Chapter 13.1 Objectives Chapter 13.1 Objectives

• Demonstrate the uniqueness of Demonstrate the uniqueness of water as a chemical substance.water as a chemical substance.

• Model the 3 dimensional geometry of Model the 3 dimensional geometry of a water molecule.a water molecule.

• Relate the physical properties of Relate the physical properties of water to the molecular level.water to the molecular level.

Chapter 13.1: Uniquely Chapter 13.1: Uniquely WaterWater• Water may be a Water may be a

common substance common substance on Earth, but its on Earth, but its properties are properties are anything but anything but common. Those common. Those properties make properties make water essential for water essential for life.life.

The Structure of WaterThe Structure of Water

• A molecule of HA molecule of H22O (water) is just 2 O (water) is just 2 hydrogen atoms and 1 oxygen atom hydrogen atoms and 1 oxygen atom linked by covalent bonds. linked by covalent bonds.

• The electron locations and 3-D shape The electron locations and 3-D shape of the water molecule are the source of the water molecule are the source of water’s unique properties.of water’s unique properties.

• Recall the Lewis Recall the Lewis structure of water. structure of water.

• There is a large There is a large electro-negativity electro-negativity differencedifference between between the covalently bonded the covalently bonded hydrogen and oxygen.hydrogen and oxygen.

• Thus the electron pair Thus the electron pair between them is between them is shared unequally, shared unequally, with the electrons with the electrons spending more time spending more time around the oxygen around the oxygen atom.atom.

• This makes water a This makes water a polar molecule.

H – O - H

Water’s 3-d shape is Water’s 3-d shape is referred to as “bent”.referred to as “bent”.

PolarityPolarity• One end of a One end of a polar polar moleculemolecule has a + has a + charge and the other charge and the other end has a – charge. end has a – charge. For water, For water, the H the H ends are + and the ends are + and the oxygen ends are -.oxygen ends are -.

• This means there are This means there are intermolecular intermolecular forcesforces – like – like attraction and attraction and repulsion – between repulsion – between water molecules.water molecules.• The attraction between the H of one The attraction between the H of one

molecule and molecule and an O of another molecule is called a an O of another molecule is called a hydrogen hydrogen bondbond..

PolarityPolarity• Because of hydrogen bonding, water is a Because of hydrogen bonding, water is a

liquidliquid at room temperature. Similarly- at room temperature. Similarly-sized nonpolar molecules are gases.sized nonpolar molecules are gases.

• Water has a Water has a high boiling point and high high boiling point and high melting pointmelting point because of the attraction because of the attraction between water molecules (H bonding).between water molecules (H bonding).

• Many of waters physical characteristics Many of waters physical characteristics are are due to the hydrogen bondingdue to the hydrogen bonding that that occurs between water molecules. occurs between water molecules.

Other Properties: DensityOther Properties: Density• Water is Water is less dense as a less dense as a

solid than as a liquidsolid than as a liquid. . That is why ice floats That is why ice floats rather than sinking.rather than sinking.

• As water cools its volume As water cools its volume decreases (like most decreases (like most substances) because the substances) because the molecules draw closer molecules draw closer together.together.

• However . . .However . . .

Ice FloatsIce Floats

• However, at about 4 However, at about 4 00C, the water molecules move apart due to H bonding and form an organized, stable arrangement called a crystal.

•The density of solid water, because the molecules have moved apart somewhat, is therefore LESS than that of liquid water.

• Since liquid water Since liquid water expands when it freezes, expands when it freezes, frozen water pipes can frozen water pipes can break.break.

• The forces involved in The forces involved in the expansion of freezing the expansion of freezing water are surprisingly water are surprisingly great. great.

• The processes of freezing The processes of freezing and thawing can and thawing can eventually break up big eventually break up big boulders, contributing to boulders, contributing to soil formation and soil formation and erosion on the surface of erosion on the surface of the Earth.the Earth.

Surface TensionSurface Tension• Have you ever Have you ever

watched water drip watched water drip from a faucet? Each from a faucet? Each drop is composed of an drop is composed of an enormous number of enormous number of water molecules, water molecules, roughly 2 x 10roughly 2 x 102121..

