水的重要性
水的重要性眾所周知8。如果沒有水，生命將不復存在6，水實際上是對我們健康最重要的營養素9。您可以不進食一個月左右，但是三天不喝水便會致命。它用以維持體溫，吸收營養，消除廢物10以及參與許多細胞過程，包括酶催化11。水在成人的平均重量為55％至75％，跟據性別和身材各有不同。良好的身體水合作用對細胞水合作用和細胞功能非常重要12,13，影響通訊，信號傳導，細胞代謝，基因表達等。體內大約有三分之二的水存在細胞內，大約三分之一存在細胞外。
水分子以104.45°彎曲19，這是因為氧原子上的兩個孤對電子（圖中黑點）具有電子排斥力，將兩個氫原子推近。亦因為氧原子具有這些電子，所以它具有高電負性，並具有部分負電荷19。氫原子具有部分正電荷，使水分子成為極性分子，俱有獨特而重要的化學性質，例如毛細作用，溶解離子鹽，高沸點等。
水的質量只有18克/摩爾，雖然很輕，但水是液體而不是氣體。以元素Radon為例，它是稀有氣體之一，但摩爾質量為222克/摩爾。雖然它的重量是水分子的12倍以上，但Radon在室溫下仍是氣體，而較輕的水是液體，這是氫鍵導致的原因20。
由於氧原子帶有部分負電荷，氫原子帶有部分正電荷，因此兩個相鄰的水分子會被吸引在一起。如圖中所示，這是極性分子之間的電磁吸引力相互作用21。每個水分子可以最多與四個水分子22形成氫鍵。但氫鍵很弱，並且會毫微微時標內（即10-15秒）與其他水分子重新排列鍵合23，使水簇在大量水中無法形成24。水分子的氫鍵與水離子化亦有關係，形成氫氧化物（OH-）和水合氫離子（H3O+）。這時水會變成酸性或鹼性25。如下圖所示，一個水分子可以將氫原子由另一個水分子中拉出，從而產生兩個離子。
水：氧氣+氫氣=生命
生命中必要的水3是由氧氣20，一種強大的氧化劑27，但是生命必不可缺的和氫氣20，一種強大的還原劑28，是生命必不可缺的29所結合形成，所以解釋了氫分子具有治療潛力30。
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