Physical Science for Armed Services Vocational Aptitude Battery (ASVAB) Study Guide (page 3)
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.
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 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:
- All reactants (starting materials) and products must be known, and their formulas must be correctly written.
- The formulas are never changed in order to balance an equation.
- The number of atoms of each element in the reactants must be equal to the number of atoms of the element in the products.
- 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
Concentration, Acids, Bases, and pH
Concentration is a measure of how much solute is in a solution. A solute is a substance that is dissolved in a medium, and a solvent is a medium in which a solute is dissolved. For instance: salt water is a solution—a homogenous mixture consisting of the solute (salt) and the solvent (water).
Acids are proton donors; they release hydrogen ions (protons). Acids have a sour taste. Bases are proton acceptors; they take up hydrogen ions. Bases have a bitter taste and feel slippery. Strong bases take up more hydrogen ions than weak bases.
The relative concentration of hydrogen ions is measured in concentration units called pH units. The pH scale runs from 0 to 14. A substance with a pH of 7 is neutral. Substances such as vinegar and orange juice, with a pH of less than 7, are considered acidic. Substances such as soaps and ammonia, with a pH of more than 7, are considered alkaline.
Although you may be more familiar with the English system of measurement (inches, pounds, and so on), the metric system is the standard system of measurement in science. The metric system is a decimal system based on multiples and fractions of ten. The meter (m) is the standard unit of length in the decimal system:
- 1 meter = 100 centimeters (cm)
- 1 kilometer (km) = 1,000 meters
The gram (g) is the metric system unit of mass:
- 1 gram = 1,000 milligrams (mg)
- 1 kilogram (kg) = 1,000 grams
Volume, the amount of space occupied by a fluid or body, is usually measured using the liter. The cubic meter (cm3) is actually the standard metric unit of volume, but it is infrequently used.
- 1 liter (L) = 1,000 milliliters (ml)
- 1 cm3 = 1 ml
In science, temperature is most often measured using degrees Celsius (°C). On the Celsius scale, the freezing point for water is 0° and the boiling point for water is 100°. This makes it much easier to use than the Fahrenheit scale, which has a freezing point of 32° and a boiling point of 212°. The two equations below show how to convert a temperature measurement from one scale to the other.
Other important equations are as follows:
- Speed is the distance covered or traveled by an object per a certain unit or amount of time:
- Momentum is the tendency of an object to continue moving in the same direction: Momentum = mass × speed
- Work is a force applied to an object which, in turn, results in movement:
- Power is the rate at which work is done. It is measured in joules (J):
Speed = distance/time
Work = force × distance
Power = work/time
Note: Mass should not be confused with weight. Mass is the measurement of the amount of matter in an object. Weight is the force by which gravity attracts a body to Earth.
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