SAT Chemistry Introduction to Chemistry - Matter
SAT Chemistry Introduction to Chemistry - MatterMatter is defined as anything that occupies space and has mass. Mass is the quantity of matter that a substance possesses and, depending on the gravitational force acting on it, has a unit of weight assigned to it. Its formula is w = mg, where m is the mass of the substance and g is a gravitational constant. Although weight then can vary as the gravitational constant does, the mass of the body is a constant and can be measured by its resistance to a change of position or motion. This property of mass to resist a change of position or motion is called inertia. Since matter does occupy space, we can compare the masses of various substances that occupy a particular unit volume. This relationship of mass to a unit volume is called the
density of the substance. It can be shown in a mathematical formula as D = m/v The unit of mass (m)
commonly used in chemistry is the gram (g), and of volume (V) is the cubic centi-meter (cm3), milliliter (mL), or liter (L).
An example of how density varies can be shown by the difference in the volumes occupied by 1 gram of a metal, such as gold, and 1 gram of Styrofoam. Both have the same mass, 1 gram, but the volume occupied by the Styrofoam is much larger. Therefore, the density of the metal will be much larger than that of the Styrofoam. In chemistry, the standard units for density of gases are grams/liter at a standard temperature and pressure. This aspect of the density of gases is discussed in Chapter 6. Basically then, density can be defined as the mass per unit volume.
States of Matter
Matter occurs in three states: solid, liquid, and gas. A solid has both a definite size and a definite shape. A liquid has a definite volume but takes the shape of the container, and a gas has neither a definite shape nor a definite volume. These states of matter can often be changed by the addition or subtraction of heat energy. An example is ice changing to liquid water and finally steam.
Composition of Matter
Matter can be subdivided into two general categories: pure substances and mixtures. A pure substance can be subdivided into the smallest particle that still has the properties of that substance. At that point, if the substance is made up of only one kind of atom, it is called an element. Atoms are considered to be the basic building blocks of matter that cannot be easily created nor destroyed. The word atom comes from the Greeks and means the smallest possible piece of something. Today, scientists recognize approximately 114 different kinds of atoms, each with its own unique composition. These atoms then are the building blocks of elements when only one kind of atom makes up the substance. If, however, two or more kinds of atoms join together in a definite grouping, this pure substance is called a compound. Compounds are made by combining atoms of two or more elements in a definite proportion (or ratio) by mass, according to the Law of Definite Composition (or Proportions). The smallest naturally occurring unit of a compound is called a molecule of that compound. A molecule of a compound has a definite shape that is determined by how the atoms are bonded to or combined with each other, as described in Chapter 3. An example is the compound water: it always occurs in a two hydrogen atoms to one oxygen atom relationship. Mixtures, however, can vary in their composition.
In general, then:
The following chart shows a classification scheme for matter.
Chemical and Physical Properties
Physical properties of matter are those properties that can usually be observed with our senses. They include everything about a substance that can be noted when no change is occurring in the type of structure that makes up its smallest component. Some common examples, are physical state, color, odor, solubility in water, density, melting point, taste, boiling point, and hardness.
Chemical properties are those properties that can be observed in regard to whether or not a substance changes chemically, often as a result of reacting with other substances. Some common examples are: iron rusts in moist air, nitrogen does not burn, gold does not rust, sodium reacts with water, silver does not react with water, and water can be decomposed by an electric current.
Chemical and Physical Changes
The changes matter undergoes are classified as either physical or chemical. In general, a physical change alters some aspect of the physical properties of matter, but the composition remains constant. The most often altered properties are form and state. Some examples of physical changes are breaking glass, cutting wood, melting ice, and magnetizing a piece of metal. In some cases, the process that caused the change can be easily reversed and the substance regains its original form. Water changing its state is a good example of physical changes. In the solid state, ice, water has a definite size and shape. As heat is added, it changes to the liquid state, where it has a definite volume but takes the shape of the container. When water is heated above its boiling point, it changes to steam. Steam, a gas, has neither a definite size, because it fills the containing space, nor shape, because it takes the shape of the container.
Chemical changes are changes in the composition and structure of a substance. They are always accompanied by energy changes. If the energy released in the formation of a new structure exceeds the chemical energy in the original substances, energy will be given off, usually in the form of heat or light or both. This is called an exothermic reaction. If, however, the new structure needs to absorb more energy than is available from the reactants, the result is an endothermic reaction. This can be shown graphically.
Notice that in Figures 1 and 2 the term activation energy is used. The activation energy is the energy necessary to get the reaction going by increasing the energy of the reactants so they can combine. You know you have to heat paper before it burns. This heat raises the energy of the reactants so that the burning can begin; then enough energy is given off from the burning so that an external source of energy is no longer necessary.
Conservation of Mass
When ordinary chemical changes occur, the mass of the reactants equals the mass of the products. This can be stated another way: In a chemical change, matter can neither be created nor destroyed, but only changed from one form to another. This is referred to as the Law of Conservation of Matter (Lavoisier—1785). This law is extended by the Einstein mass-energy relationship, which states that matter and energy are interchangeable