一、水分子的存在

????????水是生物體中含量最高的化合物，約占70％-90％。其中：水生比陸生多、幼年比老年多、男性比女性多。

????????自由水和結(jié)合水之間可相互轉(zhuǎn)換。

二、水分子的結(jié)構(gòu)

????????水分子中心原子為O，其價(jià)電子排布為2s22p4，sp3雜化，兩對(duì)孤電子分別占據(jù)四面體的兩個(gè)頂點(diǎn)，壓縮O—H鍵，使水分子整體呈V型。

????????O的電負(fù)性較大，電子云在O—H鍵中偏向O，使水分子具有較強(qiáng)的極性，易于形成氫鍵。水分子既是氫鍵的供體，又是氫鍵的受體。

三、水的性質(zhì)

注意這里是水的性質(zhì)，而不是水分子的性質(zhì)。

1.良好溶劑

????????水是良好溶劑，生物體中一切化學(xué)反應(yīng)均需在水中進(jìn)行，水分子亦作為底物或產(chǎn)物，參與許多反應(yīng)過(guò)程。

2.內(nèi)聚力與附著力

????????由于水中大量氫鍵的形成與斷裂具有協(xié)同性，使得水具有較強(qiáng)的內(nèi)聚力與附著力。

Eg：表面張力→某些昆蟲能夠在水面上行走，如水黽；毛細(xì)現(xiàn)象→植物將水分由根系向上運(yùn)輸。

3.比熱容高

????????由于水中含有大量氫鍵，使得水具有較大的比熱容，有利于維系氣候溫和與代謝穩(wěn)定。

4.冰密度小

????????水在4℃時(shí)密度最大，而0℃水在固化時(shí)，由于大量氫鍵由動(dòng)態(tài)轉(zhuǎn)化為固態(tài)，每一個(gè)水分子周圍形成4個(gè)氫鍵，與周圍水分子形成空間四面體，從而令體積增大，密度減小。這使得地球上總有液態(tài)水的存在，冰浮在水面形成隔離層，隔離層之下總有生命存在。

Water's properties

???? The polarity of water molecules results in hydrogen bonding- Hydrogen bonding is responsible for several of water's properties

???? polarity is the result of one atom in a molecule being much more electronegative than another, causing unequal sharing of the electrons; this creates a partial positive charge and a positive negative charge

???? Hydrogen bonds involve the partial positive charge of one molecule being attracted to the partial negative charge of another molecule.

???? The atoms that are electronegative enough to form partial charges with hydrogen include Nitrogen, Oxygen, and Fluorine

???? Four Important properties of water; cohesion/adhesion, specific heat, ice density, solvent properties

???? Cohesion; The hydrogen bonding holds water close together.

·?????? In plants, this cohesion allows water to move against gravity. Important for moving resources from roots to other parts of plants

·?????? Surface tension- Cohesion also makes it difficult to break the surface of water; this is exemplified in nature in some animals that can walk on water

·?????? Adhesion is the attraction of water to other substances; this includes water's adherence to cell walls which also contributes to water being able to move against gravity

???? Specific Heat/Temperature Moderation

·?????? Specific heat of a substance is the amount of heat that is needed for 1 g of a substance to change temperature by 1 degree Celsius. Measured in calories. 1000 calories is 1 Calorie/kcal.

·?????? Water's hydrogen bonds mean it takes a lot of energy to change water's temperature; this gives water a high specific heat. Thus water moderates temperature within limits that are fit for life.

·?????? Water also has a high heat of vaporization; amount of heat needed to concert 1 g of gas to liquid. This results in more temperature moderation.

·?????? Temperature moderation occurs on large and small scales. Earth's climate is moderated by water, especially along coastal regions. It also occurs on individual scale. We, for example, sweat to take advantage of water's properties to cool off. This uses evaporative cooling; hottest molecules convert to gas first and take energy away from a surface, thus cooling it.

???? Ice Density: Ice insulates bodies of water

·?????? Water is less dense as a solid than as a liquid; this is very unusual and results in ice floating in water. (Again, this is caused by hydrogen bonding).? If this were not the case, bodies of water like lakes and ponds would freeze solid. Due to this unusual property however, only a top layer of water is frozen, allowing organisms to continue living in the water.

???? Solvent Properties

·?????? Like dissolves like; polar dissolves polar and nonpolar dissolves nonpolar

·?????? Water is very polar and thus it is very good at dissolving other polar molecules and charged molecules.

·?????? Water acts as the main solvent for organisms. Blood for example, is mostly water. Cells, nutrients, and other such molecules exist within the context of this water.

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