from the heat. In films, where stuntmen have to jump through breaking windows, the window panes are made of sugar, which is safer than glass. Melting the sugar however, is a tricky business; an error of just a few degrees will start a caramelisation process that will ruin the product.

• Water, when heated to well over 2000 °C, decomposes to its constituent elements:

2 H₂O → 2 H₂ + O₂

• The compound with the highest known decomposition temperature is carbon monoxide at ≈3870 °C (≈7000 °F).^{[citation needed]}.

See also

• Ellingham diagram
• Thermochemical cycle
• Thermal depolymerization
• Chemical thermodynamics
• Pyrolysis - thermolysis of organic material