2.
The states of matter and their distinguishing properties can be explained
by a particle model of matter.
All
matter is composed of tiny particles (atoms or molecules) that are in
constant motion. These particles are much too small to be seen with an
ordinary microscope. A single drop of water, for example, is made of
approximately 3,000,000,000,000,000,000,000 (3 X 1021) water particles.
In
a solid, strong forces of attraction hold the particles in a tightly
packed and regular arrangement. Movement of particles is restricted to
vibration about a fixed position. This explains the definite shape and
definite volume of solids. It also accounts for why solids are dense (a
lot of matter packed into a space) and non-compressible (the particles are
already very close together). To form a mental model for the particles of
a solid, think about apples packed in layers in a box that is in a truck
driving on a gravel road.
In
a liquid, the particles are not so strongly held together and can slide
freely past one another, but cannot easily move apart from one another.
This explains how liquids are able to flow and take the shape of their
container while maintaining a definite or constant volume. A mental model
for the particles of a liquid may be to think about the plastic balls in a
“ball pool” in which young children are playing.
In
a gas the particles are independent of one another. There is very little
or no attraction between gas particles. Particles move in random
directions at very high speeds. Particles of a gas at room temperature may
move at about 500 meters per second (or about 1000 miles/hour). The
particles move as far apart from one another as the container space
allows, moving from an area of higher concentration to an area of lower
concentration until they are uniformly distributed in the space available.
They collide frequently with one another and with the inside surfaces of
their container. The arrangement and motion of particles of a gas explains
why gases are able to rapidly diffuse and how gases exert pressure. It
also accounts for why gases have a very low density, or small mass for
their volume. The particles are so spread out that most of the gas is
empty space. This empty space between particles is what allows a gas to be
compressed into a smaller space. To form a mental model for particles in a
gas, imagine lottery balls whizzing around inside the dispenser.
The
particle model described above is sometimes referred to as the kinetic
theory.
3.
While substances having the same physical state share the distinguishing
properties of that state, they may vary widely in other properties. Some
of these variations in properties can also be explained by the particle
model.
Solids
vary in properties such as hardness, tensile strength, density,
elasticity, conductivity, solubility, porosity, etc. Liquids vary in
properties such as cohesion, adhesion, viscosity, density,
solubility/miscibility, rate of evaporation, etc. Gases vary in rate of
diffusion, density, etc.
The
particle model can help explain some of these variations in properties.
For example, difference in the tensile strength of solids is related to a
difference in the strength of attraction between the particles
constituting the solids. Difference in the viscosity of liquids, which
results from friction between particles as they slide past one another, is
related to a difference in the length of the particles. Difference in the
rates of diffusion of gases can be explained by differences in the mass of
the particles constituting the gas. More massive particles have less
speed, so a gas that has more massive particles does not diffuse as
rapidly.
4.
Every substance has a unique set of properties that can be used to
identify the substance, to determine ways in which it may be used, and to
predict its behavior in certain situations.
Substances
differ from one another in characteristic properties such as color,
density, freezing point, and solubility. Characteristic properties do not
depend on the quantity of material present. (Mass and volume are,
consequently, not characteristic properties.)
Density
is a particularly useful characteristic property. The density of a
substance describes the amount of matter in a certain volume of the
substance. Density = mass/volume. The density of a substance depends on
both the mass of the particles of which it is made and the way in which
these particles are arranged or packed together. The density of a
substance can be used to predict sinking and floating behavior. A
substance will float in a fluid of greater density, and sink in one of
lesser density. The density of water is 1 g/ml (at 4 oC). An object with a
density greater than 1 g/ml will sink in water, while a substance with a
density of less than 1 g/ml will float in water. If an object is made of
more than one substance, the density of the object can be calculated by
dividing the total mass of the object by the total volume the object
occupies. The density of a substance may change with even slight changes
in temperature. This is because when a substance is heated, the particles
of which it is composed move faster and further part.
Characteristic
properties are useful in identifying a substance. Aluminum, for example,
is a malleable, silver metal that has a density of 2.7 g/ml and melts at a
temperature of 660oC. It doesn’t matter if you have a pea-sized pellet
of aluminum or a gigantic block of aluminum; its characteristic properties
are constant. Thus, if you have a piece of unidentified silver-colored
metal - no matter how large or small — you can determine if it is
aluminum by testing its melting point and/or its density. You can use
these same tests to determine the purity of a substance. The collective
characteristic properties of a substance also determine appropriate uses
for the substance. Aluminum is used for the body of aircraft because it is
malleable, strong, and lightweight.