• The observation that The observation that this large number of this large number of molecules can hold molecules can hold together as a single together as a single drop is more evidence drop is more evidence of H bonding.of H bonding.

• Water molecules form Water molecules form drops because of surface drops because of surface tension.tension.

• Surface tension is the Surface tension is the force needed to overcome force needed to overcome intermolecular attractions intermolecular attractions and break through the and break through the surface of a liquid or surface of a liquid or spread the liquid out.spread the liquid out.

• Water has a HIGH surface Water has a HIGH surface tension so it is resistant tension so it is resistant to having its surface to having its surface broken. broken.

Surface TensionSurface Tension

• In a drop of water, In a drop of water, water molecules at the water molecules at the surface are not surface are not surrounded surrounded on topon top by by other molecules so other molecules so there is a net force there is a net force pulling pulling inwardinward on these on these surface molecules.surface molecules.

• This inward pull makes This inward pull makes the water’s surface the water’s surface behave like a kind of behave like a kind of skin. It is “tough” to skin. It is “tough” to break through.break through.

Surface Tension in Everyday Surface Tension in Everyday LifeLife

CohesionCohesion

• So, due to H So, due to H bonding, water bonding, water molecules will molecules will essentially “stick” essentially “stick” to other water to other water molecules.molecules.

• This property is This property is called cohesion.called cohesion.

AdhesionAdhesion

• Water will also Water will also “stick” to surfaces. “stick” to surfaces. This is because of This is because of the attractive the attractive forces between the forces between the water molecules water molecules and the surface.and the surface.

• This property is This property is called adhesion.called adhesion.

CapillarityCapillarity• Adhesion explains Adhesion explains

capillarity.capillarity.

• Capillarity or capillary Capillarity or capillary action is the movement action is the movement of water through a of water through a narrow tube narrow tube against the against the force of gravityforce of gravity..

• Water molecules in the Water molecules in the tube form H bonds with tube form H bonds with the O atoms in the SiOthe O atoms in the SiO22 of the glass.of the glass.

• This draws the water up This draws the water up the sides of the tube.the sides of the tube.

Specific HeatSpecific Heat• Water has a very high specific heat. This Water has a very high specific heat. This

means A LOT of energy must be added to means A LOT of energy must be added to a sample of water before the a sample of water before the temperature of the water will rise.temperature of the water will rise.

• At the beginning of the summer, the At the beginning of the summer, the ocean water temperature is low. It takes ocean water temperature is low. It takes a lot of sunlight to raise the temperature.a lot of sunlight to raise the temperature.

• Once warmed, however, it will cool off Once warmed, however, it will cool off very slowly when the outdoor very slowly when the outdoor temperature once again drops. This is temperature once again drops. This is because the water has a great deal of because the water has a great deal of stored energy.stored energy.

Heat of VaporizationHeat of Vaporization

• The heat of vaporization of water is The heat of vaporization of water is the amount of energy that must be the amount of energy that must be added to cause the water to vaporize added to cause the water to vaporize (change to gas). (change to gas).

• Because of H bonding, water has a Because of H bonding, water has a high specific heat and a high heat of high specific heat and a high heat of vaporization.vaporization.

Heat of VaporizationHeat of Vaporization• This property of water is This property of water is

extremely important in extremely important in maintenance of body maintenance of body temperature.temperature.

• The vaporization of water The vaporization of water from our body surface from our body surface requires heat. The heat is requires heat. The heat is removed from the surface removed from the surface of the skin when our sweat of the skin when our sweat evaporates.evaporates.

• Perspiration cools us Perspiration cools us down.down.

The Universal SolventThe Universal Solvent• Water is an excellent solvent. Water is an excellent solvent.

• The + and – charges on the surface The + and – charges on the surface of the water molecule attract other of the water molecule attract other polar molecules and “pull” them into polar molecules and “pull” them into solution. Polar substances dissolve solution. Polar substances dissolve readily in water.readily in water.

• Almost all of the reactions necessary Almost all of the reactions necessary to maintain life take place in aqueous to maintain life take place in aqueous solutions, solutions with water as the solutions, solutions with water as the solvent. solvent.