The fourth and final part of our free ASVAB Science Study Guide covers chemistry. Subtopics include matter, atomic structure, chemical equations, and the pH scale. It’s important to know the basic concepts of chemistry as well the most common vocabulary terms.
Atoms: The smallest unit of ordinary matter which has the properties of a chemical element. Atoms are made of protons, neutrons and electrons.
Elements: A chemical element is a substance that cannot be broken down into any simpler chemical substances. Each element is distinguished by its atomic number (number of protons). There are 118 elements and they are usually organized on a chart called the periodic table.
Molecules: A molecule is a particle made up of two or more atoms that are chemically bonded together.
Compounds: Compounds are molecules made up of two or more elements.
States of Matter
States of matter can be determined by a substance’s shape and volume.
Solid materials have a definite shape and volume. Cinderblocks are solid.
Liquid materials have a definite volume but an indefinite shape. If you pour milk from the carton into your cereal bowl, the milk’s shape will change, but the amount (volume) will not.
Gases have neither a definite shape nor a definite volume. Gases take up the shape and volume of their container. For example, helium fills the shape and size of a balloon when inflated.
When a substance changes from one state of matter to another, it is referred to as a phase change. A phase change is reversible. For example, water can easily change between being a solid, liquid and gas an infinite number of times. There are six common phase changes:
- Freezing: liquid to solid
- Melting: solid to liquid
- Condensation: gas to liquid
- Vaporization: liquid to gas
- Sublimation: solid to gas
- Deposition: gas to solid
Atoms are made of smaller, subatomic particles—protons, electrons and neutrons.
Protons are positively charged subatomic particles found in the nucleus of an atom. An atom’s nucleus may contain numerous protons. Each proton is assigned a 1+ charge.
Electrons are negatively charged subatomic particles. Electrons are found in clouds that surround the nucleus of an atom. Each electron has a 1− charge.
Neutrons are found in the nucleus, and they do not have a charge (they are neutral).
Protons and neutrons have almost exactly the same mass. Electrons are extremely small compared to the other parts of the atom. The mass of an electron is almost 1,000 times smaller than a proton.
Ions are atoms that have positive or negative charges. As a result of gaining an electron or losing of an electron, an atom becomes an ion.
Atomic Number and Mass Number
Atomic number: The atomic number of an element is defined by the number of protons in one atom of the element. Each element has a different number of protons. For example, hydrogen (H) has an atomic number 1 because it has one proton, and is the only element with one proton.
Example: Silicon has 14 neutrons, 14 protons, and 14 electrons. What is the atomic number of Silicon?
The answer is 14 since there are 14 protons.
You can also find this number in the periodic table. Below you can see Silicon (Si) in the periodic table. The number in the top right corner is 14, indicating that it’s the atomic number.
Mass number: The mass number is the number of protons plus the number of neutrons in the nucleus of an atom. If you know the mass number and the atomic number, you can calculate the number of neutrons in an atom.
Number of neutrons = Mass number – Atomic number
Chemical equations describe a chemical reaction.
Reactants are the substances in a chemical reaction that undergo a change. They are shown on the left side of the arrow.
Products are the new substances formed as a result of the reaction. They are shown on the right side of the arrow.
Reactants → Products
Example of a Chemical Equation:
Carbon + Oxygen → Carbon Dioxide
C + O2 → CO2
Reduction & Oxidation Reactions
Reduction and oxidation describes chemical reactions in which atoms have their oxidation number changed. They are also known as Redox reactions. Oxidation numbers are used to keep track of how many electrons an atom has. If electrons are lost by one atom, they must be gained by another element. This means that oxidation and reduction cannot occur alone.
- Oxidation describes the loss of electrons by a molecule, atom or ion
- Reduction describes the gain of electrons by a molecule, atom or ion
- OIL: Oxidation Is Loss
- RIG: Reduction Is Gain
The pH Scale
The pH scale is a number scale that describes the concentration of hydronium ions in a solution; it is used to show if a solution is acidic or basic. The pH scale runs from 0 to 14.
Acids have a pH less than 7, and a greater concentration of hydronium ions. Examples include vinegar and tomatoes.
Bases have a pH greater than 7, and a lower concentration of hydronium ions. Examples include baking soda and ammonia.
Neutral substances have a pH equal to 7. Distilled water is neutral.
Part 4 Review Quiz:
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______ are made up of ______
Which of the following is a phase change from a solid to a gas?
Which of the following is true of an ion?
it has a positive charge
it has a negative charge
it occurs after a movement of electrons
it occurs when an atom does not have the same amount of protons and neutrons
Boron has an atomic number of 5 and an atomic mass of 11. How many neutrons does Boron have?
not enough information
When an ion loses an electron, it is known as
Lemon juice has a pH of 2. This means that lemon juice is a(n)