您所在的位置:黃岡中學廣州學校 > 師生園地 > 語言文字工作專欄 > > 【國際部•教師科普】暢談雪花
                【國際部•教師科普】暢談雪花

                *此文為科普文,其中有部分內容借鑒相關書籍、資料

                 

                近日,傳來一則神奇的喜訊——南方大部分地區都降雪啦!

                雪花對于長期在陽光雨露之下成長的南方的孩子來說是新鮮的。同學們熱烈地討論著,甚至還存在著小小的期盼與幻想:廣州會不會也有機會下雪呢?如果真有那么一天,我們一定要打雪仗堆雪人!





                不過,這漫天飛舞的雪花這么好看,它究竟是如何形成的呢?這其中就涉及到很多化學知識。

                雪花是一種美麗的晶體,又名未央花和六出,是由雪晶互相碰并、粘合和鉤連在一起而形成的雪晶聚合物。目前,已知的雪花形狀大約有兩萬多種,可以說是形形色色,絢麗多姿。




                雪花的形成,首先要具備兩個條件:一是水汽飽和,另一個條件就是空氣里必須有凝結核。只有空氣中的水蒸氣附在凝結核上凝聚成水滴,才能形成降雨和降雪。

                凝結核(condensation nuclei),是物質由氣態轉化為液態或固態,或由液態轉化為固態的凝結過程中,起凝結核心作用的顆粒。這些顆粒對水分子格外具有“吸引力”,在水汽接近飽和的空氣中,水分子遇上它們,便會附著上去,并迅速聚集起來,凝結成小水滴。所以說,我們看到的雪花其實都是有“核”的。

                 

                雪花的形狀基本上以六角形為主,如六角枝狀、六角片狀等,由它們變化而來的雪花也多呈現出“六角”的特征。古人很早就發現雪花呈現六邊形,所謂“草木之花多五出,獨雪花六出”。那么,這是什么原因形成的呢?

                這主要是和水的結晶習性有關。當水結成冰的時候,由于水分子中的氫鍵作用,會有序排列形成具有一定剛性的六方形晶體結構。天然水凍結的冰和大氣中水汽凝華結晶的雪,都屬于六方晶系。其實,六方晶系就像是幾何學上的一個正六面柱體。當水汽凝華結晶的時候,如果主晶軸比其它三個輔軸發育緩慢,并且較短,那么,雪的形狀就成為六角形雪片;要是主晶軸發育很快,延伸較長,那么,雪的形狀就成為一個六個側面,一個上面和一個下面的六棱柱體。

                正是由于水分子通過化學鍵結合成六邊形的網狀結構。隨著氣溫的降低,新生的分支尖端開始變窄,并出現側向分支。冰晶遭受了交替的冷暖氣流,就會出現更多的分支,這時晶體變得又長又細。當晶體降落到更暖和的空氣中,就不再生長,尖端卻越來越寬闊,最終形成了六瓣雪花的形態。



                雪花——這冬日飛舞的精靈,呈現出千姿百態、瞬息萬變的美麗景象。

                 

                不過,在北方銀裝素裹的精致固然美麗,卻也為交通帶來了不便。于是,人們通常會將融雪劑撒在路面上以除冰除雪。那么融雪劑究竟是什么物質?它又是如何實現融雪化冰的呢?

                融雪劑,主要成份是醋酸鉀和氯鹽等化學品。當它溶于水后,相對來說,水中離子濃度會上升,從而使水的液相蒸氣壓比飽和時的蒸汽壓降低,但冰的固態蒸氣壓不變。為了使冰水混和物固液蒸氣壓相等,冰就開始融化了,其本質就是使冰雪的熔點降低。

                目前,融雪劑主要分為兩大類:一類是以“氯鹽”為主要成分的無機融雪劑,也叫“化冰鹽”,如氯化鈉(NaCl)、氯化鈣(CaCl2)、氯化鎂(MgCl2)、氯化鉀(KCl)等。其中最常用的就是是氯化鈉(即工業鹽),其價格便宜,使用方便。但是,由于鹽類遇水發生鹽漲現象,會破壞道路路基;另外,由于鹽類融雪劑和路基上的鐵等金屬形成原電池,也會加快路面破損;而且,“氯鹽”還會增加道路兩旁農田的鹽堿性,造成植被難以生長。

                另一類融雪劑,則是以醋酸鉀為主要成分的有機融雪劑,這類融雪劑融雪效果好,基本不會造成腐蝕損害, 但其成本價格較高,一般只用于機場、高爾夫球場等重要場所。

                一片小小的雪花,竟凝聚了如此眾多的化學知識。這正應了那句詩:“一粒沙里看世界,半瓣花上說人情”。也許,化學的魅力正是體現在這些細微之處,生活之中。

                【文/國際部 顧雪雅老師】

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