Hydrogen gas is lighter than air. So theoretical logic says that it can be collected by downward displacement of air.
But here exists a practical inconvenience regarding implementation of this theoretical logic.
If a jar full of air is kept its mouth inverted over the mouth of the tube from which reaction generated hydrogen gas is coming out (as shown in the figure) , the less dense hydrogen gas ascends towards the bottom of the jar and is is collected displacing the denser air within it downward.
But hydrogen gas being colorless and odorless we cannot visualize to what extent the jar has been filled by hydrogen . There is no suitable chemical test to detect whether it is filled up or not.
To test with a burning jute stick may be dangerous one as hydrogen is highly inflammable . If the jar is partially filled then the air-hydrogen mixture burns with explosion when comes in contact with fire causing an accident. So it should be avoided for the sake of safety.
In case of preparation of
Formation of white fumes of ammonium chloride in case of glass rod moistened with conc. HCl or turning of red litmus paper to blue in case of litmus paper confirms the complete filling of the jar.
If we use the process of downward displacement of water for collection of