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Biology
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2-2 Properties of Water
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The Water Molecule
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
The unequal sharing of electrons causes the water molecule to be polar. The hydrogen end of the molecule is slightly positive and the oxygen end is slightly negative.
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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
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
Adhesion is an attraction between molecules of different substances.
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8. 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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9. Solutions and Suspensions
Two types of mixtures can be made with water
solutions
suspensions
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10. Solutions and Suspensions
All the components of a solution are evenly distributed throughout the solution.
solute—the substance that is dissolved.
solvent—the substance in which the solute dissolves.
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11. Solutions and Suspensions
When a crystal of table salt is placed in warm water, sodium and chloride ions are attracted to the polar water molecules.
Na+
Cl -
Water
When an ionic compound such as sodium chloride is placed in water, water molecules surround and separate the positive and negative ions.
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12. Solutions and Suspensions
Some materials do not dissolve when placed in water but separate into pieces so small that they do not settle out easily.
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Acids, Bases, and pH
Acids, Bases, and pH
A water molecule is neutral, but can react to form hydrogen and hydroxide ions.
H2O   H+ + OH-
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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.
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
Acids
An acid is any compound that forms H+ ions in solution. 
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Acids, Bases, and pH
Bases
A base is a compound that produces hydroxide ions (OH- ions) in solution. 
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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 6.5 and 7.5.
Controlling pH is important for maintaining homeostasis.
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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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25. END OF SECTION