Water

WaterWater•A water molecule (H2O), is

made up of threethree atoms --- one oxygen and two hydrogen.

H

HO

Properties of Water• Polar molecule• Cohesion and

adhesion• High specific heat• Density – greatest

at 4oC• Universal solvent of

life

In each water molecule, the oxygen atom attracts moreoxygen atom attracts more than its "fair share" of electronselectronsThe oxygenoxygen end “acts” negativenegativeThe hydrogenhydrogen end “acts” positivepositiveCauses the water to be POLARPOLARHowever, Water is neutralneutral (equal number of e- and p+) --- Zero Net ChargeZero Net Charge

Polarity of Water• In a water molecule two

hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids– Because oxygen is more

electronegative, the region around oxygen has a partial negative charge.

– The region near the two hydrogen atoms has a partial positive charge.

• A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.

HYDROGEN BONDS• Hold water molecules

together

• Each water molecule can form a maximum of 4 hydrogen bonds

• They form, break, and reform with great frequency

• Extraordinary Properties that are a result of hydrogen bonds.

– Cohesive behavior– Resists changes in

temperature– High heat of

vaporization– Expands when it

freezes– Versatile solvent

Organisms Depend on Cohesion

• Attraction between particles Attraction between particles of the same substance of the same substance (( why why water is attracted to itself)water is attracted to itself)

• Results in Results in Surface tensionSurface tension (a (a measure of the strength of measure of the strength of water’s surface)water’s surface)

• Produces a Produces a surface film surface film on on water that water that allows insects to allows insects to walk on the surfacewalk on the surface of water of water

Hydrogen bonds hold the substance together, a phenomenon called cohesion

Adhesion

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Fig. 3.3

•Attraction between two Attraction between two different substancesdifferent substances..•Water will make Water will make hydrogen hydrogen bonds with other surfacesbonds with other surfaces such such as glass, soil, plant tissues, and as glass, soil, plant tissues, and cotton. cotton. •CapillaryCapillary actionaction-water -water molecules will “tow” each other molecules will “tow” each other along when in a thin glass tube.along when in a thin glass tube. Which gives water the ability to “climb” structuresExample: Example: transpirationtranspiration process process which plants and trees remove which plants and trees remove water from the soil, and paper water from the soil, and paper towels soak up water.towels soak up water.

Moderates Temperatures on Earth

• What is kinetic energy?• Heat?• Temperature?• What is specific heat?

Celsius Scale at Sea Level

100oC Water boils

37oC Human body temperature

23oC Room temperature

0oC Water freezes

 

Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air.Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature.

Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for:

1. Prevention of temperature fluctuations that are outside the range suitable for life.

2. Coastal areas having a mild climate

3. A stable marine environment

Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to

change its temperature by 1oC.

Evaporative Cooling• The cooling of a surface

occurs when the liquid evaporates (Under Armor)

• This is responsible for:

– Moderating earth’s climate

– Stabilizes temperature in aquatic ecosystems

– Preventing organisms from overheating

Density of Water• Most dense at 4oC• Contracts until 4oC• Expands from 4oC to

0oC

The density of water:

1. Prevents water from freezing from the bottom up.

2. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.

3. Makes transition between season less abrupt.

– When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners.

– As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state.

– Ice is about 10% less dense than water at 4oC.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 3.5

Water is Less Dense as a Water is Less Dense as a SolidSolid

WaterWater IceIce

Buoyancy• Buoyancy is the ability of a

fluid to exert an upward force on an object floating in it.

• Buoyant force (upward) must be greater than gravitational force (downward) in order for an object to float

• Objects with a lower density than water will float in water.

• Metal is more dense than water, so the shape of a boat helps it float.

Buoyancy and the Titanic

Solvent for Life• Solution

– Solute– Solvent

• Hydrophilic– Ionic compounds

dissolve in water– Polar molecules

(generally) are water soluble

• Hydrophobic– Nonpolar compounds

Solutions & SuspensionsSolutions & Suspensions

•Water is usually part of a Water is usually part of a mixture.mixture.

• There are two types of There are two types of mixtures:mixtures:–SolutionsSolutions–SuspensionsSuspensions

SolutionSolution

• Ionic compounds disperse as Ionic compounds disperse as ionsions in in waterwater

• Evenly distributedEvenly distributed• SOLUTESOLUTE

– Substance that is being dissolvedSubstance that is being dissolved• SOLVENTSOLVENT

– Substance into which the solute Substance into which the solute dissolvesdissolves

SolutionSolution

SuspensionsSuspensions

• Substances that don’t dissolve don’t dissolve but separate but separate into tiny pieces.

• Water keeps Water keeps the pieces the pieces suspended suspended so they don’t settle out.

The pH ScaleThe pH Scale

• Indicates the concentration of Hconcentration of H++ ions ions• Ranges from 0 – 140 – 14• pH of 7 is neutral7 is neutral• pH 0 up to 7 is acid 0 up to 7 is acid … H+ • pH above 7 – 14 is babove 7 – 14 is basic… OH- • Each pH unit represents a factor of

10X 10X change in concentration• pH 3 is 10 x 10 x 10 (1000) stronger pH 3 is 10 x 10 x 10 (1000) stronger

than a pH of 6than a pH of 6

Acids and Bases• An acid is a substance that

increases the hydrogen ion concentration in a solution.

• Any substance that reduces the hydrogen ion concentration in a solution is a base.– Some bases reduce H+ directly by

accepting hydrogen ions.

• Strong acids and bases complete

dissociate in water.

• Weak acids and bases dissociate only partially and reversibly.

AcidsAcids

•StrongStrong Acids Acids have a have a pH of pH of 1-31-3

•Produce Produce lots of lots of H H+ + ionsions

BasesBases

• Strong Strong Bases Bases have a pH of 11 to 14 11 to 14

• Contain lots of OHlots of OH--

ions ions and fewer H+ ions

BuffersBuffers• Weak acids or bases that react Weak acids or bases that react

with strong acids or bases to with strong acids or bases to prevent sharp, sudden changes prevent sharp, sudden changes in pH in pH (neutralization).(neutralization).

• Produced naturally by the body Produced naturally by the body to maintain homeostasisto maintain homeostasis

Weak AcidWeak Acid Weak BaseWeak Base