End Show Slide 1 of 44 Copyright Pearson Prentice Hall Outline 2-2: Properties of Water.

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Outline 2-2: Properties of Water


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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.

Polarity

Oxygen atoms, with 8 protons, has a much stronger attraction for electrons than does the hydrogen atom with one proton.

Thus, the electrons are more likely to be near oxygen than hydrogen.


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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 attract each other.

Attractions between the H atoms on one molecule and the O 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.


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The Water Molecule

Some special properties of water that are due to its ability to form hydrogen bonds:

1. Cohesion

This is an attraction between molecules of the same substance.

a. Cohesion explains why drops of water form beads on a smooth surface.

b. Cohesion also explains why some insects and spiders can walk on water.


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The Water Molecule

2. Adhesion

This is an attraction between molecules of different substances.

a. Adhesion explains why the water in the center of a cylinder dips lower than the edges.

b. It also is the reason water will rise in a narrow tube. This effect is called capillary action.

c. Adhesion and cohesion together explain why water goes up in roots & stems of

plants.


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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.

Two types of mixtures:

1. Solutions

2. 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 salt is placed in water it separates into sodium and chloride ions.

1. The sodium and chloride ions are attracted to the polar water molecules.

2. The ions get surrounded by water molecules thus creating 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. Thus, water is the greatest solvent on Earth.

However, some compounds will NOT dissolve in water. These are called nonpolar compounds.

If you try to dissolve oil in water, it will separate out because:

1. Water molecules are attracted to each other

2. Nonpolar oil molecules will be shoved apart as water molecules come together.


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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.

The movement of water molecules keeps the small particles suspended.

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

Examples: jello, gravy

Blood is both a solution of dissolved solutes like sugar and a suspension of cells & other substances


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Acids, Bases, and pH


A water molecule can break apart to form hydrogen (H+) and hydroxide ions (OH-) . This is called dissociation.

Water is neutral because the number of H+ ions produced is equal to the number of OH- produced.

An acid is any compound that forms H+ ions in solution. Acids turn pH paper red.

A base is a compound that produces OH- ions in solution. Bases turn pH paper blue.


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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.

Strong acids have a pH between 1 and 3.

Hydrochloric acid in stomach is a strong acid.

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.

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

The pH Scale

Soap

Bleach

Oven cleaner

Ammonia solution


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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.

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 Show Slide 1 of 44 Copyright Pearson Prentice Hall Outline 2-2: Properties of Water.

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