Matter is anything that occupies space and has mass, and is the building block of
the physical world. Energy is required to cause matter to change course or speed.
The diver, the diver’s air supply, everything that supports him or her, and the
surrounding environment is composed of matter.
An element is the simplest form of matter that exhibits distinct physical
and chemical properties. An element cannot be broken down by chemical means
into other, more basic forms. Scientists have identified more than 100 elements in
the physical universe. Elements combine to form the more than four million
substances known to man.
The atom is the smallest particle of matter that carries the specific properties
of an element. Atoms are made up of electrically charged particles known as
protons, neutrons, and electrons. Protons have a positive charge, neutrons have a
neutral charge, and electrons have a negative charge.
Molucules are formed when atoms group together (Figure 2-1). Molecules
usually exhibit properties different from any of the contributing atoms. For
example, when two hydrogen atoms combine with one oxygen atom, a new
substance—water—is formed. Some molecules are active and try to combine with
many of the other molecules that surround them. Other molecules are inert and do
not naturally combine with other substances. The presence of inert elements in breathing mixtures is important when calculating a diver’s decompression
obligations.
Figure 2-1. Molecules. Two similar atoms
combine to form an oxygen molecule while the
atoms of two different elements, hydrogen and
oxygen, combine to form a water molecule.
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The Three States of Matter Matter can exist in one of three natural states: solid,
liquid, or gas (Figure 2-2). A solid has a definite size and shape. A liquid has a
definite volume, but takes the shape of the container. Gas has neither definite
shape nor volume, but will expand to fill a container. Gases and liquids are collectively
referred to as fluids.
The physical state of a substance depends primarily upon temperature and
partially upon pressure. A solid is the coolest of the three states, with its molecules
rigidly aligned in fixed patterns. The molecules move, but their motion is like a
constant vibration. As heat is added the molecules increase their motion, slip apart
from each other and move around; the solid becomes a liquid. A few of the molecules
will spontaneously leave the surface of the liquid and become a gas. When
the substance reaches its boiling point, the molecules are moving very rapidly in
all directions and the liquid is quickly transformed into a gas. Lowering the
temperature reverses the sequence. As the gas molecules cool, their motion is
reduced and the gas condenses into a liquid. As the temperature continues to fall,
the liquid reaches the freezing point and transforms to a solid state.
Figure 2-2. The Three States of Matter.
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