[{"data":1,"prerenderedAt":73},["ShallowReactive",2],{"q-fire-108-1-fire-science-023":3},{"subject":4,"subjectSlug":5,"subjectKicker":6,"subjectShort":7,"question":8,"related":27,"sameNumber":52,"hasEssay":26},"火災學概要","fire-science","火災學 · Fire Science","火災學",{"id":9,"webId":10,"year":11,"session":12,"subject":4,"number":13,"stem":14,"options":15,"answer":20,"answerNote":21,"images":22,"imagesPending":23,"lawTimestamp":11,"explanation":24,"explanationDeep":25,"freq":12,"indexable":26},"fire-108-1-火災學概要-023","fire-108-1-fire-science-023",108,1,23,"乙醇在閃火點時的飽和蒸汽壓為 25 mmHg，則乙醇在常壓下的燃燒下限約為何？",{"A":16,"B":17,"C":18,"D":19},"3.29%","1.8%","5.28%","6.8%","A",null,[],false,"本題考點：閃火點的定義（液面蒸氣濃度恰達燃燒下限），並以道爾頓分壓定律把飽和蒸氣壓換算成體積百分比。\n【正解理由】依道爾頓分壓定律，混合氣體中某成分的體積百分比等於其分壓除以總壓；而閃火點的定義即液面蒸氣濃度恰好等於燃燒下限，故燃燒下限＝閃火點時的飽和蒸氣壓÷常壓（760 mmHg）。以 25 mmHg 代入得 3.29%，故選 A。\n【逐項排除】\n(A) 3.29%：25÷760＝0.0329，與題給的閃火點飽和蒸氣壓完全對應，也與乙醇燃燒範圍下限約 3.3% 相符，為正解。\n(B) 1.8%：須飽和蒸氣壓約 14 mmHg（0.018×760）才成立，與題給的 25 mmHg 不符。\n(C) 5.28%：須飽和蒸氣壓約 40 mmHg 才成立，該數值對應的液溫已明顯高於乙醇閃火點。\n(D) 6.8%：須飽和蒸氣壓約 52 mmHg 才成立，此濃度已落在乙醇燃燒範圍的中段，並非下限。\n【演算步驟】燃燒下限（%）＝閃火點之飽和蒸氣壓÷大氣壓×100%＝25 mmHg÷760 mmHg×100%＝0.03289×100%≈3.29%。\n【記憶點】閃火點的蒸氣壓除以 760 mmHg，算出來的就是燃燒下限。","閃火點的嚴謹定義是：液體表面上方的蒸氣與空氣混合氣，恰好達到燃燒下限而能被引火源點燃的最低液溫。由這個定義出發，只要知道閃火點溫度下的飽和蒸氣壓，就能用道爾頓分壓定律把「壓力」換算成「體積百分比」——氣相中乙醇的莫耳分率等於分壓除以總壓，理想氣體下莫耳分率即體積分率，故下限為 25÷760＝3.29%。乙醇的閃火點約在 13℃ 附近，教科書所列燃燒範圍約 3.3～19%，與本題算得的下限一致，可互相驗證。\n單位換算務必熟練：1 atm＝760 mmHg＝101.3 kPa＝1.033 kgf\u002Fcm²≈10.33 m 水頭。若題目改給 kPa，只要把分母換成 101.3 kPa，公式完全不變。同一邏輯反過來用在燃燒上限：液體到達沸點時飽和蒸氣壓等於大氣壓、蒸氣濃度達 100%，故沸點以上必然超出燃燒上限而過濃。飽和蒸氣壓與溫度的關係由 Antoine 方程式描述，溫度升高時蒸氣壓呈指數成長，這說明為何閃火點只要相差幾度，危險程度差距就很明顯。\n易混淆對照：閃火點是「有火源才燒、移開即熄」，燃點（著火點）是「移開火源仍能持續燃燒」，發火點（自燃溫度）則是「不需火源即自行起火」，三者溫度依序升高。延伸考點包括以勒沙特列公式求混合蒸氣的燃燒下限、由閃火點反推密閉空間形成爆炸性混合氣的最低環境溫度，以及第四類公共危險物品依閃火點高低分級的邏輯——閃火點愈低，代表只要很少的加熱就能讓液面蒸氣達到可燃濃度。",true,[28,32,36,40,44,48],{"webId":29,"stem":30,"number":31,"year":11,"session":12},"fire-108-1-fire-science-022","物體在 327℃時的輻射強度為 E，則當溫度升高到 927℃時的輻射強度為何？",22,{"webId":33,"stem":34,"number":35,"year":11,"session":12},"fire-108-1-fire-science-024","下列有關燃燒上下限之敘述，何者錯誤？",24,{"webId":37,"stem":38,"number":39,"year":11,"session":12},"fire-108-1-fire-science-021","依據建築物火災 t2 成長理論，在一定義為快速成長的火災中，釋熱率達到 1MW 的時間應為多少秒？",21,{"webId":41,"stem":42,"number":43,"year":11,"session":12},"fire-108-1-fire-science-025","木材受熱分解進而燃燒的過程中，當加熱到引火溫度時，可燃性氣體釋出量迅速增加，試問木材的引火溫度約為多少℃？",25,{"webId":45,"stem":46,"number":47,"year":11,"session":12},"fire-108-1-fire-science-020","下列何種滅火劑所含成分之一，於放射後有刺激呼吸之作用，可有效降低場內人員的窒息危害？",20,{"webId":49,"stem":50,"number":51,"year":11,"session":12},"fire-108-1-fire-science-026","乙炔加壓後容易產生分解爆炸，為避免此現象，乙炔鋼瓶常利用浸泡何種溶劑之多孔性物質，使乙炔溶解於其中，防止分解爆炸？",26,[53,57,61,65,69],{"webId":54,"year":55,"stem":56,"number":13},"fire-111-1-fire-science-023",111,"建築物起火後，對於防止火焰由下層窗戶往上層延燒，下列敘述何者錯誤？",{"webId":58,"year":59,"stem":60,"number":13},"fire-110-1-fire-science-023",110,"請問 2 公斤的氫氣（H2）完全燃燒，所需要的理論空氣量為多少公斤？",{"webId":62,"year":63,"stem":64,"number":13},"fire-109-1-fire-science-023",109,"若要看清某一物體，該物體與其背景間有一定之對比度。對比度之計算：C=(B\u002FB0)-1，有關對比度下列何者正確？",{"webId":66,"year":67,"stem":68,"number":13},"fire-107-1-fire-science-023",107,"熱傳導之熱流動方向為何？",{"webId":70,"year":71,"stem":72,"number":13},"fire-106-1-fire-science-023",106,"燃燒範圍是研判物質物性之重要因素，今有某液化石油氣，其成分為丙烷（C3H8 ）40%燃燒範圍為 2.4%~9.5%、丁烷（C4H10）60%燃燒範圍為 1.8%~8.4%，試求該液化石油氣之燃燒下限為何？",1785123163271]