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The Burning Question - and the nature of science.

The Burning Question - and the nature of science. Dr Bert Sorsby University of Hull. Why do things burn? The modern idea. Burning is the release of light and heat energy ( and new substances ) when oxygen reacts chemically with a fuel.

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The Burning Question - and the nature of science.

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  1. The Burning Question - and the nature of science. Dr Bert Sorsby University of Hull

  2. Why do things burn? The modern idea. • Burning is the release of light and heat energy ( and new substances ) when oxygen reacts chemically with a fuel. • A small amount of energy is generally needed to start the reaction. • A flame is the region where this takes place.

  3. Why do things burn? Early ideas • Empedocles (490-430 BC) and Aristotle (384-322 BC) • Four Element Theory: Earth; Air; Fire; Water • Paracelsus (1494-1541) - ‘iatrochemist’ • Three Principles ; Salt; Sulphur; Mercury

  4. Why do things burn? Another idea. • J.J. Becher (1635-82) & G.E. Stahl (1660-1734) • The Phlogiston Theory • S.Hales (1677-1761); J.Priestley (1733-1804); H. Cavendish (1771-1810) • Different types of ‘air’ - qualitative and quantitative study.

  5. Priestley and ‘dephlogisticated air.(1774) • When mercury is heated in air it produces red calx • When red calx is heated strongly, it produces a new gas, which Priestly called ‘dephlogisticated air’.

  6. Cavendish (1770+) and inflammable air • metal + acid  salt + inflammable air(calx+Ph) + acid  (calx +acid) + phlogiston • inflammable air burns to produce water • Volta and Priestley also recognised this. • Controversy between Priestley, Cavendish and Watt about the composition of water.

  7. Antoine Laurent Lavoisier (1743-1794) and the oxygen theory • Autumn 1774 Priestley met Lavoisier at a dinner in Paris • For the next three years Lavoisier checked and re-interpreted Priesley’s results. • 1777 he produced 9 papers for the Academy in Paris.

  8. Lavoisier’s investigations (Part 1) • He heated 4 ounces mercury in a flask with 50 cubic inches of air. • 8 cubic inches of air were used up (weighing 3 grains). • Red powder on surface of remaining mercury and flask and contests were 3 grains heavier. • New ‘air’ in the flask could not support life or burning.

  9. Lavoisier’s investigations (Part 2) • He removed all the red powder and heated it in another flask • A new gas was produced - volume 8 cubic inches and weight 3 grains • The new gas supported life and burning. • Substances burned in it produced acids therefore ‘oxygen’ or ‘acid maker’ • Published in Traité élémentaire de chimie in 1789 including work on the composition of water etc.

  10. Phlogiston and Oxygen Theories compared • Burning • Metal (Calx +)  calx +  • Metal + O  Metal oxide • Metals and acid ( Before 1789) • metal (calx+ ) + acid  salt +  • metal + acid  salt (calx?+acid) + inflammable air(?) • After 1789 • Metal + water (hydrogen oxide) + acid  salt (metal oxide +acid) + hydrogen

  11. Reception of Oxygen Theory • In France it was largely accepted • Berthollet 1787 ‘oxymuriatic acid’ • Fourcroy and Guyton de Morveau taught oxygen theory and new nomenclature 1780s. • In Britain only accepted at first in Scotland (Joseph Black 1784). Davy was still sceptical in early 19th century. • In Germany there was resistance. Especially from Gren ( is a form of matter with negative weight)

  12. How do both theories explain other observations? For example:- • Candle(s) under bell jar. • Candle (s) under bell jar over water.

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