Steamed and boiled food finish at the exact same temperature — both are capped at 212°F (100°C) at sea level, so heat isn't what separates the two methods. What actually differs is contact: boiling submerges food in liquid water that vitamins and flavor compounds can dissolve into and drain away with, while steaming only ever touches food with vapor that condenses on the surface and mostly stays there.
Why identical temperatures don't mean identical heat
Water boiling at sea level and the steam rising off it are both stuck at 212°F (100°C). Steam doesn't get hotter than the water it came from — it just carries its heat differently. Liquid water hands over energy the plain way, through direct contact: dense, tightly packed molecules bumping straight into food and passing on heat at the moment of impact. Steam does that too, but only after it condenses back into liquid first, and that condensation releases something extra — the latent heat of vaporization, close to 970 BTU per pound, far more energy than the same pound of water gives up just by cooling from 212°F down to skin temperature. That's the whole reason a steam burn does more damage than a splash of boiling water at the identical reading on a thermometer: the steam dumps its jolt of energy the instant it turns back to liquid, before it's even begun to cool.
Why boiled vegetables lose more of their vitamins
Boiling drops food into a pot holding many times its own weight in water, and compounds that dissolve easily — vitamin C, several B vitamins, potassium — leach out of a vegetable's cut surfaces for as long as it sits in there. Whatever moves out stays in the pot. And the pot usually goes down the drain. Steaming never gives those compounds anywhere to go: the food sits above the waterline, touched only by vapor that condenses on it and largely drips right back onto the food or the plate underneath. There's no reservoir for anything to diffuse into. Nothing about steam is gentler chemically — it's just that submersion is the only setup that actually pulls nutrients out in volume.
Why boiling still finishes a few minutes ahead
Boiling usually reaches doneness a little before steaming does for the same vegetable, because a rolling boil keeps swapping in fresh, undiminished hot water against every surface of the food, nonstop. Steam has to condense before it can hand over any heat at all, and the film of condensed water that builds up on the food's surface acts like a thin blanket the next wave of steam has to push through to reach it. It's a small effect. A couple of minutes on most vegetables, not a different outcome — which is why a recipe calling for "steam until tender" almost always asks for more time than "boil until tender."
How to tell it's actually working
A boil is unmistakable to the eye: the whole surface breaks in rolling, overlapping bubbles, not the occasional bubble sneaking up from the bottom that shows up well before a true boil. Steaming is quieter to watch. Steam should be rising steadily and visibly around the lid; lift it and see only a thin, unsteady wisp, and the water has dropped too low or the burner is turned down too far to keep making vapor fast enough. Doneness is judged the same way either way — a knife tip should slide into a vegetable with no resistance, fish should flake apart at a gentle push, and green vegetables turn a brighter, more saturated color in the first minute or two before dulling if left too long.
For vegetables, pasta, and dumplings, stopping a minute short is only a texture problem — a firmer bite and nothing else. Chicken is the exception. Whether it's boiled or steamed, a thigh needs to hit 165°F to be called safe, and neither color nor time on the stove proves it got there.
Which one to reach for
None of this makes boiling the wrong choice most of the time. Pasta, blanched green beans, hard-boiled eggs — none of them are prized for retained vitamin C, and full submersion cooks them faster and more evenly than vapor ever could. Steaming earns its place on food where the nutrient loss or the beating from moving water actually costs something: broccoli and other vegetables eaten mainly for what's in them, fish fillets and dumplings too delicate to survive a rolling boil, anything that would otherwise turn waterlogged by the time it's done.