Concrete and sand are highly unreactive to most of the chemical attacks. But chlorine triflouride is so dangerous that it will ignite concrete, sand and other fire retarding substances. Which property does ClF3 possess to cause this ignition and why?

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    $\begingroup$ The related posts (1, 2) by Derek Lowe on his blog In the Pipeline are practically required reading for anyone interested in chemistry! His prose is interesting and very enjoyable. $\endgroup$ Jul 21, 2015 at 13:57
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    $\begingroup$ @NicolauSakerNeto From that first post: "It is also hypergolic with such things as cloth, wood, and test engineers...." Not often one laughs and shudders at the same time...! $\endgroup$
    – hBy2Py
    Jul 21, 2015 at 14:45

2 Answers 2


Concrete, sand, and asbestos are all, in large part, silicon dioxide (SiO₂). It's energetically favorable for available fluorine to take the place of oxygen, producing SiF₄. This occurs even in weak solutions of hydrogen fluoride, and it's why HF solutions are used for etching glass.

ClF₃ has a much greater amount of potential bond energy than HF or even F₂. This means that, when it decomposes, such as upon contact with silicon dioxide, it gives off a lot of energy, which is released as fire.


Chlorine trifluoride is very unstable so it's energetically favourable to react with almost anything to produce more stable fluorides; releasing oxygen, chlorine and compounds of the two.

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    $\begingroup$ You really should elaborate. $\endgroup$
    – Mithoron
    Jul 21, 2015 at 19:14

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