Test-papers

Test-papers are made by dipping unsized paper into an alcoholic solution of a vegetable colouring matter which changes colour when exposed to the action of an acid or alkaline solution. The paper, after being gently dried, is cut into slips of a suitable size. Hence, by dipping the appropriate test papers into any solution, we can ascertain whether it is acid, alkaline, or neutral. Litmus and turmeric are most commonly used as the colouring matters; litmus for the detection of acids, and turmeric for that of alkalies. Test-papers are also employed for detecting sulphuretted hydrogen, &c., and for such a purpose the paper must be dipped in the solution of an appropriate substance. Thus acetate of lead paper becomes black in presence of sulphuretted hydrogen, while starch paper becomes blue when touched with iodine.