Physical Science: GED Test Prep (page 2)
Physical science includes the disciplines of chemistry (the study of matter) and physics (the study of energy and how energy affects matter). The questions on the physical science section of the GED will cover topics taught in high school chemistry and physics courses. This article reviews the basic concepts of physical science—the structure of atoms, the structure and properties of matter, chemical reactions, motions and forces, conservation of energy, increase in disorder, and interactions of energy and matter.
The Structure of Atoms
You and everything around you are composed of tiny particles called atoms. The book you are reading from, the neurons in your brain, and the air that you are breathing can all be described as a collection of various atoms.
History of the Atom
The term atom, which means indivisible, was coined by Greek philosopher Democritus (460–370 B.C.). He disagreed with Plato and Aristotle—who believed that matter could infinitely be divided into smaller and smaller pieces—and postulated that matter is composed of tiny indivisible particles. In spite of Democritus, the belief that matter could be infinitely divided lingered until the early 1800s, when John Dalton formulated a meaningful atomic theory. It stated:
- Matter is composed of atoms.
- All atoms of a given element are identical.
- Atoms of different elements are different and have different properties.
- Atoms are neither created nor destroyed in a chemical reaction.
- Compounds are formed when atoms of more than one element combine.
- A given compound always has the same relative number and kind of atoms.
These postulates remain at the core of physical science today, and we will explore them in more detail in the following sections.
Protons, Neutrons, and Electrons
An atom is the smallest unit of matter that has a property of a chemical element. It consists of a nucleus surrounded by electrons. The nucleus contains positively charged particles called protons, and uncharged neutrons. Each neutron and each proton have a mass of about 1 atomic mass unit, abbreviated amu. An amu is equivalent to about 1.66 × 10–24 g. The number of protons in an element is called the atomic number. Electrons are negatively charged and orbit the nucleus in what are called electron shells.
Electrons in the outermost shell are called valence electrons. Valence electrons are most responsible for the properties and reaction patterns of an element. The mass of an electron is more than 1,800 times smaller than the mass of a proton or a neutron. When calculating atomic mass, the mass of electrons can safely be neglected. In a neutral atom, the number of protons and electrons is equal. The negatively charged electrons are attracted to the positively charged nucleus. This attractive force holds an atom together. The nucleus is held together by strong nuclear forces.
The number of protons in an element is always the same. In fact, the number of protons is what defines an element. However, the number of neutrons in the atomic nucleus, and thus the atomic weight, can vary. Atoms that contain the same number of protons and electrons, but a different number of neutrons are called isotopes. The atomic masses of elements in the periodic table are weighted averages for different isotopes. This explains why the atomic mass (the number of protons plus the number of neutrons) is not a whole number. For example, most carbon atoms have 6 protons and 6 neutrons, giving it a mass of 12 amu. This isotope of carbon is called "carbon twelve" (carbon-12).But the atomic mass of carbon in the periodic table is listed as 12.011. The mass is not simply 12, because other isotopes of carbon have 5, 7, or 8 neutrons, and all of the isotopes and their abundance are considered when the average atomic mass is reported.
An atom can lose or gain electrons and become charged. An atom that has lost or gained one or more electrons is called an ion. If an atom loses an electron, it becomes a positively charged ion. If it gains an electron, it becomes a negatively charged ion. For example, calcium (Ca), a biologically important element, can lose two electrons to become an ion with a positive charge of +2 (Ca2+). Chlorine (Cl) can gain an electron to become an ion with a negative charge of –1(Cl–).
The Periodic Table
The periodic table is an organized list of all known elements, arranged in order of increasing atomic number, such that elements with the same number of valence electrons, and therefore similar chemical properties, are found in the same column, called group. For example, the last column in the periodic table lists the inert (noble) gases, such as helium and neon—highly unreactive elements. A row in the periodic table is called a period. Elements that share the same row all have the same number of electron shells.
Some elements are frequently encountered in biologically important molecules and everyday life. Here, you will find a list of common elements, their symbols, and common uses.
H—Hydrogen: involved in the nuclear process that produces energy in the Sun, found in many organic molecules within our bodies (like fats and carbohydrates) and in gases (like methane)
He—Helium: used to make balloons fly
C—Carbon: found in all living organisms; pure carbon exists as graphite and diamonds
N—Nitrogen: used as a coolant to rapidly freeze food, found in many biologically important molecules, such as proteins
O—Oxygen: essential for respiration (breathing) and combustion (burning)
Si—Silicon: used in making transistors and solar cells
Cl—Chlorine: used as a disinfectant in pools and as a cleaning agent in bleach, and is also important physiologically as well, for example within the nervous system
Ca—Calcium: necessary for bone formation and muscle contraction
Fe—Iron: used as a building material; carries oxygen in the blood
Cu—Copper: a U.S. penny is made of copper; good conductor of electricity
I—Iodine: lack in the diet results in an enlarged thyroid gland, or goiter
Hg—Mercury: used in thermometers; ingestion can cause brain damage and poisoning
Pb—Lead: used for X-ray shielding in a dentist office
Na—Sodium: Found in table salt (NaCl), also important biologically within the nervous system and is a key player in the active transport process that occurs across cell membranes
Some elements exist in diatomic form (two atoms of such an element are bonded), and technically, they are molecules. These elements include hydrogen (H2), nitrogen (N2), oxygen (O2), fluorine (F2), chlorine (Cl2), bromine (Br2), and iodine (I2).
Structure and Properties of Matter
Matter has mass and takes up space. The building blocks of matter are atoms and molecules. Matter can interact with other matter and with energy. These interactions form the basis of chemical and physical reactions.
