Life Depends on the Unique Life Depends on the Unique Properties of WaterProperties of Water

4.44.4

All living things are dependent on All living things are dependent on water. water.

Inside your body, your cells are Inside your body, your cells are surrounded by a fluid that is mostly surrounded by a fluid that is mostly water, and your cells themselves are water, and your cells themselves are 70 to 95 percent water. 70 to 95 percent water.

The abundance of water is a major The abundance of water is a major reason Earth can support life. reason Earth can support life.

The Structure of WaterThe Structure of Water

A water molecule A water molecule at first may seem at first may seem pretty simple. pretty simple.

Its two hydrogen Its two hydrogen atoms are each atoms are each joined to an joined to an oxygen atom by a oxygen atom by a single covalent single covalent bond. bond.

The key to water's unusual The key to water's unusual properties is that the electrons of properties is that the electrons of each covalent bond are not shared each covalent bond are not shared equally between oxygen and equally between oxygen and hydrogen atoms. hydrogen atoms.

Oxygen pulls electrons much more Oxygen pulls electrons much more strongly than does hydrogen. strongly than does hydrogen.

Part of the reason is that the oxygen Part of the reason is that the oxygen nucleus has eight protons, and nucleus has eight protons, and therefore has a stronger positive therefore has a stronger positive charge than the hydrogen nucleus, charge than the hydrogen nucleus, which has one proton. which has one proton.

This unequal pull results in the This unequal pull results in the shared electrons spending more of shared electrons spending more of their time in the "neighborhood" of their time in the "neighborhood" of the oxygen atom. the oxygen atom.

The unequal sharing of electrons causes The unequal sharing of electrons causes the oxygen end of the molecule to have a the oxygen end of the molecule to have a slight negative charge, while the end with slight negative charge, while the end with the two hydrogen atoms is slightly the two hydrogen atoms is slightly positive. positive.

A molecule in which opposite ends have A molecule in which opposite ends have opposite electric charges is called a opposite electric charges is called a polar moleculepolar molecule. .

Water is a compound consisting of polar Water is a compound consisting of polar molecules. molecules.

Water molecules are attracted to one Water molecules are attracted to one another in a specific way. another in a specific way.

The slightly negative oxygen end of The slightly negative oxygen end of one molecule attracts the slightly one molecule attracts the slightly positive hydrogen ends of adjacent positive hydrogen ends of adjacent water molecules, causing the water molecules, causing the molecules to become arranged as molecules to become arranged as you see in Figure 4-12. you see in Figure 4-12.

This type of weak attraction between the This type of weak attraction between the hydrogen atom of one molecule and a hydrogen atom of one molecule and a slightly negative atom within another slightly negative atom within another molecule is a type of chemical bond called molecule is a type of chemical bond called a a hydrogen bondhydrogen bond. .

Because the atoms within the water Because the atoms within the water molecules have not transferred an molecules have not transferred an electron (and thus a full unit of charge) to electron (and thus a full unit of charge) to another atom, the attraction in a hydrogen another atom, the attraction in a hydrogen bond is not as strong as that in an ionic bond is not as strong as that in an ionic bond. bond.

Water's Life-Supporting Water's Life-Supporting PropertiesProperties

The polar nature of water and the The polar nature of water and the effects of hydrogen bonding explain effects of hydrogen bonding explain most of water's unique properties. most of water's unique properties.

These properties include cohesion These properties include cohesion and adhesion, temperature and adhesion, temperature moderation, the lower density of ice moderation, the lower density of ice compared to liquid water, and compared to liquid water, and water's ability to dissolve other water's ability to dissolve other substances. substances.

Cohesion and AdhesionCohesion and Adhesion

Each hydrogen bond between Each hydrogen bond between molecules of liquid water lasts for molecules of liquid water lasts for only a few trillionths of a second. only a few trillionths of a second.

Yet, at any instant most of the Yet, at any instant most of the molecules are involved in hydrogen molecules are involved in hydrogen bonding with other molecules bonding with other molecules because new hydrogen bonds form because new hydrogen bonds form as fast as old ones break. as fast as old ones break.

This tendency of molecules of the same This tendency of molecules of the same kind to stick to one another is called kind to stick to one another is called cohesioncohesion. .

Cohesion is much stronger for water than Cohesion is much stronger for water than for most other liquids. for most other liquids.

Water molecules are also attracted to Water molecules are also attracted to certain other molecules. certain other molecules.

The type of attraction that occurs between The type of attraction that occurs between unlike molecules is called unlike molecules is called adhesionadhesion. .

Both cohesion and adhesion are Both cohesion and adhesion are important in the living world. important in the living world.

One of the most important effects of One of the most important effects of these forces is keeping large these forces is keeping large molecules organized and arranged in molecules organized and arranged in a way that enables them to function a way that enables them to function properly in cells. properly in cells.

Trees depend on cohesion and adhesion to Trees depend on cohesion and adhesion to help transport water from their roots to help transport water from their roots to their leaves.their leaves.

