Water

A. Structure of waterB. Ionization of waterC. Solvent properties of waterD. Thermal properties of waterE. Monomers and polymers

A. Structure of Water

A water molecule is composed of two hydrogen atoms covalently bonded to an oxygen atomThe hydrogen atoms form an angle of about 110°, so the molecule is “bent”

A. Structure of Water

The oxygen nucleus exerts a greater “pull” on the electrons in the covalent bonds– Therefore, the oxygen atom has a partial negative

charge– And the hydrogen atoms have partial positive

charges

Because opposite charges attract each other, water molecules are attracted to each other and to other charged molecules or ions(This is an exampleof a hydrogen bond.)

A. Structure of Water

The bent geometry of water and the attraction between water molecules gives rise to unique properties that are essential for its role in living organisms and the environment

A. Structure of Water

B. Ionization of Water

Consider a glass of pure water:– In a tiny fraction of the water molecules

(1 out of 10 million), one of the hydrogen nuclei is completely pulled off the molecule

– This forms two ions:• A hydrogen ion (H+)• And a hydroxyl ion (OH–)

– This is caused by the attraction of the water molecules for each other

Water molecules are continuously splitting into ions and rejoining to form water molecules

B. Ionization of Water

In chemically pure water, the number of H+ and OH– ions are the sameCertain chemical substances, when dissolved in water, can change the amounts of H+ or OH–

B. Ionization of Water

Acid– A substance that increases the amount of H+

(and decreases the amount of OH–)Base (Alkaline)– A substance that increases the amount of OH–

(and decreases the amount of H+)Neutral substance– A substance that does not change the amounts of

H+ and OH– (so H+ remains equal to OH–)

B. Ionization of Water

Acidity and alkalinity are represented by a value called “pH”– pH = -log[H+]– Acids: pH value is less than 7– Bases: pH value is greater than 7– Neutral substances: pH value is equal to 7– Each pH value represents a 10-fold change in the

amount of H+ in the solution– So a substance with pH = 5 has a 10 times greater

amount of H+ than a substance with pH = 6

B. Ionization of Water

C. Solvent Properties of Water

Solution– A mixture of two (or more) different substances in

which the particles of one substance are completely interspersed with the particles of the other substance(s)

– Solvent: The substance that is present in the largest amount

– Solute: The substance(s) that are present in smaller amounts

Hydrophilic substances– Substances that can be dissolved in water– Water molecules are attracted to ions or to other

molecules that have partial positive and negative charges

– Examples of hydrophilic substances:• Sodium chloride (table salt): This substance consists of

sodium ions and chloride ions• Sucrose (table sugar): This substance is a compound with

many -OH groups in its structure, with many partial positive and negative charges

C. Solvent Properties of Water

Hydrophobic substances– Substances that cannot be dissolved in water– Water molecules have difficulty interacting with

uncharged molecules. These substances tend to separate from water.

– Example of a hydrophobic substance:• Cooking oil: The molecules of cooking oil have long

chains of carbon atoms bonded to hydrogen. The atoms do not have the “bent” geometry of water, so there are no partial charges to attract the water.Therefore, oil and water don’t mix!

C. Solvent Properties of Water

Amphipathic substances– Substances in which part of the molecule is

hydrophobic, and part of the molecule is hydrophilic– When amphipathic substances are mixed in water,

its molecules form into clusters called “micelles” • with the hydrophilic part on the outside of the micelle in

contact with water• and the hydrophobic part on the inside of the micelle,

away from the water .

C. Solvent Properties of Water

Amphipathic substances (cont.)– Example of an amphipathic substance:

• Soap: Soap molecules have an ionic group attached to one end, and an oily hydrocarbon chain attached to the other end. When soap is mixed with water, it forms micelles that trap oily dirt molecules.

C. Solvent Properties of Water

D. Thermal Properties of WaterMolecules are in constant motion due to the heat energy (kinetic energy) they containPhases of matter:– Solid

• Limited movement of molecules; non-fluid– Liquid

• Molecules can move freely around each other; fluid– Gas

• Molecules have greatest freedom of movement; substance can expand to fill the available space

Water has unusual thermal properties because of the attraction of water molecules for each other– Water has relatively high melting and boiling points– Water remains in a liquid state over a wide

temperature range– Water has a high heat capacity: it can absorb a large

amount of heat with a small change in temperature– The solid form of water (ice) is less dense than the

liquid, so ice floats on water

D. Thermal Properties of Water

E. Monomers and Polymers

Monomer– An organic molecule that serves as a “building block”

to build larger organic molecules

Polymer– An organic molecule composed of two or more

monomer units linked together by covalent bonds

Condensation reaction– Polymers are often formed by the process of

condensation– In this process, two hydrogen atoms and an oxygen

atom are removed from two monomer units– And a covalent bond forms between the monomers

E. Monomers and Polymers

E. Monomers and Polymers

Hydrolysis reaction– Polymers are often broken down by the process of

hydrolysis– In this process, a water molecule is inserted between

the monomer units of a polymer– To split the polymer into its monomer units

E. Monomers and Polymers

E. Monomers and Polymers