What indicates the presence of hydrogen gas?

The presence of hydrogen gas can be indicated by a squeaky pop sound when it is ignited. This characteristic noise occurs because hydrogen gas reacts rapidly with oxygen in the air when ignited, leading to an exothermic reaction that produces water. The sound is a result of the rapid expansion of gases and the creation of shock waves from the quick combustion.

Other methods, such as formation of bubbles, may suggest gas production but do not specifically identify hydrogen. Color change in litmus paper is related to acidity or basicity and is not indicative of hydrogen gas directly. A loud bang can signify a more vigorous reaction or presence of other gases but is not specific for hydrogen. Therefore, the squeaky pop sound stands out as the most reliable indicator of hydrogen gas.