Molecules are composed of two or more atoms. Atoms are held together in molecules by chemical bonds. Chemical bonds can be ionic or covalent. Ionic bonds form when one atom donates one or more electrons to another. Covalent bonds form when the electrons are shared between atoms. The mass of a molecule can be calculated by adding the masses of the atoms that it is composed of. The number of atoms of a given element in a molecule is designated in a chemical formula by a subscript after the symbol for that element. For example, the glucose (blood sugar) molecule is represented as C6H12O6. This formula tells you that the glucose molecule contains six carbon atoms (C), twelve hydrogen atoms (H), and six oxygen atoms (O).
Organic and Inorganic Molecules
Molecules are often classified as organic or inorganic. Organic chemistry is technically defined as the study of carbon compounds. However, traditionally, certain compounds that contain carbon were considered inorganic (such as CO, carbon monoxide and CO2, carbon dioxide). In fact, a lot of chemists still consider these compounds to be inorganic. Many modern chemists consider organic molecules to be those that contain carbon and one or more other elements (such as hydrogen, nitrogen, and oxygen). Examples of organic compounds are methane (natural gas, CH4), glycine (an amino acid, NH2CH2COOH), and ethanol (an alcohol, C2H5OH). Inorganic compounds include sodium chloride (table salt, NaCl), ammonia (NH3), and water (H2O).
States of Matter
Matter is held together by intermolecular forces—forces between different molecules. Three common states of matter are solid, liquid, and gas. Matter is an atom, a molecule (compound), or a mixture. Examples of matter in solid form are diamonds (carbon atoms), ice (water molecules), and metal alloys (mixtures of different metals). A solid has a fixed shape and a fixed volume. The molecules in a solid have a regular, ordered arrangement and vibrate in place, but are unable to move far.
Examples of matter in liquid form are mercury (mercury atoms), vinegar (molecules of acetic acid), and perfume (a mixture of liquids made of different molecules). Liquids have a fixed volume, but take the shape of the container they are in. Liquids flow and their density (mass per unit volume) is usually lower than the density of solids. The molecules in a liquid are not ordered and can move from one region to another, through a process called diffusion.
Examples of matter in gaseous form include helium gas used in balloons (helium atoms), water vapor (molecules of water), and air (mixture of different molecules including nitrogen, oxygen, carbon dioxide, and water vapor). Gases take the shape and volume of the container they are in. They can be compressed when pressure is applied. The molecules in gases are completely disordered and move very quickly. Gas density is much lower than the density of a liquid.
Change of phase involves the transition from one state of matter into another. Freezing water to make ice for cooling your drink, condensation of water vapor as morning dew, and sublimation of dry ice (CO2) are examples of phase changes. A phase change is a physical process. No chemical bonds are being formed or broken. Only the intermolecular (physical) forces are affected.
Freezing is the process of changing a liquid into a solid by removing heat. The opposite process whereby heat energy is added to the solid until it changes into a liquid is called melting. Boiling is the change of phase from a liquid to a gas and also requires the input of energy. Condensation is the change from gas to liquid. Some substances sublime—change directly from the solid phase to the gas phase, without forming the liquid state first. Carbon dioxide is such a substance. Solid carbon dioxide, called dry ice, evaporates into the gas phase when heated. When gas changes directly into a solid, the process is called deposition.
The stronger the intermolecular forces are, the easier it is for the molecule to exist in one of the condensed states (liquid or gas) because these interactions among the molecules hold the solid or liquid together. For example, some neutral molecules have a positive end and a negative end even though, overall, the molecules have no net charge. Molecules such as these are considered polar and are attracted to each other by dipole-dipole forces. Molecules in which intermolecular forces are strong tend to have high boiling points, since these forces need to be overcome in order to turn the molecules into the gaseous state. This necessitates the input of more energy (heat).
Compounds and Mixtures
When two or more elements combine chemically, the result is a compound. Examples of compounds include carbon dioxide (a product of respiration),sucrose (table sugar), seratonin (a human brain chemical), and acetic acid (a component of vinegar). In each of these compounds, there is more than one type of atom, chemically bonded to other atoms in a definite proportion. The combination of these atoms also result in a fixed, definite structure.
When two or more elements combine physically, the result is a mixture. In a homogeneous mixture, the components can't be visually separated. Homogeneous mixtures also have the same composition (ratio of components) throughout their volume. An example is a mixture of a small amount of salt in water. A uniform mixture is often called a solution. In a solution, one substance (solute) is dissolved in another (solvent). In the salt and water mixture, the salt is the solute, and the water is the solvent. In a heterogeneous mixture, the components can often be visually identified, and the composition may vary from one point of the mixture to another. A collection of dimes and pennies is a heterogeneous mixture. A mixture of sugar and flour is also heterogeneous. While both components (sugar and flour) are white, the sugar crystals are larger and can be identified.
Miscibility is the term used to describe the ability of two substances to form a homogeneous mixture. Water and alcohol are miscible. They can be mixed in such a way that the mixture will be uniform throughout the sample. At each point, it will look, smell, and taste the same. Oil and water are not miscible. A mixture of these two substances is not homogeneous, since the oil floats on water. In a mixture of oil and water, two layers containing the two components are clearly visible. Each layer looks, smells, tastes, and behaves differently.
Removing stains from clothes, digesting food, and burning wood in a fireplace are all examples of chemical reactions. Chemical reactions involve changes in the chemical arrangement of atoms. In a chemical reaction, the atoms of reactants combine, recombine or dissociate to form products. The number of atoms of a particular element remains the same before and after a chemical reaction. The total mass is also preserved. Similarly, energy is never created or destroyed by a chemical reaction. If chemical bonds are broken, energy from those bonds can be liberated into the surroundings as heat. However, this liberation of energy does not constitute creation, since the energy only changes form—from chemical to heat.