中国科技网10月13日报道（张微 编译）它是宇宙中第二最丰富的物质。它比任何其他溶剂都能溶解更多的物质。它还能存储大量的能量。如果没有它，生命就不可能存在。尽管它覆盖地球表面70%以上的面积，世界许多地区却因为缺乏它而陷入困境。它是什么?

答案当然是水。在某些方面，水是我们最了解的物质，因为人体的组成75%是水分。我们每天都喝水，用水洗澡，用它做清洁，用它来处理废物。然而，科学家们仍然在努力了解有关水的非凡特性，21世纪将迫使我们去思考我们之前没有认真研究过的水。

是什么让水如此引人注目?

对于人类历史来说，水与空气，土和火一道被认为是四大元素之一。在18世纪，化学家安托万 拉瓦锡在水中通电，使水释放出两种气体：氢(字面意思“造水器”)和氧。

水的化学公式是H2O——两个氢原子和一个氧原子。水最显著的特性之一是，在带负电荷的氧和带正电荷的氢之间不断地组合以及重新组成易形成的氢键。由于这些氢键，水分子间相互吸引力远远超过其他物质。

这些氢键给了水非常高的比热，这意味着需要大量的能量才能加热水。比起和它化学上类似的分子，如硫化氢，水也有一个异乎寻常的高沸点。这些特性使得人类在运动中通过出汗，可以消耗大量的热量。

氢键的另一个重要作用是毛细管作用，液体附着在画笔的毛刷上就是毛细管作用。水分子间的相互吸引力非常强，它们可以抗拒引力的力量。一棵树上最高处的叶子上水分蒸发了，那么它就能把根部以下土壤里的水分吸上来。

氢键还有一个作用就是水的表面张力。这使得水能够形成水滴，使昆虫在水面上行走。水的表面张力非常强大，早产儿如果缺乏表面活性剂(表面活性剂能降低水的表面张力)就会引起肺部感染。幸运的是，表面活性剂目前已经成为临床药物。

水拥有轻微和正电荷和负电荷双极，使得水成为“万能溶剂”，能够完全溶解盐、糖、酸类，碱以及像二氧化碳这样，在苏打水中吱吱作响的气体。这些物质被称为亲水物质，因为它们在水中非常容易溶解。

相比之下，脂肪和油被归类为疏水物质，因为它们的末端没有电荷。因此，它们之间的吸引力比和水分子的吸引力强。从我们的手上或衣服上洗去这些物质，需要肥皂，因为肥皂具有疏水端和亲水端，有助于把它们分解成能够被水冲走的微小液滴。

从一种状态到另一种状态

更加奇特的是，水是目前人类已知的唯一一种物质，当它冷却时能够从液态变成固态，并伴随体积膨胀(这违背了热胀冷缩的常规)。几乎所有的其他物质会因为“冻结”导致密度变大，而水却不是这样，正是由于这一特性，冰块才能漂浮在我们的饮料上。更重要的对于生物体、湖泊和其他水体来说，冰冻过程是从上到下进行的。

冰的密度非常低是因为，水分子需要热量来维持液态水状态时的随机结构、当温度降低时，水分子开始排布导致成一种敞开结构，也就是说冰的结构中有较大的空隙，所以冰的密度反比同温度的水低9%左右。

俗话说没有两片雪花是完全一样的，这似乎很难让人相信，但是如果你考虑到水分子结冰的方式取决于温度和湿度因素时，就不难理解了。平均每个雪晶大概包含10万兆(10后面有18个零)个水分子，你就会知道组合的数量大到难以想象。

一个连续的循环

水有巨大的活力，以蒸发、凝结、沉淀和径流回到海洋和湖泊的循环方式，不断地在地球上运动。在生物体中的水也是这样，组成水的氢和氧，通过光合作用和呼吸过程，不断地组合和重组。

虽然没有水我们没法生活，但是也可以说，我们是水的生产者。我们每次分解一个葡萄糖分子，就生产了6个水分子，这个发生在人体内的反应高达6百万的7乘方次/每天(6后面跟着24个零)。即便如此，我们产生的水也不够满足自身需求。

尽管美国西部的干旱在今天获得了相当大的关注，但是在本世纪，水很可能成为一个更加热门的话题。首先，地球上只有约3%的水是淡水，其余97%的水存在于海洋中。而且大约70%的淡水存在与南极洲的冰川和冰盖中。

我们这蓝色的地球所有的水资源都汇集到一起，能形成一个直径860英里(约1384千米)的水泡，但只有其中的一小部分能够为人类所用，而且这部分可用水还在日益短缺。一些科学家预言，在21世纪的某个时候，淡水将成为比石油更值钱的商品。

有一句话经常被错当成是阿尔伯特 爱因斯坦所说的，活着有两种状态。第一个是，好像没有什么是奇迹，第二是，仿佛一切都是奇迹。水完全是天然的，非常丰富的，也是我们的生命所必须的，因为人体细胞浸润在水中。而且它也是非同寻常的，作为一名医生和科学家，我认为水简直就是一个奇迹。

英文原文：

The universe's most miraculous molecule

It's the second most abundant substance in the universe. It dissolves more materials than any other solvent. It stores incredible amounts of energy. Life as we know it would not be possible without it. And although it covers more than 70% of the Earth's surface, many parts of the world are in dire straits for lack of it. What is it?

