properties of water - mr. meagher's science · • discuss the unique properties of water ......
TRANSCRIPT
Properties of Water
SC.912.L.18.12 Discuss the special properties of water that contribute to Earth's suitability as an
environment for life: cohesive behavior, ability to moderate temperature, expansion upon freezing,
and versatility as a solvent.
• discuss the unique properties of water
• explain that the polarity of water is the basis for its unique properties
• differentiate between solutions and suspensions
• explain that water is known as the universal solvent due to its ability to dissolve many
compounds
Terms to know
1. Atom
2. Proton
3. Neutron
4. Electron
5. Element
6. Compound
7. Ionic bond
8. Covalent bond
9. Isotope
10.Polarity
11.Hydrogen bonds
12.Cohesion
13.Adhesion
14.Mixture
15.Solution
16.Solute
17.Solvent
18.Suspension
19.pH scale
20.Acids
21.Bases
22.Buffers
The Atom
• The study of chemistry begins with
the basic unit of matter, the atom.
• Nucleus - the center of the atom.
• Electron - negatively charged
particle with only 1/1840 the mass
of a proton.
The Atom
• Element is a pure substance that consists entirely of one type of atom.
• A chemical compound is a substance formed by the chemical combination
of two or more elements in definite proportions.
The Atom
• An ionic bond is formed when
one or more electrons are
transferred from one atom to
another.
• Ions – a positively or negatively
charged atom.
• An atom that loses electrons
becomes positively charged.
An atom that gains electrons
has a negative charge.
• The moving electrons travel
about the nuclei of both atoms,
forming a covalent bond.
The Atom
Atoms of the same element that differ in the number of neutrons they
contain are known as isotopes.
The Water Molecule
• Water is one of the few compounds
found in a liquid state over most of
Earth’s surface.
• Like other molecules, water (H2O) is
neutral. The positive charges on its
10 protons balance out the negative
charges on its 10 electrons.
The Water Molecule
• Polarity – when molecules have
different charges are each end.
• Water is considered polar. It’s
oxygen end is negative while its
hydrogen ends are positive.
Hydrogen Bonding
• Because of their partial positive and
negative charges, polar molecules
such as water can attract each other.
• Hydrogen bonds - The attraction
between a hydrogen atom on one
water molecule and the oxygen atom
on another.
• Hydrogen bonds are not as strong as
covalent or ionic bonds, and they can
form in other compounds besides
water.
Hydrogen Bonding
• Cohesion is an attraction between
molecules of the same substance.
• Because a single water molecule may
be involved in as many as four
hydrogen bonds at the same time,
water is extremely cohesive.
• Cohesion also produces surface
tension, explaining why some insects
and spiders can walk on a pond’s
surface.
Hydrogen Bonding
• Adhesion is an attraction between
molecules of different substances.
• The surface of water in a graduated
cylinder dips slightly in the center,
forming a curve called a meniscus,
because the adhesion between water
molecules and glass molecules is
stronger than the cohesion between
water molecules.
Heat Capacity
• Heat capacity - the amount of heat energy required to increase its temperature.
• Water’s heat capacity is high. This is mainly because of the multiple hydrogen bonds water
molecules form.
• Large bodies of water, such as oceans and lakes, can absorb large amounts of heat with
only small changes in temperature. This protects organisms living within from drastic
changes in temperature.
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.
• Solution – a mixture in which ions are dispersed in water. Each ion is
surrounded by water molecules.
Solutions and Suspensions
• Solute—the substance that is dissolved. (In this case, salt).
• Solvent—the substance in which the solute dissolves. (Almost always
water).
Solutions and Suspensions
• Suspensions - mixture of water and nondissolved material. This occurs
when materials do not dissolve when placed in water, but separate into
pieces so small that they do not settle out.
The pH Scale
• pH scale – used to indicate the
concentration of H+ ions in
solution. It ranges from 0 to 14.
• 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
• Solutions with a pH below 7 are
called acidic because they have
more H+ ions than OH– ions.
• Solutions with a pH above 7 are
called basic because they have
more OH– ions than H+ ions.
• Each step on the pH scale
represents a factor of 10.
• For example, a liter of a solution
with a pH of 4 has 10 times as
many H+ ions as a liter of a
solution with a pH of 5.
The pH Scale
• Each step on the pH scale
represents a factor of 10. For
example, a liter of a solution
with a pH of 4 has 10 times as
many H+ ions as a liter of a
solution with a pH of 5.
The pH Scale
• Acid - any compound that
forms H+ ions in solution.
• Base - a compound that
produces hydroxide (OH–) ions
in solution.
Buffers
• Fluids in the human body must be kept between 6.5 and 7.5 in order to
maintain homeostasis.
• If the pH is lower or higher, it will affect the chemical reactions that take place
within the cells.
• Buffers - weak acids or bases that prevent sudden changes in pH.
Buffers
• Adding acid to an unbuffered solution causes the pH of the unbuffered
solution to drop. If the solution contains a buffer, however, adding the acid
will cause only a slight change in pH.
