Life Depends on the Unique Properties of Waer生命取決于水的獨(dú)特性能

1、life depends on the unique properties of water4.4uall living things are dependent on water. uinside your body, your cells are surrounded by a fluid that is mostly water, and your cells themselves are 70 to 95 percent water. uthe abundance of water is a major reason earth can support life. ua water m

2、olecule at first may seem pretty simple. uits two hydrogen atoms are each joined to an oxygen atom by a single covalent bond. uthe key to waters unusual properties is that the electrons of each covalent bond are not shared equally between oxygen and hydrogen atoms. uoxygen pulls electrons much more

3、strongly than does hydrogen. upart of the reason is that the oxygen nucleus has eight protons, and therefore has a stronger positive charge than the hydrogen nucleus, which has one proton. uthis unequal pull results in the shared electrons spending more of their time in the neighborhood of the oxyge

4、n atom. uthe unequal sharing of electrons causes the oxygen end of the molecule to have a slight negative charge, while the end with the two hydrogen atoms is slightly positive. ua molecule in which opposite ends have opposite electric charges is called a polar molecule. uwater is a compound consist

5、ing of polar molecules. uwater molecules are attracted to one another in a specific way. uthe slightly negative oxygen end of one molecule attracts the slightly positive hydrogen ends of adjacent water molecules, causing the molecules to become arranged as you see in figure 4-12. uthis type of weak

6、attraction between the hydrogen atom of one molecule and a slightly negative atom within another molecule is a type of chemical bond called a hydrogen bond. ubecause the atoms within the water molecules have not transferred an electron (and thus a full unit of charge) to another atom, the attraction

7、 in a hydrogen bond is not as strong as that in an ionic bond. uthe polar nature of water and the effects of hydrogen bonding explain most of waters unique properties. uthese properties include cohesion and adhesion, temperature moderation, the lower density of ice compared to liquid water, and wate

8、rs ability to dissolve other substances. ueach hydrogen bond between molecules of liquid water lasts for only a few trillionths of a second. uyet, at any instant most of the molecules are involved in hydrogen bonding with other molecules because new hydrogen bonds form as fast as old ones break. uth

9、is tendency of molecules of the same kind to stick to one another is called cohesion. ucohesion is much stronger for water than for most other liquids. uwater molecules are also attracted to certain other molecules. uthe type of attraction that occurs between unlike molecules is called adhesion. ubo

10、th cohesion and adhesion are important in the living world. uone of the most important effects of these forces is keeping large molecules organized and arranged in a way that enables them to function properly in cells. utrees depend on cohesion and adhesion to help transport water from their roots t

11、o their leaves.uthe evaporation of water from leaves pulls water upward from the roots through narrow tubes in the trunk of the tree. uas a result of cohesion, water moves against the force of gravity even to the top of a very tall tree. uthermal energy is the total amount of energy associated with

12、the random movement of atoms and molecules in a sample of matter. utemperature is a measure of the average energy of random motion of the particles in a substance. uwhen two substances differ in temperature, thermal energy in the form of heat is transferred from the warmer substance to the cooler on

13、e. uwhen you heat a substancesuch as a metal pan or waterits temperature rises because its molecules move faster. ubut in water, some of the thermal energy that is absorbed goes to break hydrogen bonds. uthat doesnt happen in the metal pan, which has no hydrogen bonds. uas a result, the water absorb

14、s the same amount of thermal energy but undergoes less temperature change than the metal. uconversely, when you cool a substance, the molecules slow and the temperature drops. ubut as water cools, it forms hydrogen bonds. uthis releases thermal energy in the form of heat, so there is less of a drop

15、in temperature than in metal. uone result of this property is that it causes oceans and large lakes to moderate the temperatures of nearby land areas. uin other words, coastal areas generally have less extreme temperatures than inland areas. uwater also moderates temperature through evaporation, suc

16、h as when you sweat. uevaporation occurs when molecules at the surface of a liquid escape to the air. uas water molecules evaporate, the remaining liquid becomes cooler. udensity is the amount of matter in a given volume. ua high-density substance is more tightly packed than a low-density substance.

17、 uin most substances, the solid state is more dense than the liquid state. uwater is just the oppositeits solid form (ice) is less dense than the cold liquid form. uonce again, hydrogen bonds are the reason. ubecause the molecules in liquid water are moving faster than those in ice, there are fewer

18、and more short-lived hydrogen bonds between molecules. uthe liquid water molecules can fit more closely together than the molecules in ice. usince substances of lesser density float in substances of greater density, ice floats in liquid water.ice floats because its molecules are less densely packed

19、than those in liquid water. uhow is the fact that ice floats important to living things? uif ice sank, it would form on the bottom of a body of water as the water was cooling. uponds and lakes would freeze from the bottom up, trapping the fish and other organisms in a shrinking layer of water withou

20、t access to the nutrients from the muddy bottom. uwhen you stir table salt into a glass of water, you are forming a solution, a uniform mixture of two or more substances. uthe substance that dissolves the other substance and is present in the greater amount is the solvent (in this case, water). uthe

21、 substance that is dissolved and is present in a lesser amount is the solute (in this case, salt). uwhen water is the solvent, the result is called an aqueous solution (from the latin word water). sodium chloride dissolves as na+ and cl- ions become attracted to water molecules and break away from t

22、he surface of the solid. uwater is the main solvent inside all cells, in blood, and in plant sap. uwater dissolves an enormous variety of solutes necessary for life. figure 4-16 illustrates how water dissolves ionic compounds such as table salt (sodium chloride). uin aqueous solutions, a very small

23、percentage of the water molecules themselves break apart into ions. uthe ions formed are positively charged hydrogen ions (h+) and negatively charged hydroxide ions (oh-). usome chemical compounds contribute additional h+ ions to an aqueous solution while others remove h+ ions from it. ua compound t

24、hat donates h+ ions to a solution is called an acid. uan example is hydrochloric acid (hcl), the acid in your stomach. uin an aqueous solution, hydrochloric acid breaks apart completely into h+ and cl- ions. ua compound that removes h+ ions from an aqueous solution is called a base. usome bases, suc

25、h as sodium hydroxide (naoh), do this by adding oh- ions, which then combine with h+ ions and form water molecules. uthe ph scale describes how acidic or basic a solution is. the scale ranges from 0 (most acidic) to 14 (most basic).ueach ph unit represents a tenfold change in the concentration of h+

26、 ions. ufor example, lemon juice at ph 2 has 10 times more h+ ions than an equal amount of grapefruit juice at ph 3. upure water and aqueous solutions that have equal amounts of h+ and oh- ions are said to be neutral. uthey have a ph of 7 and are neither acidic nor basic. uthe ph of the solution inside most living cells is close to 7. a solution having a ph of 7 is neutral. many fruits have ph values less than 7,