For the experiment used hydrogen is collected by displacement of air. Without changing the position of the test tube bring a lighted match to a test tube and sharply turn the tube so the hole was in flames. If you hear a sharp “barking” sound, with gas (hydrogen) you can not work because it contains the admixture of air. Need some time to wait until the test-tube will be pushed out all the air. If you hear a light sound resembling “t - groin”, with hydrogen to operate.