The evaporation of water from leaves pulls The evaporation of water from leaves pulls water upward from the roots through water upward from the roots through narrow tubes in the trunk of the tree. narrow tubes in the trunk of the tree.

As a result of cohesion, water moves As a result of cohesion, water moves against the force of gravity even to the top against the force of gravity even to the top of a very tall tree. of a very tall tree.

Temperature ModerationTemperature Moderation

Thermal energyThermal energy is the total amount of is the total amount of energy associated with the random energy associated with the random movement of atoms and molecules in a movement of atoms and molecules in a sample of matter. sample of matter.

TemperatureTemperature is a measure of the average is a measure of the average energy of random motion of the particles energy of random motion of the particles in a substance. in a substance.

When two substances differ in When two substances differ in temperature, thermal energy in the form temperature, thermal energy in the form of heat is transferred from the warmer of heat is transferred from the warmer substance to the cooler one. substance to the cooler one.

When you heat a substance—such as a When you heat a substance—such as a metal pan or water—its temperature rises metal pan or water—its temperature rises because its molecules move faster. because its molecules move faster.

But in water, some of the thermal energy But in water, some of the thermal energy that is absorbed goes to break hydrogen that is absorbed goes to break hydrogen bonds. bonds.

That doesn't happen in the metal pan, That doesn't happen in the metal pan, which has no hydrogen bonds. which has no hydrogen bonds.

As a result, the water absorbs the same As a result, the water absorbs the same amount of thermal energy but undergoes amount of thermal energy but undergoes less temperature change than the metal. less temperature change than the metal.

Conversely, when you cool a Conversely, when you cool a substance, the molecules slow and substance, the molecules slow and the temperature drops. the temperature drops.

But as water cools, it forms hydrogen But as water cools, it forms hydrogen bonds. bonds.

This releases thermal energy in the This releases thermal energy in the form of heat, so there is less of a form of heat, so there is less of a drop in temperature than in metal. drop in temperature than in metal.

One result of this property is that it One result of this property is that it causes oceans and large lakes to causes oceans and large lakes to moderate the temperatures of moderate the temperatures of nearby land areas. nearby land areas.

In other words, coastal areas In other words, coastal areas generally have less extreme generally have less extreme temperatures than inland areas. temperatures than inland areas.

Water also moderates temperature Water also moderates temperature through evaporation, such as when through evaporation, such as when you sweat. you sweat.

Evaporation occurs when molecules Evaporation occurs when molecules at the surface of a liquid escape to at the surface of a liquid escape to the air. the air.

As water molecules evaporate, the As water molecules evaporate, the remaining liquid becomes cooler. remaining liquid becomes cooler.

Low Density of IceLow Density of Ice

Density is the amount of matter in a Density is the amount of matter in a given volume. given volume.

A high-density substance is more A high-density substance is more tightly "packed" than a low-density tightly "packed" than a low-density substance. substance.

In most substances, the solid state is In most substances, the solid state is more dense than the liquid state. more dense than the liquid state.

Water is just the opposite—its solid form Water is just the opposite—its solid form (ice) is less dense than the cold liquid (ice) is less dense than the cold liquid form. form.

Once again, hydrogen bonds are the Once again, hydrogen bonds are the reason. reason.

Because the molecules in liquid water are Because the molecules in liquid water are moving faster than those in ice, there are moving faster than those in ice, there are fewer and more short-lived hydrogen fewer and more short-lived hydrogen bonds between molecules. bonds between molecules.

The liquid water molecules can fit The liquid water molecules can fit more closely together than the more closely together than the molecules in ice. molecules in ice.

Since substances of lesser density Since substances of lesser density float in substances of greater float in substances of greater density, ice floats in liquid water.density, ice floats in liquid water.

Ice floats because its molecules are less Ice floats because its molecules are less

densely packed than those in liquid water.densely packed than those in liquid water.

How is the fact that ice floats important to How is the fact that ice floats important to living things? living things?

If ice sank, it would form on the bottom of If ice sank, it would form on the bottom of a body of water as the water was cooling. a body of water as the water was cooling.

Ponds and lakes would freeze from the Ponds and lakes would freeze from the bottom up, trapping the fish and other bottom up, trapping the fish and other organisms in a shrinking layer of water organisms in a shrinking layer of water without access to the nutrients from the without access to the nutrients from the muddy bottom. muddy bottom.

Water's Ability to Dissolve Other Water's Ability to Dissolve Other SubstancesSubstances

When you stir table salt into a glass of water, you When you stir table salt into a glass of water, you are forming a are forming a solutionsolution, a uniform mixture of two , a uniform mixture of two or more substances. or more substances.

The substance that dissolves the other substance The substance that dissolves the other substance and is present in the greater amount is the and is present in the greater amount is the solventsolvent (in this case, water). (in this case, water).

