12PREPARING COLLECTING AND TESTING GASES b Hydrogen Be careful A dangerous explosion may occur if you use any vessel bigger than a small testtube when igniting the gas particularly if it is mixed with air Never dry hydrogen gas with concentrated sulphuric acid i Place a few pieces of granulated zinc or zinc foil from the casing of an old dry cell in a boiling tube add 2 drops of copper sulphate solution An alternative to the thistle funnel at A is a syringe as shown at B Pour enough 1 M sulphuric acid down the thistle funnel on to the zinc to cover the bottom of the funnel tube Discard the first two or three test tubes of hydrogen as they will contain displaced air Collect test tubes of the gas and stopper them Test the third testtube of gas by holding a lighted taper or splint over the mouth as soon as you take out the stopper Pure hydrogen burns with a quiet pop sound ii Alternatively add sulphuric acid from a syringe Gas cannot escape through the syringe so you do not need to cover the tube of the syringe with acid iii Hydrogen burns in air to form water vapour When hydrogen ignites in a dry testtube note any vapour or mist on the sides of the testtube iv Investigate whether hydrogen is lighter than air by pouring the gas into a testtube held either above the first tube or below it Use a lighted taper to investigate where the hydrogen has gone v Blow soap bubbles by holding the delivery tube of the apparatus in detergent or soap solution The hydrogen bubbles will rise into the air showing the low density of hydrogen gas