Physical Science for Armed Services Vocational Aptitude Battery (ASVAB) Study Guide (page 3)

Chemistry

Chemistry is the study of elements and the compounds they form. Matter can take the form of an element, a compound, or a mixture.

An element is the basic form of matter, incapable of being decomposed by chemical means into simpler substances. Each element has distinct chemical and physical characteristics. Hydrogen (H), oxygen (O), and carbon (C) are elements.

A compound is a combination of two or more elements chemically combined in a specific proportion. Compounds can be separated by chemical means, and are represented by chemical formulas that include the symbols of all the elements present. Examples of familiar compounds are water (H2O) and table salt (NaCl). In order to be considered organic, a compound must contain carbon.

A mixture is a combination of two or more substances that are not chemically combined. Dissolving salt in water results in a mixture. The two compounds don't react with each other and can be separated by physical means—in this case, heating the water so it evaporates, leaving the salt behind.

All matter is made up of atoms. The following terms are used to define atomic structure:

• Atom: the smallest unit of an element that retains all of the element's chemical properties. An atom is composed of three primary particles: electrons, protons, and neutrons.
• Electron: found outside the nucleus (the center of an atom), it has a negligible mass and a charge of –1.
• Proton: found in the nucleus, it has a mass of 1 amu (atomic mass unit) and a charge of +1.
• Neutron: found in the nucleus, it has a mass of 1 amu and no charge.

An atom contains an identical number of protons and electrons, making it electrically neutral.

Atoms of the same element generally have the same properties, unless they are isotopes, which can behave differently. Atoms of different elements have different properties and different masses. Atoms of elements combine in simple whole number ratios.

Periodic Table

The periodic table (next page) lists all of the known elements according to their atomic numbers.

Atomic number is the number of protons in the atom. The atomic number determines the element.

Mass number is the total number of protons and neutrons in one atom of an element. Mass number can vary because the number of neutrons in an atom can change.

The horizontal rows of elements in the periodic table are called periods. There are seven periods in all. Moving from left to right across a period, the atomic number increases by one from one element to the next. Each successive element has one more electron in its outer shell. All elements in the same period have the same number of shells.

The vertical columns of elements in the periodic table are groups. Elements in the same group have the same number of electrons in their outer shell. They therefore have similar chemical properties.

Electronic Structure of Atoms

According to the periodic table, the atomic number of hydrogen (H) is 1, because it has one proton in its nucleus. The number of protons in an atom determines its atomic nucleus. The atomic number of nitrogen (N) is 7; nitrogen has seven protons in its nucleus.

Niels Bohr, a Danish physicist, proposed a model of the atom that had a nucleus surrounded by concentric orbits

Electrons are found in the orbits. When electrons become excited, they absorb energy and move to an orbit farther from the nucleus. When they release energy, they fall to an orbit with lower energy closer to the nucleus. The energy level (also called electron shell) is a region of space in which electrons move around the nucleus.

Chemical Equations

Chemical equations are used to show the result of a physical or chemical change in matter. A physical change is when the shape or size of matter is changed, but the molecules remain unchanged. Steam from a boiling pot of water is an example of a physical change. A chemical change is one in which two or more molecules interact to form new molecules. Paper burning is an example of a chemical change—the paper changes to carbon.

The sum of the atomic weights of all the atoms in a formula is called the formula weight. A mole is the amount of substance containing Avogadro's number of particles, 6.02 × 1023 atoms of an element. The abbreviation is mol.

Use the following rules in balancing equations:

1. All reactants (starting materials) and products must be known, and their formulas must be correctly written.
2. The formulas are never changed in order to balance an equation.
3. The number of atoms of each element in the reactants must be equal to the number of atoms of the element in the products.
4. The numbers to the left of each formula (coefficients) must be reduced to the smallest possible whole numbers.

Types of chemical reactions include:

• Combination or synthesis reactions: two chemicals combine to form a new substance
• Decomposition reactions: one substance breaks down to create two or more substances

Energy

Energy, the capacity to do work, is never created or destroyed—it may only be changed in form. There are many forms of energy, including light, sound, heat, chemical, mechanical, and electrical. Energy, like power, is measured in joules.

The two forms of energy are potential and kinetic energy. Potential energy is the energy stored in a body or system as a consequence of its position, shape, or state. Kinetic energy is the energy of motion, and is usually defined as the work that will be done by the body possessing the energy when it is brought to rest.

Sound and light are measured by their wavelength. A wave is a periodic disturbance in a medium or space. Sound is a vibration at a frequency and intensity audible to the normal human ear (20–20,000 Hertz). Vibrations that have a lower frequency are called infrasounds, and those with a higher frequency are called ultrasounds. Light is a form of electromagnetic radiation. Light can pass through a vacuum, but sound cannot. The electromagnetic spectrum is displayed in the following diagram.

Refraction is the bending of light, especially when it moves from one material to another. The refraction of sunlight results in a spectrum of colors, such as a rainbow. The colors in a spectrum are red, orange, yellow, green, blue, indigo, and violet.

Another form of energy—heat—may be transferred via conduction, convection, or radiation. Conduction is the transmission of heat from a region of high temperature to a region of lower temperature. Putting a cool kettle of water on a stove to boil is a good example. Convection is the process by which parts of a fluid or gas change density due to the uneven application of heat. For instance, as a stove heats a cool room, the warmer (less dense) air rises and the cold air remains lower. Radiation is heat transmitted via electromagnetic waves. The sun's heat warming the petals of a flower is an example of radiation.