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.
坐在營火的旁邊, 你可以感受到它的熱度, 聞到燒木材的煙味, 聽見木材被燒裂的聲音。 如果你太靠近, 你的眼睛會感到灼燒, 你的鼻孔會感到灼痛。 你可以目不轉睛一直 盯著明亮的火焰看, 因為它們會在扭曲、 閃爍中無限循環。 但你到底是在看什麼? 很明顯,火焰並不是固體, 它們也不是液體。 和空氣混合在一起, 它們比較像是氣體, 但比空氣更可見——也更短暫。 就科學上來說,火和氣體不同, 因為在同樣的狀態下 氣體可以無限期地存在, 而火最終都會燒盡。
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.
有一種誤解是認為火是電漿, 物質的第四種相,在這種相中, 原子被剝去了電子。 和火一樣,但和其他物質不一樣, 電漿不會在地球上 存於穩定的狀態中。 電漿的形成,只會發生在 氣體接觸到電場時, 或是加熱到數千、 數萬度的高溫時。 相對之下,用木材 和紙當燃料來燃燒, 只會有幾百度的高溫—— 遠低於一般認定的電漿標準。
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.
所以,如果火不是 固體、液體、氣體, 或電漿,那還剩什麼可能性? 結果發現,其實火根本不是物質。 火是我們對某種 化學反應的感官體驗, 這種化學反應叫做燃燒反應。 某種方面來說,火就像是 秋天樹葉改變顏色、 水果成熟時的氣味, 或是螢火蟲的閃光。 上述這些都是感官線索, 表示有一項化學反應正在發生。 火的差別在於它同時連結了 我們的好幾種感官, 創造出一種生動的體驗, 讓我們以為是來自實體。
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.
燃燒創造出那感官體驗, 用的是燃料、熱,以及氧氣。 在營火中,當木料被加溫到 它們的點火溫度時, 它們的細胞壁會分解, 把糖以及其他分子釋放到空氣中。 這些分子接著會和空氣 傳播的氧氣發生作用, 創造出二氧化碳和水。 同時,木料中含有的任何水份 都會蒸發、擴張,並造成 它周圍的木頭破裂, 然後跑出來, 發出的令人滿足的霹啪聲。 隨著火的溫度上升,燃燒造成的 二氧化碳和水氣會擴張。 現在它們的密度比較低, 會以越來越細的柱狀形式上升。 地心引力造成這種擴張和上升, 所以火焰的特徵之一 就是頂端是尖細的。 若沒有地心引力, 分子不會因為密度而分開, 火焰的形狀就會完全不同。
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.
我們能看到這些, 是因為燃燒也會產生光。 分子被加熱時會發射出光, 光的顏色會依分子的 溫度而有所不同。 最熱的火焰是白色或藍色。 火中的分子類型也會 影響火焰的顏色。 比如,木料中若有 未作用的碳原子, 就會形成一叢一叢的煤煙, 上升到火焰中, 並發散出我們在想像營火時 會想到的橘黃色光。 像是銅、氯化鈣, 以及氯化鉀這些物質 會有它們自己獨特的顏色, 造成不同的混合色。
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.
除了多彩的火焰之外, 火一邊燒也會一邊持續產生熱。 這些熱就會讓燃料 保持在點火溫度之上, 進而維持著火焰。 不過,最終,再熱的火 也會燃料或氧氣用盡。 接著,那些扭曲的火焰就會 發出最後的嘶嘶聲, 隨著一縷煙消失, 好像從來沒有在那裡存在過一樣。