Sitting around a campfire, you can feel its heat, smell the woody smoke, and hear it crackle. If you get too close, it burns your eyes and stings your nostrils. You could stare at the bright flames forever as they twist and flicker in endless incarnations. But what exactly are you looking at? The flames are obviously not solid, nor are they liquid. Mingling with the air, they’re more like a gas, but more visible--and more fleeting. And on a scientific level, fire differs from gas because gases can exist in the same state indefinitely while fires always burn out eventually.
坐在篝火旁, 你能感受到它的热度, 闻到木头冒烟的味道, 还能听到它劈啪作响。 要是离得太近, 火会烧到你的眼睛, 熏疼你的鼻子。 你毫不厌倦的盯着这明亮的火焰, 看它闪烁摇曳, 不断变换身姿。 但是, 你在看的到底是什么? 火焰显然不是固体, 也不是液体。 当与空气混合, 它们更像一种气体, 但更显眼—也更短暂。 从科学的角度来说, 火不同于气体, 因为气体能以一种恒定的状态存在, 而火最终总会熄灭。
One misconception is that fire is a plasma, the fourth state of matter in which atoms are stripped of their electrons. Like fire and unlike the other kinds of matter, plasmas don’t exist in a stable state on earth. They only form when gas is exposed to an electric field or superheated to temperatures of thousands or tens of thousands of degrees. By contrast, fuels like wood and paper burn at a few hundred degrees —far below the threshold of what's usually considered a plasma.
有一种错误认识, 认为火是一种等离子体, 后者是原子被剥夺电子后 产生的第四种物质形态。 与其他物质形态不一样, 等离子体更像火, 它们不是以一种稳定的状态 存在于地球上。 它们只在气体暴露于电场, 或者被过度加热到 成千上万度时才会产生。 相比之下, 类似木柴和纸张之类的燃料 几百度时就能燃烧— 远低于通常认为的 等离子体存在的临界温度。
So if fire isn’t a solid, liquid, gas, or a plasma, what does that leave? It turns out fire isn’t actually matter at all. Instead, it’s our sensory experience of a chemical reaction called combustion. In a way, fire is like the leaves changing color in fall, the smell of fruit as it ripens, or a firefly’s blinking light. All of these are sensory clues that a chemical reaction is taking place. What differs about fire is that it engages a lot of our senses at the same time, creating the kind of vivid experience we expect to come from a physical thing.
如果火不是固体、液体、气体, 也不是等离子体, 那它到底是什么呢? 事实上, 火都称不上是一种物质。 相反,它是我们对一种 名为燃烧的化学反应 所产生的感官体验。 某种程度上, 火就像叶子在秋天的颜色变化、 果实成熟时散发的气息, 或是萤火虫发出的亮光。 这些都是感官上的提示, 表明一种化学反应正在发生。 火的不同之处在于 它同时调动人体的多项感官, 创造出一种我们希望从实体中获得的 生动的体验。
Combustion creates that sensory experience using fuel, heat, and oxygen. In a campfire, when the logs are heated to their ignition temperature, the walls of their cells decompose, releasing sugars and other molecules into the air. These molecules then react with airborne oxygen to create carbon dioxide and water. At the same time, any trapped water in the logs vaporizes, expands, ruptures the wood around it, and escapes with a satisfying crackle. As the fire heats up, the carbon dioxide and water vapor created by combustion expand. Now that they’re less dense, they rise in a thinning column. Gravity causes this expansion and rising, which gives flames their characteristic taper. Without gravity, molecules don’t separate by density and the flames have a totally different shape.
燃烧能创造那样的感官体验, 只需要燃料、温度、氧气。 当篝火中的木材被加热到燃点, 其内部的细胞壁分解开来, 向空气中释放糖类和其他分子。 这些分子随后与空气中的氧气反应, 生成二氧化碳和水。 同时, 被困在木材中的水分 蒸发、扩散、 撑破周围的木质, 最终伴着满意的噼啪声逃脱。 随着火温度升高, 燃烧所产生的 二氧化碳和水开始扩散。 现在它们没那么拥挤, 它们从一条细长的圆柱体中上升。 重力导致了这种扩散和上升, 形成了特有的锥形火焰。 如果没有重力, 分子就不会因密度不同而分离, 火焰也会是全然不同的形状。
We can see all of this because combustion also generates light. Molecules emit light when heated, and the color of the light depends on the temperature of the molecules. The hottest flames are white or blue. The type of molecules in a fire can also influence flame color. For instance, any unreacted carbon atoms from the logs form little clumps of soot that rise into the flames and emit the yellow-orange light we associate with a campfire. Substances like copper, calcium chloride, and potassium chloride can add their own characteristic hues to the mix.
我们能观察到这些, 是因为燃烧也会产生光。 分子被加热时会发光, 而光的颜色 取决于分子的温度。 温度最高的火焰是白色或蓝色的。 火中分子的种类 也会影响火焰颜色。 例如, 木料中未发生反应的碳原子 会生成小簇煤烟, 从火焰中蹿升, 发出我们在篝火中常看到的 橙黄色光芒。 而像铜、氯化钙、 氯化钾一类的化学物质, 能把它们特有的颜色混合进去。
Besides colorful flames, fire also continues to generate heat as it burns. This heat sustains the flames by keeping the fuel at or above ignition temperature. Eventually, though, even the hottest fires run out of fuel or oxygen. Then, those twisting flames give a final hiss and disappear with a wisp of smoke as if they were never there at all.
除了多彩的火焰, 火还会在燃烧中持续产生热量。 这种热量能将燃料温度 保持在燃点以上, 从而维持火焰不灭。 然而,即使是温度最高的烈火, 最终也会耗尽燃料或者氧气。 接着,这些摇曳的火焰 发出最后的咝咝声, 消失在一缕轻烟中, 仿佛它们从没来过。