Hydrogen reacting with oxygen in air. In this experiment a sample of hydrogen gas and a sample of oxygen gas have been mixed together in the balloon. Notice that the mixture of the two elements is stable. That is mixing hydrogen and oxygen gas does not cause a reaction to occur immediately. To get the to start a spark, or in this case, a match is used to initiate the reaction. Once the the reaction is initiated it has NO problem continuing. Do you know what the product of this reaction is?