¨ Three fourths of the Earth’s surface is covered by water
¨ Water is the single most abundant compound in most living things.
The Water Molecule
¨ The water molecule (H2O) is neutral.
Polarity
¨ A water molecule has a bent shape with the oxygen atom on one end
of the molecule and the hydrogen atoms on the other.
¨ The oxygen end has a slight negative charge and the hydrogen end
has a slight positive charge.
¨ A molecule in which the charges are unevenly distributed is called
a polar molecule.
¨ The molecule is like a magnet with poles.
Hydrogen Bonds
¨ Polar molecules such as water can attract each other.
¨ The charges on a polar molecule are written with parentheses, (-) or (+), to show
they are weaker than the charges on ions such as Na+ and Cl- .
¨ The attraction between the hydrogen atom on one water molecule and the oxygen
atom on another molecule is an example of a hydrogen bond.
¨ The process of cohesion is an attraction between molecules of the same substance.
¨ Water is extremely cohesive because of its ability to form multiple
hydrogen bonds.
¨ 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.
¨ The process of adhesion is an attraction between molecules of different substances.
¨ The surface of the water in a graduated cylinder dips slightly in
the center because the adhesion between water molecules and glass molecules is stronger than the cohesion between water molecules.
Solutions and Suspensions
¨ A mixture is a material
composed of two or more elements or compounds that are physically mixed together but not chemically combined.
Ex)
salt and pepper stirred together, sugar and sand,
Earth’s
atmosphere is a mixture of gases
¨ Two types of mixtures that can be made with water are solutions
and suspensions.
Solutions
¨ When an ionic compound such as sodium chloride is placed in water,
water molecules surround and separate the positive and negative ions.
¨ The ions gradually become dispersed in the water, forming a type
of mixture called a solution.
¨ 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 dissolved.
¨ Water is the greatest solvent on Earth.
Suspensions
¨ Some materials do not dissolve when placed in water but separate
into pieces so small that they do not settle out.
¨ A suspension
is a mixture of water and nondissolved material.
Ex)
Blood that circulates through your body is mostly
water,
which contains many dissolved compounds and undissolved particles that remain in suspension as the blood moves through the
body.
Acids, Bases, and pH
The pH scale
¨ The pH scale
is a measurement device used to indicate the concentration of H+ ions in solution.
¨ The pH scale ranges from 0 to 14.
¨ A pH of 7, the concentration of H+ ions and OH-
ions is equal.
¨ Solutions with a pH below 7 are acidic because they have more H+
ions than OH- ions.
¨ The lower the pH, the greater the acidity.
¨ Solutions with a pH above 7 are basic because they have more OH- ions than H+ ions.
¨ The higher the pH, the more basic the solution.
Acids
¨ An acid is any compound that forms H+ ions in solution.
¨ Strong acids tend to have pH values that range from 1 to 3.
Ex) Hydrochloric acid produced by the stomach to help
digest food is a strong acid.
Bases
¨ A base is a compound that produces hydroxide ions (OH-
ions) in solution.
¨ Strong bases, such as lye, tend to have pH values ranging from 11 to 14.
Buffers
¨ The pH of the fluids within most cells in the human body must generally be kept
between 6.5 and 7.5.
¨ The pH will affect chemical reactions that take place within the cells.
¨ pH is important for maintaining homeostasis.
¨ A buffer is a weak acid or base that can react with strong acids or bases to prevent
sharp, sudden changes in pH.
