on the surface of a pond,

能夠在池塘水面上行走

but you quickly sink to the bottom

可是你想嘗試在水面上行走

when you try to walk on water?

卻會噗通沉下去？

And why do lakes freeze from the top down in winter?

為什麼冬天湖面會先結冰？

In a word, the answer to all these questions is polarity.

簡單的說，答案就是「極性」

Water is a simple molecule

水是一個簡單的分子

made up of one oxygen atom and two hydrogen atoms,

由一個氧和兩個氫原子組成

but it is essential to life.

但水是生命不可或缺的要素

In fact, water makes up approximately 60%

事實上，水佔了將近

of the adult human being's body weight.

成人 60% 的身體重量

The polarity within those water molecules

水分子間的極性

gives this common substance the properties

賦予這常見的物質

that make it unique and life-sustaining.

得以維持生命的一些獨特性質

Polarity refers to the unequal sharing

極性指的是分子中

of electrons within a molecule.

電子的不平均分配

For water, the bonding between the oxygen atom

一個水分子裡面

and two hydrogen atoms within a single water molecule

氧原子和兩個氫原子之間的鍵結

is like a tug-of-war between a big, strong football player

就像是在拔河 一邊是又高又壯的美式足球球員

and a cute little toddler.

另一邊是蹣跚學步的小孩

Oxygen is a larger atom,

氧原子是一個大原子

with more protons in its nucleus than hydrogen.

原子核裡的質子比氫原子更多

These positive charges are like a person's physical strength.

這些正電荷好像人的力氣

They're able to attract

可以吸引

the negatively charged electrons in the bond,

鍵結中帶負電的電子

just like a strong individual is able to overpower

就像是拔河比賽中的強者

a weaker individual in a tug of war.

可輕鬆把弱者拉過來

So, oxygen is able to attract

氧原子會吸引電子

more than its fair share of electrons.

比本身原有的電子還多

Because hydrogen is smaller and has less strength,

因為氫原子個子較小、力量較弱

or fewer protons,

也就是質子比較少

it loses the tug of war

在拔河比賽成了輸家

and attracts fewer than its fair share of electrons.

因而吸引電子的量，比原有的還少

So, the oxygen in water behaves as though it's negative,

所以水分子裡的氧，就像帶了負電

and the hydrogens behave as though they're positive.

而氫就像帶了正電一樣

The bonds within a single water molecule

水分子中的這種鍵結

are called polar covalent bonds.

就叫做「極性共價鍵」

Covalent means that the electrons are shared.

共價的意思是，電子由彼此共享

But, as we just learned,

而我們剛學過

polar means that these electrons are not shared equally.

極性的意思是，電子分配不均

In water, the oxygen acts negative

水分子中，氧就像帶負電

and the hydrogens act positive.

氫就像帶正電

Since negative and positive attract,

因為正負電會相吸

that oxygen is attracted to the hydrogen atoms

氧原子也會吸引

in neighboring water molecules.

鄰近水分子裡的氫原子

A special type of bond forms between water molecules,

水分子間形成了一種特殊的鍵結

known as a hydrogen bond.

稱為「氫鍵」

Hydrogen bonds don't just happen in water either.

氫鍵並不只出現在水分子間

They can form between a water molecule

也會出現在水分子和

and different substances that are polar or ionic.

有極性、或離子化的物質之間 （糖－極性）　（牙膏－離子化）

Water's ability to stick to itself is called cohesion,

水會黏在一起的能力叫「內聚力」

while water's ability to stick to other substances

而水附著在其他物質的能力

is called adhesion.

叫做「附著力」

Now, think back to the initial questions.

現在回想一下最初的問題

First, why are some insects able to walk on water?

為何有些昆蟲可在水面行走？

Surface tension due to hydrogen bonding

由氫鍵造成的表面張力

creates a thin film on the surface of water

在水面形成一層薄膜

that gives enough resistance

提供了足夠的阻力

for super-light insects to walk on.

讓很輕的昆蟲可以行走

You can't walk on it

我們沒辦法這樣

because the hydrogen bonds aren't strong enough

因為氫鍵沒有強到

to hold you up.

可以支撐我們的體重

Why does ice float on top of liquid water?

為什麼冰會浮在水面上呢？

For most other substances,

對大部份物質來說

the solid state is more dense than the liquid state,

固態的密度比液態更高

but that is not the case for water!

但水是個例外！

Hydrogen bonds keep water molecules

水分子的氫鍵

farther apart in frozen water than in liquid water.

在水固態時的距離比液態更遠

The farther apart the molecules are,

分子之間的距離愈遠

the less dense that solid is.

固體的密度就愈小

So ice is about 9% less dense than water,

冰的密度比水低了 9%

which means it floats on the top.

這表示冰會浮在水上

That's why lakes freeze from the top down

因而湖面會從表面開始結冰

and aquatic life is able to survive

讓水中生物可以

through a cold winter every year.

熬過每年寒冬

It is the polarity of the water molecule

水分子的極性

and the resulting hydrogen bonding

產生了氫鍵

that account for water's unique properties.

造成了水的獨特性質

So, the reason that water is so special,

水之所以這麼特別

from inside your cells

小至細胞

to the world's oceans,

大至海洋

is simply because it is a polar molecule.

就因為它是個極性分子