Classification and types of Matter in Chemistry
Matter is anything that has mass and occupies space. It exists in three phases: solids, liquid and gas. A solid has a fixed shape and volume. A liquid has a fixed volume but is not rigid in shape, it takes the shape of the container. A gas has neither a fixed volume nor a rigid shape, it takes on both the volume and the shape of the container.
Matter can be classified into two categories.
Pure substances, each of which has a fixed composition and a unique set of properties, mixtures, composed of two or more substances.
Pure substances are either elements or compounds, where as mixtures can be either homogeneous or heterogeneous.
An element is a type of matter that cannot be broken down into two or more pure substances. There are 115 elements of which 91 occur naturally.
Many elements are familiar to all of us. The charcoal used in outdoor grills is nearly pure carbon. Electrical wiring, jewelry and water pipes are often made from copper, a metallic element. Another such element, aluminum, is used in many household utensils.
Some elements come in and out of fashion, so to speak. Sixty years ago, elemental silicon was a chemical curiosity. Today ultra pure silicon has become the basis for multibillion dollar semiconductor industry. Lead, on the other hand, is an element in the other direction. A generation ago, it was widely used to make paint pigments, plumbing connections and gasoline additives. Today, because of the toxicity of lead compounds, all of these applications have been banned in the United States.
In chemistry, an element is defined by its symbol. This consists of one or two letters, usually derived from the name of the element. Thus, the symbol for carbon is C. That for aluminum is Al. Sometimes the symbol comes from the Latin name of the element or one of its compounds. The two elements copper and mercury, which were known in ancient times, have the symbols Cu (cuprum) and Hg (Hydrargyrum).
Curiously, several of the most familiar elements are really quite rare. An example is mercury, which has been known since at least 500 B.C., even though its abundance is only 0.00005%. It can easily prepared by heating the red mineral cinnabar. Beyond that, its unique properties make it extremely useful. Because of its high density, mercury is the liquid used in barometers and manometers. Mercury dissolves many metals, forming solutions (amalgams). A silver-mercury-tin amalgam is used in filling teeth.
In contrast, aluminum (abundance 7.5%), despite its usefulness, was little more than a chemical curiosity until about a century ago. It occurs in combined form in clays and rocks, from which it cannot be extracted. In 1886 two young chemists, Charles Hall in the United States and Paul Herroult in France, independently worked out a process for extracting aluminum from relatively rare ore, bauxite. That process is still in use today to produce the element.
A compound is a pure substance that contains more than one element. Water is a compound of hydrogen and oxygen. The compounds methane, acetylene and naphthalene all contain the elements carbon and hydrogen, in different proportions.
Compounds have fixed compositions. That is, a given compound always contains the same elements in the same percentage by mass. A sample of pure water contains precisely 11.19% of hydrogen and 88.81% oxygen. In contrast, mixtures can vary in composition. For example, a mixture of hydrogen and oxygen might contain 5, 10, 25 or 60% hydrogen, along with 95, 90, 75 and 40% oxygen.
The properties of compounds are very different from those of the elements that they contain. Ordinary table salt, sodium chloride, is a white, unreactive solid. As one can guess from its name, it contains two elements sodium and chlorine. Sodium (Na) is a shiny, extremely reactive metal. Chlorine (Cl) is a poisonous, greenish-yellow gas. Clearly, when these two elements combine to form sodium chloride, a profound change takes place. Sodium, a metallic element that is soft enough to be cut with a knife and chlorine, a nonmetallic element that is a gas. Both sodium and chlorine combine to form the crystalline compound.
A mixture contains two or more substances combined in such a way that each substance retains its chemical identity. When you shake copper sulfate with sand, the two substances do not react with one another.
There are two types of mixtures
Homogenous or uniform mixtures are ones in which the composition is the same throughout. Another name for a homogenous mixture is solution, which is made up of a solvent, usually taken to be the substance present in largest amount, and one or more solutes. Most commonly, the solvent is liquid, whereas solutes may be solids, liquids or gases. Soda water is a solution of carbon dioxide (solute) and water (solvent). Seawater is a more complex solution in which there are several solid solutes, including sodium chloride; the solvent, is water. It is also possible to have solutions in the solid state. Brass is a solid solution containing the two metals copper and zinc in proportions of 67-90% and 10-33% respectively.
Heterogeneous or nonuniform mixtures are those in which the composition varies throughout. Most rocks fall into this category. In a piece of granite, several components can be distinguished, differing from one another in color.
Many different methods can be used to separate the components of a mixture from one another. A couple of methods that you may have carried out in the laboratory are
Filtration, used to separate a heterogeneous solid liquid mixture. The mixture is passed through a barrier with fine pores, such as filter paper. Copper sulfate, which is water soluble, can be separated from sand by shaking with water. On filtration the sand remains on the paper and the copper sulfate solution passes through it.
Distillation, used to resolve a homogenous solid-liquid mixture. The liquid vaporizes, leaving a residue of the solid in the distilling flask. The liquid is obtained by condensing the vapor. Distillation can be used to separate the components of a water solution of copper sulfate.
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