Can a soap bubble explode? Fill bubbles with hydrogen gas, light them on fire, and the answer depends entirely on their size. This project uses a tank of hydrogen gas to blow soap bubbles of different sizes, from less than 1 mm to over 18 mm across. Each bubble is ignited with a lighter while you observe and compare the flames. Bubbles smaller than 4 mm produce no flame at all. As size increases past that threshold, the flames grow larger. Above 18 mm, the combustion turns explosive.