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2.2 Properties of Water
Earth, the “blue planet”, has water covering nearly ¾ of its surface.
The very presence of liquid water tells a scientist that life may also be present on such a planet.
Why is this so? There is something very special about water and the role it plays in living things!
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H2O
Water is a molecule made up of 2 hydrogen atoms that are bonded through single covalent bonds to 1 oxygen atom. Like all molecules, a water molecule is neutral.
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The Water Molecule
Polarity
However the oxygen atom, with 8 protons in its nucleus, has a much stronger attraction for electrons than does the hydrogen atom with a single proton.
Therefore, in each of the covalent bonds, the shared electrons spend a greater amount of time near the oxygen atom than near the hydrogen atom.
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This type of covalent bond, where there is a greater probability of finding the shared electrons closer to one atom in the bond, is called a polar covalent bond.
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The Water Molecule
Why are water molecules polar?
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
Since electrons in each oxygen to hydrogen bond will spend more time with the oxygen atom, the oxygen end of the molecule develops a slight negative charge and the hydrogen end develops a slight positive charge. - + +
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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 a special Van der Waals force called a hydrogen bond.
Hydrogen bonds are not true bonds and, so, are not as strong as covalent or ionic bonds.
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The Water Molecule
Hydrogen bonds form between water molecules.
Hydrogen bonds form between water molecules.
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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
Cohesion is an attraction between molecules of the same substance.
Because of hydrogen bonding, water is extremely cohesive.
Water's cohesion causes molecules on the surface of water to be drawn inward. That is, drops of water form beads on a smooth surface.
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Cohesion also explains why some insects and spiders can walk on a pond's surface. There is a increase in attraction between water molecules at the surface of a sample. Water therefore has a high surface tension.
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The Water Molecule
Adhesion is an attraction between molecules of different substances.
Water adheres to many surfaces due to hydrogen bonding between the water molecules and the surface molecules.
The meniscus in a graduated cylinder exists because of the adhesion between water molecules and glass molecules.
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The Water Molecule
Adhesion between water and glass also causes water to rise in a narrow tube against the force of gravity.
This effect is called capillary action.
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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The Water Molecule
Heat Capacity
Because of hydrogen bonding and the attraction water molecules have for each other, large amounts of heat energy must be added for them to move faster (which is what causes an increase in temperature.) This means, water’s heat capacity (the amount of heat energy required to increase temperature) is relatively high.
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The water molecule
Large bodies of water (lakes, oceans) then can absorb a lot of heat with little change in temperature. At the cell level, water can absorb heat and regulate the temperature of the cell.
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17. Solutions and Suspensions
Water, being a polar molecule, has electric charges on its surface. This allows water to be an excellent solvent – it readily dissolves most other substances!
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18. Solutions and Suspensions
Living systems are in part composed of mixtures involving water. 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 can be made with water.
solutions
suspensions
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19. Solutions and Suspensions
All the components of a solution are evenly distributed throughout the solution.
In a salt–water solution, salt is evenly distributed throughout the water.
Salt is the solute—the substance that is dissolved.
Water is the solvent—the substance in which the solute dissolves.
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20. Solutions and Suspensions
How Substances Dissolve
When a crystal of table salt is placed in warm water, sodium and chloride ions are attracted to the polar water molecules.
Cl -
Cl -
Na+
When an ionic compound such as sodium chloride is placed in water, water molecules surround and separate the positive and negative ions.
Na+
Water
Water
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21. Solutions and Suspensions
Ions break away from the crystal and are surrounded by water molecules.
Cl -
Cl -
Na+
Na+
Water
Water
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22. Solutions and Suspensions
The ions gradually become evenly dispersed in the water, forming a solution.
Cl -
Cl -
Na+
Na+
Water
Water
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23. Solutions and Suspensions
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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24. Solutions and 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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25. Solutions and Suspensions
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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26. Solutions and Suspensions
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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Acids, Bases, and pH
Acids, Bases, and pH
Some water molecules in every sample dissociate (break apart) naturally to form hydrogen (H+) and hydroxide ions (OH-).
Water remains neutral because the number of positive hydrogen ions (H+) produced is equal to the number of negative hydroxide ions (OH-) produced.
When water reacts with other substances or other substances dissolve in water, this balance between ions can be upset.
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Acids, Bases, and pH
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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Acids, Bases, and pH
The pH Scale
At a pH of 7, the concentration of H+ ions and OH- ions is equal.
Pure water has a pH of 7.
Sea water
Human blood
Pure water
Milk
Normal rainfall
The concentration of H+ ions determines whether solutions are acidic or basic.
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Acids, Bases, and pH
The pH Scale
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.
The concentration of H+ ions determines whether solutions are acidic or basic.
Acid rain
Tomato juice
Lemon juice
Stomach acids
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Acids, Bases, and pH
The pH Scale
Oven cleaner
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.
Bleach
Ammonia solution
Soap
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Acids, Bases, and pH
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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Acids, Bases, and pH
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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Acids, Bases, and pH
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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Acids, Bases, and pH
Strong bases, such as lye, tend to have pH values ranging from 11 to 14.
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Acids, Bases, and pH
Buffers 
The pH of the fluids within most cells in the human body must generally be kept between pH 6.5 and pH 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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Acids, Bases, and pH
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.
Buffers play an important role in maintaining homeostasis in organisms.
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2-2
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2-2
A molecule in which the charges are unevenly distributed is called a
polar molecule.
cohesive molecule.
hydrogen molecule.
covalent molecule.
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2-2
A dissolved substance is called a
solvent.
solution.
solute.
Suspension.
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2-2
A compound that produces hydroxide ions in solution is called a(an)
base.
buffer.
acid.
salt.
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2-2
Hydrogen bonds between water molecules result from
adhesion between water molecules.
magnetic attractions between water molecules.
uneven electron distribution in each water molecule.
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
equal concentrations of H+ and OH- ions.
the same concentration of H+ ions as pure water.
higher concentration of H+ than in pure water.
lower concentration of H+ than in pure water.
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