Past exam of the medicine course of the University of Oxford / 1865 / First examination / Chemistry / 7 / Solution

“Carbonic oxide” means carbon monoxide, $\mathrm{CO}$: a combustible, reducing, colourless gas distinct from carbon dioxide. Several preparations remove part of the oxygen from a carbon–oxygen compound or prevent complete oxidation of carbon.

Passing carbon dioxide over sufficiently hot carbon givesHigh temperature favours substantial conversion. The reaction is important in furnaces, but gas emerging from such a process need not be pure carbon monoxide; residual carbon dioxide and other gases must be considered. Incomplete combustion of carbonaceous fuel is another source, with the ideal net balance $2\mathrm{C}+{\mathrm{O}}_2\to 2\mathrm{CO}$. It is a source principle rather than a selective method guaranteeing a pure sample.

For a historical laboratory preparation, formic acid can be dehydrated by a suitable strongly dehydrating medium:Oxalic acid under an appropriate dehydrating treatment yields a mixture:An alkaline absorption stage removes much of the carbon dioxide, while carbon monoxide passes on. A generator, droplet trap, purifier and collecting vessel illustrate why preparation and purification are separate operations. The gas is only slightly soluble in water, so collection over water is possible in principle. Concentrated sulfuric acid acting on carbonaceous material can also give mixed carbon oxides and sulfur dioxide; that route requires additional purification and should not be presented as clean CO evolution without by-products.

The relative density of carbon monoxide against dry air is approximately $28.01/28.97\simeq 0.967$. It has no reliable warning odour. It burns with a blue flame when supplied with suitable oxygen:A CO sample itself does not ordinarily support combustion of a taper. Nor does it immediately give the usual limewater test of carbon dioxide; oxidation of the CO first gives a gas that does. This distinguishes fuel behaviour from the property of supporting combustion of another material.

Its reducing power is illustrated byand by reduction of suitable iron oxides in metallurgy. The products reflect transfer of oxygen from the oxide to carbon monoxide. Its ability to bind metal centres is also chemically important. In biological exposure, binding to haemoglobin and other effects disrupt oxygen delivery and use; it is a serious poison, not merely an innocuous replacement of atmospheric oxygen. This modern explanation must not turn an historical gas-preparation answer into a proposed exposure experiment.^[ref][ref]