The substance that is dissolved and is present in The substance that is dissolved and is present in a lesser amount is the solute (in this case, salt). a lesser amount is the solute (in this case, salt).

When water is the solvent, the result is called an When water is the solvent, the result is called an aqueous solution (from the Latin word aqueous solution (from the Latin word aqua,aqua, "water"). "water").

Sodium chloride dissolves as Na+ and Cl- ions become Sodium chloride dissolves as Na+ and Cl- ions become attracted to water molecules and break away from the attracted to water molecules and break away from the

surface of the solid.surface of the solid. Water is the main Water is the main

solvent inside all cells, solvent inside all cells, in blood, and in plant in blood, and in plant sap. sap.

Water dissolves an Water dissolves an enormous variety of enormous variety of solutes necessary for solutes necessary for life. Figure 4-16 life. Figure 4-16 illustrates how water illustrates how water dissolves ionic dissolves ionic compounds such as compounds such as table salt (sodium table salt (sodium chloride). chloride).

Acids, Bases, and pHAcids, Bases, and pH

In aqueous solutions, a very small In aqueous solutions, a very small percentage of the water molecules percentage of the water molecules themselves break apart into ions. themselves break apart into ions.

The ions formed are positively The ions formed are positively charged hydrogen ions (H+) and charged hydrogen ions (H+) and negatively charged hydroxide ions negatively charged hydroxide ions (OH-). (OH-).

Some chemical compounds Some chemical compounds contribute additional H+ ions to an contribute additional H+ ions to an aqueous solution while others aqueous solution while others remove H+ ions from it. remove H+ ions from it.

A compound that donates H+ ions to A compound that donates H+ ions to a solution is called an acid. a solution is called an acid.

An example is hydrochloric acid An example is hydrochloric acid (HCl), the acid in your stomach. (HCl), the acid in your stomach.

In an aqueous solution, hydrochloric acid In an aqueous solution, hydrochloric acid breaks apart completely into H+ and Cl- breaks apart completely into H+ and Cl- ions. ions.

A compound that removes H+ ions from A compound that removes H+ ions from an aqueous solution is called a base. an aqueous solution is called a base.

Some bases, such as sodium hydroxide Some bases, such as sodium hydroxide (NaOH), do this by adding OH- ions, which (NaOH), do this by adding OH- ions, which then combine with H+ ions and form water then combine with H+ ions and form water molecules. molecules.

The pH ScaleThe pH Scale

The pH scale describes how acidic or basic The pH scale describes how acidic or basic a solution is. The scale ranges from 0 a solution is. The scale ranges from 0 (most acidic) to 14 (most basic).(most acidic) to 14 (most basic).

Each pH unit represents a tenfold change Each pH unit represents a tenfold change in the concentration of H+ ions. in the concentration of H+ ions.

For example, lemon juice at pH 2 has 10 For example, lemon juice at pH 2 has 10 times more H+ ions than an equal amount times more H+ ions than an equal amount of grapefruit juice at pH 3. of grapefruit juice at pH 3.

Pure water and aqueous solutions Pure water and aqueous solutions that have equal amounts of H+ and that have equal amounts of H+ and OH- ions are said to be neutral. OH- ions are said to be neutral.

They have a pH of 7 and are neither They have a pH of 7 and are neither acidic nor basic. acidic nor basic.

The pH of the solution inside most The pH of the solution inside most living cells is close to 7. living cells is close to 7.

A solution having a pH of 7 is neutral. Many fruits have pH A solution having a pH of 7 is neutral. Many fruits have pH values less than 7, making them acidic. Various household values less than 7, making them acidic. Various household

cleaners have pH values greater than 7, making them cleaners have pH values greater than 7, making them

basic.basic.

BuffersBuffers

Because the molecules in cells are Because the molecules in cells are very sensitive to concentrations of very sensitive to concentrations of H+ and OH- ions, even a slight H+ and OH- ions, even a slight change in pH can be harmful to change in pH can be harmful to organisms. organisms.

Many biological fluids contain Many biological fluids contain buffers, substances that cause a buffers, substances that cause a solution to resist changes in pH. solution to resist changes in pH.

A buffer works by accepting H+ ions A buffer works by accepting H+ ions when their levels rise and donating when their levels rise and donating H+ ions when their levels fall, H+ ions when their levels fall, thereby maintaining a fairly constant thereby maintaining a fairly constant pH in the solution. pH in the solution.

An example of the importance of An example of the importance of buffers is their role in regulating the buffers is their role in regulating the pH of the blood. pH of the blood.

Human blood normally has a pH of about Human blood normally has a pH of about 7.4. 7.4.

Certain chemical reactions within your Certain chemical reactions within your cells can lead to an increase in the amount cells can lead to an increase in the amount of H+ ions. of H+ ions.

When these ions move into the blood, When these ions move into the blood, buffers take up some of them, preventing buffers take up some of them, preventing the blood from becoming acidic enough to the blood from becoming acidic enough to endanger cell function. endanger cell function.