4.1 Properties of Substances
Properties are classified as either physical
or chemical.
Physical properties are characteristics which
depend upon the physical change of matter
such as the conversion of a liquid to a gas at
it's boiling point, or the conversion of a solid
to a liquid at it's melting point. As well, the
color texture or other similar items which
leave the chemical nature of the material
unchanged.
Chemical properties are those characteristics
of a substance which are involved in the
formation of other substances by chemical.
Physical changes are those which can alter
the state of the material without changing
it's chemical nature. An example would be
the freezing of liquid water. The liquid
becomes a solid(ice) which is still water that
can be recovered by simply melting the ice.
Chemical changes are those which change the
chemical nature of the material. An example
of a chemical change is the burning of paper.
The cellulose in the paper is converted to
carbon dioxide and water. The chemical
nature of the paper has been altered to form
water and carbon dioxide and it is no longer
cellulose.
Mass is neither created nor destroyed in a physical
or chemical change. All the matter that went into the
change whether physical or chemical change must be
present at the end of the reaction. The only thing that
changes is the form of the matter. The absolute
amounts of mass are conserved.
Energy is often defined as the ability to do work
or to move matter. This is a general definition and
can be further refined to describe that form of energy
kinetic energy and the form potential energy.
Kinetic energy is the energy of motion.
Potential energy is the energy stored in a system.
Typically because of the position of an object in
gravitational field. An example of this is a rock
at the top of a hill which has potential energy
due to it's position at the top of the hill. Another
example is the potential energy stored in a spring
after it has been stretched or compressed. For our
purpose the energy stored in the chemical bonds
of a carbohydrate used later as a source of food.
4.6 Heat: Quantitative Measurements
Heat is the energy of motion of molecules. It is
measured in joules and calories.
One calorie is the amount of heat requires to
raise the temperature of one gram of water one
degree Celsius. Every substance has a different
capacity to absorb heat energy.
Specific heat is the amount of heat energy gained
or lost required to change the temperature of
one gram of the substance one degree Celsius.
*The specific heat of water is 1.00 cal/(g.0°C).
The specific heat of aluminum is 0.900 cal/(g.0°C).
This means that the amount of heat necessary to
raise 5.00 grams of aluminum 5.00degrees Celsius
can be calculated as follows:
| mass of the aluminum |
| x specific heat of the aluminum |
| x total change in temperature |
| = total amount of energy in calories |
4.7 Energy in Chemical Changes
All chemical changes have either a net absorption of
energy from their environment or a net release of
energy to their environment.
The energy may be absorbed or released as heat ,
light, or converted to the potential energy found in
the chemical bonds of various molecules.
*Energy can neither be created nor destroyed only
transformed from one type to another in a chemical
or physical change. In a Chemical reaction the energy
stored in the bonds between the atoms forming the
various molecules is released or alternatively these
bonds are created by the conversion of energy input
to the system. An example of this is the burning of
hydrogen in an oxygen atmosphere. The hydrogen
molecules are taken apart and the oxygen molecules
are taken apart. This requires energy. Each oxygen
atom combines with two hydrogen atoms.This releases
energy. The amount of energy that is released is
greater than the amount of energy that is used, so
we sense the excess as the heat of the reaction.
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In the nuclear reaction some of the mass is converted
to energy and the above statement has been modified
to show that mass/energy can neither be created nor
destroyed.