The Sulphur Trioxide Gas Laboratory experiments starts when you pour about 10 c.c. of strong Nordhausen sulphuric acid (H2S2O7) into a small retort to which is attached a receiver; heat the retort carefully, and fumes of sulphur trioxide will pass over and condense to a white crystalline solid in the receiver.

H2S2O7 = H2SO4 + SO3

Allow a drop of water to fall on one of the crystals; the chemical combination is so intense that the water hisses as it does when falling on red-hot iron. If you test this solution you will find it is sulphuric acid.

SO3+H2O = H2SO4

Heat a little strong sulphuric acid and pour it on to a small piece of sugar in a porcelain dish; the sugar is instantly charred ; steam is given off, while a black mass of charcoal is left in the dish.

C12H22O — 11H2O = C12
(Sugar)

The sulphuric acid robs the sugar of its elements of water and leaves only carbon.

INSTRUCTIONS TO THE STUDENT

The student has now obtained a fair knowledge of simple glass-working and of the preparation and properties of the more common gases. In the following sections he proceeds to examine solids and liquids (generally solids which are put into solution). First he examines an unknown substance by ‘dry tests’ for ‘ bases ’ and ‘ acids ’; the results so obtained are then confirmed by systematic ‘ wet tests.’

The order of work laid down is to be followed with most careful and patient attention to all details; the student has then done his part, but unless this is supplemented by equal care on the part of the demonstrator, the best results cannot be expected. The demonstrator, besides supervising the actual testing, must prepare a carefully graded series of substances leading from simple salts containing one ‘base’ and one ‘ acid ’ to complex mixtures containing four or five ‘bases’ and several ‘ acids ’ (including insolubles). The substances must all be carefully selected with the definite object of teaching the student some important point in every mixture he analyses. Indiscriminate preparation of mixtures leads to waste of time and bad work.

In his first tests the student may be given salts of known composition, and his work is then checked by unknown salts from the demonstrator’s set. For instance, on the next page he may take ZnCl2, SnCl2, Pb, Bi2S3, and so on for practice, and when fairly confident, his proficiency or otherwise is checked on ‘unknown’ salts given by the demonstrator.

The demonstrator’s ‘Record Book’ should show full details of the substances given to each student, the results obtained, the time taken with the analysis, and general remarks where necessary. A somewhat similar record must be kept of the work done in Quantitative Analysis and Assaying.