The answer, of course, is water. In some ways, water is one of the substances we know best, in part because it makes up 75% of our bodies. Every day we drink it, bathe in it, clean with it and use it to dispose of our wastes. Yet scientists are still striving to understand many of water's remarkable properties, and the 21st century will force us to think about water like we never have before.

What makes water so remarkable?

For most of human history, water was considered to be one of the four elements, along with air, earth and fire. It was only in the 18th century that chemist Antoine Lavoisier passed an electrical current through water and realized that it gives off two gases: hydrogen (literally, "water maker") and oxygen.

The formula of water is H2O – two atoms of hydrogen and one of oxygen. One of water's most remarkable properties is traceable to the hydrogen bonds that continually form and reform between its slightly negatively charged oxygen and slightly positively charged hydrogen components. Thanks to these bonds, water molecules attract one another far more strongly than those of almost any other substance.

These hydrogen bonds give water a very high specific heat, meaning that it takes a great amount of energy to warm it. It also has a remarkably high boiling point compared to other chemically similar molecules, such as hydrogen sulfide. These properties enable human beings to dissipate large amounts of heat during exercise by perspiring.

Another consequence of hydrogen bonding is capillary action, which occurs, for example, when a liquid is drawn up between the hairs of a paintbrush. The water molecules attract one another so strongly that they defy the force of gravity. When water evaporates from the highest leaves of a tree, it draws up other water molecules from the roots far below.

Still another consequence of hydrogen bonding is water's high surface tension. This accounts for its tendency to form droplets and enables some insects literally to walk on water. This force can be so strong that premature infants, who lack surfactant, a substance that lessens it, can become exhausted just trying to inflate their lungs. Fortunately, surfactant is now available as a medication.

The fact that water has slightly positively and negatively charged poles also makes it the "universal solvent," perfect for dissolving salts, sugars, acids, alkalis and even gases such as carbon dioxide, accounting for the fizz in sodas. Such substances are known as hydrophilic (water-loving), precisely because they dissolve so easily in water.

By contrast, fats and oils are classified as hydrophobic, because they do not have electrical charges at their ends. As a result, they are attracted more strongly to one another than to water. To wash such substances from our hands or clothes, we need soaps, which have both hydrophobic and hydrophilic ends that help break them up into tiny droplets that can be carried away by water.

From one state to another

Even more remarkably, water is practically the only substance known to man that, as it cools from its liquid to solid state, actually expands. Virtually every other substance becomes denser as it "freezes," but thanks to this remarkable property, ice cubes float in our drinks. More importantly for living organisms, lakes and other bodies of water freeze from the top down.

Ice's remarkably low density is attributable to the fact that water molecules need thermal energy to maintain the random orientations they assume in liquid water. As the temperature decreases, the molecules begin to line up in a regular latticework. To do so, however, the distance between them must increase. As a result, ice is about 9% less dense than liquid water.

The adage that no two snowflakes are alike seems hard to believe until you consider the fact that the patterns in which water molecules freeze vary depending on temperature and humidity. When you add the fact that the average snow crystal contains about 10 quintillion (10 followed by 18 zeroes) water molecules, it is easy to see why the number of possible combinations is unimaginably large.

A continuous cycle

Water is also incredibly dynamic, continuously moving all over the Earth in a cycle of evaporation, condensation, precipitation and runoff back to seas and lakes. The same is true among living organisms, where the hydrogen and oxygen constituents of water are continually combining and recombining through the processes of photosynthesis and respiration.

And while we cannot live without water, it should also be said that we are water producers. Each time we break down a molecule of glucose, we produce six molecules of water, a reaction that takes place in the typical human body about six septillion (6 followed by 24 zeroes) times per day. Even so, we still don't produce enough water to meet our own needs.

Although droughts in the western US are garnering considerable attention today, it is likely that water will become an even hotter topic over the course of this century. For one thing, only about 3% of the Earth's water is fresh water, the other 97% being found in the oceans. And about 70% of this fresh water is found in glaciers and the ice caps of Antarctica.

As a result, even though the Earth holds enough water to make a sphere about 860 miles in diameter, only a tiny percentage of this water is easily accessible to human beings, and increasing shortages loom in the future. Some scientists have predicted that, as some point in the 21st century, fresh water will become a more valuable commodity than petroleum.

A saying often misattributed to Albert Einstein claims there are two ways to lead a life. The first is as though nothing is a miracle, and the second is as though everything is a miracle. Water is entirely natural, hugely abundant and so necessary to life that our cells are bathed in it. Yet it is also so remarkable that, as a physician and scientist, I regard it as little short of miraculous.

来源：中国科技网