Table of Contents
Our Hydrochloric Acid Gas experiment starts when you place a little sodium chloride in a test-tube and add a drop or two of strong sulphuric acid; an effervescence follows and a gas is given off with a strongly acid pungent odour.
NaCl + H2SO4 = NaHSO4 + HCl
Set up your apparatus in the same way as for the preparation of carbon dioxide. Put about 20 grams of fused sodium chloride into the flask and pour through the thistle funnel sufficient strong sulphuric acid to cover the salt in the flask and collect the gas by downward displacement. To test when the jars are full, bring a lighted taper near the mouth; if it is extinguished remove the jar and cover it with a greased cover-glass. If the gas slackens in coming off, heat the flask gently. Collect two jars of the gas.
Laboratory Hydrochloric Acid Gas Experiment I
Put a piece of blue litmus paper in a jar of the gas; it immediately turns red, showing the acid property of the gas.
Laboratory Experiment Hydrochloric Acid Gas II
Invert the other jar of the gas and place it over the hole in the tray of a pneumatic trough filled with water, remove the cover- glass and you will see the water rise quickly in the jar, showing the solubility of hydrochloric acid gas.
Hydrochloric Acid Gas Laboratory Experiment III
Take a few crystals of potassium bichromate, put them in a test-tube, and add a little strong hydrochloric acid, and heat; chlorine, which can be detected by its smell and colour, is evolved.
K2Cr2O7 + 14HCl = Cr2Cl6 + 2KCl + 7H2O + 6Cl
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 don