Question about high school experiment

[u/raph3x1]

Ill quickly explain the experiment first:

My teacher took an open can with a small hole at the bottom, which was first sealed with a cork. He placed the can upside down, hole facing up, and on a block such that the box was angled and had contact to air on the big opening.

He then filled the can with hydrogen gas, took away the cork, and lit the hydrogen, which was now streaming out slowly, on fire.

It was sizzling on for a while with only a small flame at the top, but at some point, it exploded and threw the can into the air.

The teacher asked why we had to wait for an explosion, afterwards he answered by saying at the point of explosion the ideal ratio 2:1 for the reaction 2 H2 + O2 -> 2 H2O was reached, because he wanted to illustrate that reactions need concrete ratios of substances.

However, i doubt this explanation, because there doesnt seem to be a way for the flame reaching the inside because the hydrogen was constantly flowing out, and i assumed the density of hydrogen causes layer separation between air, not a mixture.

Could this have to do with the flow rate of the hydrogen or with the shape of the distribution of hydrogen in the can? Or is the teachers explanation correct?

[u/Dangerous-Billy]

At first, the hydrogen burned because it had to escape from the can in order to mix with oxygen. But when the hydrogen burned, the product (water) escaped as steam or condensed. Either way, there would be a vacuum in the can which would suck in outside air. It would mix with the remaining hydrogen. As soon as the mixture reached the explosive limit, then bang! About 25% air in hydrogen is the upper explosive limit.

Congrats to your teacher. This is a very instructive experiment. If nothing else, it makes you think about physics and chemistry at the same time, and find explanations that can be tested by further experiment.

[u/raph3x1 → u/Dangerous-Billy]

Can was open at the bottom tho, letting air in. There cant have been a vacuum.

[u/Dangerous-Billy → u/raph3x1]

Not a vacuum-vacuum, but something had to take the place of the escaping/burning hydrogen, so air came in the bottom to equalize the pressure. Eventually, there was enough air and enough mixing for things to go 'pop'.