- 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.
- CaCO3 → CaO + CO2
- The reaction is used to make quick lime, which when hydrated becomes slaked lime and is used as a building material.
Equipment used by Priestley in his experiments on gases
- 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 traditionally were breakaway glass made of sugar, which is safer than real glass. Melting the sugar is a tricky business, however; an error of just a few degrees will start a caramelisation process that will ruin the product, so suitable plastics are commonly used instead nowadays.
- When water is heated to well over 2000 °C, a small percentage of it will decompose into its constituent elements:
- 2 H2O → 2 H2 + O2
Decomposition of nitrates, nitrites and ammonium compounds
Ease of decomposition
When metals are near the bottom of the reactivity series, their compounds generally decompose easily at high temperatures. This is because stronger bonds form between atoms towards the top of the reactivity series, and strong bonds break less easily. For example, copper is near the bottom of the reactivity series, and copper sulfate (CuSO4), begins to decompose at about 200°C, increasing rapidly at higher temperatures to about 560°C. In contrast potassium is near the top of the reactivity series, and potassium sulfate (K2SO4) does not decompose at its melting point of about 1069°C.
- ^ Johnson, John J. J. Cheap Tricks and Class Acts: Special Effects, Makeup and Stunts from the Films of the Fantastic Fifties. Publisher: Mcfarland & Co 1995. ISBN: 978-0786400935
- ^ "Inventions made to order" Popular Mechanics Vol. 73, No. 5. May 1940. Pages 696-699, 126A-127A
- ^ Kemmerer, Jack B. "TV Trickery" Popular Mechanics Vol. 109, No. 1 January 1958. Pages 127-129
- ^ Lide, David R. CRC Handbook of Chemistry and Physics, 90th Edition. Publisher: CRC Press 2009. ISBN: 978-1420090840