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Copyright Pearson Prentice Hall

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Biology


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2-2 Properties of Water



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The Water Molecule

The Water Molecule

Like all molecules, a water molecule is neutral.



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The Water Molecule

Why are water molecules polar?



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The Water Molecule

Polarity

However an oxygen atom, with 8 protons in its nucleus, has a much stronger attraction for electrons than does the hydrogen atom with a single proton.

Thus, there is a greater probability of finding the shared electrons near the oxygen atom than near the hydrogen atom.



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The Water Molecule

As a result, the oxygen end of the molecule has a slight negative charge and the hydrogen end has a slight positive charge.

A water molecule is polar because there is an uneven distribution of electrons between the oxygen and hydrogen atoms.



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The Water Molecule

Water Molecule



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The Water Molecule

Hydrogen Bonds

Because of their partial positive and negative charges, polar molecules can attract each other.



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The Water Molecule

The attraction between the hydrogen atoms on one molecule and the oxygen atom on another water molecule is an example of a hydrogen bond.

Hydrogen bonds are not as strong as covalent or ionic bonds.



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The Water Molecule

A single water molecule may be involved in as many as four hydrogen bonds at a time.

Water’s ability to form multiple hydrogen bonds is responsible for many of its special properties.



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The Water Molecule

Hydrogen bonds form between water molecules.



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The Water Molecule

Cohesion is an attraction between molecules of the same substance.

Because of hydrogen bonding, water is extremely cohesive.



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The Water Molecule

Water's cohesion causes molecules on the surface of water to be drawn inward, which is why drops of water form beads on a smooth surface.

Cohesion also explains why some insects and spiders can walk on a pond's surface.



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The Water Molecule

Adhesion is an attraction between molecules of different substances.

The surface of water in a graduated cylinder dips in the center because adhesion between water molecules and glass molecules is stronger than cohesion between water molecules.



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The Water Molecule

Adhesion between water and glass causes water to rise in a narrow tube against the force of gravity.

This effect is called capillary action.



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The Water Molecule

Capillary action is one of the forces that draw water out of the roots of a plant and up into its stems and leaves.

Cohesion holds the column of water together as it rises.



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Solutions and Suspensions


A mixture is a material composed of two or more elements or compounds that are physically mixed but not chemically combined.



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Two types of mixtures can be made with water

• solutions

• suspensions



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Solutions

All the components of a solution are evenly distributed throughout the solution.

In a salt–water solution, table salt is the solute—the substance that is dissolved.

Water is the solvent—the substance in which the solute dissolves.



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When a crystal of table salt is placed in warm water, sodium and chloride ions are attracted to the polar water molecules.

Na+

Na+

Cl -

Cl -

WaterWater



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Ions break away from the crystal and are surrounded by water molecules.

Na+

Na+

Cl -

Cl -

WaterWater



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The ions gradually become dispersed in the water, forming a solution.

Na+

Na+

Cl -

Cl -

WaterWater



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Water's polarity gives it the ability to dissolve both ionic compounds and other polar molecules, such as sugar.

Water is the greatest solvent on Earth.



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Suspensions

Some materials do not dissolve when placed in water but separate into pieces so small that they do not settle out.



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The movement of water molecules keeps the small particles suspended.

Such mixtures of water and nondissolved material are known as suspensions.



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Some of the most important biological fluids are both solutions and suspensions.

The blood that circulates through your body is mostly water, which contains many dissolved compounds.

Blood also contains cells and other undissolved particles that remain in suspension as the blood moves through the body.



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Acids, Bases, and pH

What are acidic solutions? What are basic solutions?



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A water molecule can react to form hydrogen and hydroxide ions.

Water is neutral because the number of positive hydrogen ions (H+) produced is equal to the number of negative hydroxide ions (OH-) produced.



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The pH scale

Chemists devised a measurement system called the pH scale to indicate the concentration of H+ ions in solution.

The pH scale ranges from 0 to 14.



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At a pH of 7, the concentration of H+

ions and OH- ions is equal.

Pure water has a pH of 7.

The pH Scale

Human blood

Milk

Sea water

Normal rainfall

Pure water



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Solutions with a pH below 7 are called acidic because they have more H+ ions than OH- ions.

The lower the pH, the greater the acidity.

Acid rainTomato juice

Lemon juice

Stomach acids

The pH Scale



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Solutions with a pH above 7 are called basic because they have more OH- ions than H+ ions.

The higher the pH, the more basic the solution.

The pH Scale

Soap

Bleach

Oven cleaner

Ammonia solution



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Acids

An acid is any compound that forms H+ ions in solution.

Acidic solutions contain higher concentrations of H+ ions than pure water and have pH values below 7.



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Strong acids tend to have pH values that range from 1 to 3.

The hydrochloric acid produced by the stomach to help digest food is a strong acid.



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Bases

A base is a compound that produces hydroxide ions (OH- ions) in solution.

Basic, or alkaline, solutions contain lower concentrations of H+ ions than pure water and have pH values above 7.



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Strong bases, such as lye, tend to have pH values ranging from 11 to 14.



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Buffers

The pH of the fluids within most cells in the human body must generally be kept between 6.5 and 7.5.

If the pH is lower or higher, it will affect the chemical reactions that take place within the cells.



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Controlling pH is important for maintaining homeostasis.

One of the ways that the body controls pH is through dissolved compounds called buffers.

Buffers are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH.


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2-2


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2-2

A molecule in which the charges are unevenly distributed is called a

a. polar molecule.

b. cohesive molecule.

c. hydrogen molecule.

d. covalent molecule.


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2-2

A dissolved substance is called a

a. solvent.

b. solution.

c. solute.

d. suspension.


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2-2

A compound that produces hydroxide ions in solution is called a(an)

a. base.

b. buffer.

c. acid.

d. salt.


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2-2

Hydrogen bonds between water molecules result from

a. adhesion between water molecules.

b. magnetic attractions between water molecules.

c. uneven electron distribution in each water molecule.

d. ionic bonds in the water molecule.


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2-2

On a pH scale, a value of 2 means that the solution has

a. equal concentrations of H+ and OH- ions.

b. the same concentration of H+ ions as pure water.

c. higher concentration of H+ than in pure water.

d. lower concentration of H+ than in pure water.

END OF SECTION

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Documents

water molecules

negative ions

hydrogen end

concentration of h ions

oxygen end

hydrogen bonds form

sodium chloride

ionic compound