[{"data":1,"prerenderedAt":73},["ShallowReactive",2],{"q-fire-112-1-fire-science-005":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-112-1-火災學概要-005","fire-112-1-fire-science-005",112,1,5,"鄰二甲苯的閃火點為 157℃，若鄰二甲苯於 157℃時的飽和蒸氣壓為 2.0 mmHg，試求其燃燒下限為何？",{"A":16,"B":17,"C":18,"D":19},"1.52%","2.0%","0.26%","0.52%","C",null,[],false,"本題考點：閃火點的物理定義——液體加熱至閃火點時，液面上方的飽和蒸氣濃度恰好等於燃燒下限，故可由該溫度下的飽和蒸氣壓與大氣壓之比，換算出燃燒下限的體積百分比。\n【正解理由】依閃火點定義與道爾頓分壓定律，理想氣體混合物中某成分的體積分率等於其分壓與總壓之比。題給 157℃（閃火點）時鄰二甲苯之飽和蒸氣壓為 2.0 mmHg,總壓取一大氣壓 760 mmHg,則此時液面上方的蒸氣體積濃度為 2.0÷760＝0.263%,而該濃度依定義即為燃燒下限，取兩位有效數字為 0.26%,故選 C。\n【逐項排除】\n(A) 1.52%:此值恰為 760×0.2%,是把百分率與分壓的角色顛倒所得，量級比正解大近六倍。\n(B) 2.0%:直接把飽和蒸氣壓的數字 2.0 當成百分率，漏掉除以總壓 760 mmHg 這一步換算，是本題最典型的陷阱。\n(C) 0.26%:與 2.0÷760＝0.263% 相符，為本題應選者。\n(D) 0.52%:恰為正解的兩倍，係把分壓誤乘 2 或把 380 mmHg 當成一大氣壓所致。\n【演算步驟】燃燒下限 LFL＝（閃火點時之飽和蒸氣壓 ÷ 大氣壓）×100%＝（2.0 mmHg ÷ 760 mmHg）×100%＝0.00263×100%＝0.263%≒0.26%。\n【記憶點】閃火點時的飽和蒸氣壓除以 760 mmHg,就是該液體的燃燒下限。","閃火點與燃燒下限的換算，是火災學計算題最常出現的一組關係。閃火點的定義是「液體表面所生蒸氣與空氣混合後，一遇火源即產生閃燃（一閃即熄）的最低液溫」,此時蒸氣濃度恰好抵達燃燒下限；若液溫再升高到蒸氣供應速率足以維持連續燃燒，才稱為著火點（燃點）,著火點通常比閃火點高數度至數十度。由於飽和蒸氣壓隨溫度呈指數上升（可由克勞修斯-克拉佩龍方程式描述）,閃火點越低的液體，常溫下蒸氣濃度就越接近下限，危險性越高。\n單位換算務必記牢:1 atm＝760 mmHg＝760 torr＝101.325 kPa＝1.0332 kgf\u002Fcm²＝約 10.33 m 水頭。若題目改以 kPa 給蒸氣壓，只要把總壓換成 101.3 kPa,算式形式不變:LFL＝P（飽和蒸氣壓）÷P（總壓）×100%。反向命題也很常見：給燃燒下限回推閃火點時的飽和蒸氣壓（例：下限 1.4% → 0.014×760＝10.6 mmHg）;若為兩種以上蒸氣的混合氣體，則改用勒沙特列公式 100÷LFL（混合）＝Σ（Ci÷LFLi）求混合下限。\n數值直覺可以這樣建立:760 mmHg 的 1% 是 7.6 mmHg,所以 7.6 mmHg 對應 1%、15.2 mmHg 對應 2%、2.0 mmHg 約為 0.26%。本題四個選項正是圍繞這組換算佈置陷阱，分別對應「漏除總壓」「反乘總壓」「分壓加倍」三種錯誤。\n易混淆概念也要一併分清：燃燒下限（LFL）與最小氧濃度（MOC）不同,MOC＝LFL×完全燃燒所需氧莫耳數；閃火點是液體的性質，自燃（發火）溫度則是不需明火即著火的溫度，兩者物理意義不同。相關考點還包括以閃火點區分我國第四類易燃液體的分級（特殊易燃物與第一至第四石油類）,以及蒸氣密度大於空氣時，蒸氣沿地面流動而擴大危害範圍的現象。",true,[28,32,36,40,44,48],{"webId":29,"stem":30,"number":31,"year":11,"session":12},"fire-112-1-fire-science-004","若空氣中的氧氣體積百分比為 21%，則甲烷完全燃燒的化學理論濃度為多少%？",4,{"webId":33,"stem":34,"number":35,"year":11,"session":12},"fire-112-1-fire-science-006","在標準狀態下，下列氣體中何者之爆炸範圍最廣？",6,{"webId":37,"stem":38,"number":39,"year":11,"session":12},"fire-112-1-fire-science-003","下列何者非為火災之特性？",3,{"webId":41,"stem":42,"number":43,"year":11,"session":12},"fire-112-1-fire-science-007","高壓氣體之爆炸界限，通常較常壓下為廣，下列何者氣體之爆炸界限，會呈現加壓時上、下限初呈狹窄，但繼續加壓後又變廣的現象？",7,{"webId":45,"stem":46,"number":47,"year":11,"session":12},"fire-112-1-fire-science-002","下列何者屬於準自然發火性物質？",2,{"webId":49,"stem":50,"number":51,"year":11,"session":12},"fire-112-1-fire-science-008","赤磷和黃磷分屬我國法定公共危險物品那一類？",8,[53,57,61,65,69],{"webId":54,"year":55,"stem":56,"number":13},"fire-114-1-fire-science-005",114,"火災煙毒氣體之危害主要是下列何種氣體？",{"webId":58,"year":59,"stem":60,"number":13},"fire-113-1-fire-science-005",113,"大樓火災常伴隨煙囪效應。所謂煙囪效應是指建築物內部受熱後熱氣流上升，冷空氣由下方填補，使得火災產生的煙霧會快速瀰漫整個建築物的現象。針對以上敘述，下列何者錯誤？",{"webId":62,"year":63,"stem":64,"number":13},"fire-111-1-fire-science-005",111,"在苯中置入油酸鎂等金屬鹼，係採取下列何種防止靜電發生的方法？",{"webId":66,"year":67,"stem":68,"number":13},"fire-110-1-fire-science-005",110,"已知乙烷、乙烯、乙炔的發火能量分別為 a、b、c，則 a、b、c 的大小關係為：",{"webId":70,"year":71,"stem":72,"number":13},"fire-109-1-fire-science-005",109,"下列有關二氧化碳（CO2）滅火劑之敘述，何者錯誤？",1785129966436]