Chemistry 1-1

Chemistry

Lecture 1-1

Classification of Matter

Chemistry is the study of matter. Matter is anything that has mass and takes up space (volume). How do chemists classify the various types of matter?

In nature most pieces of matter are actually mixtures.  Mixtures are combinations of two or more substances in which each substance retains its own chemical identity.  Much of the chemical industry deals with the separation and purification of pure substances from these mixtures.  A piece of matter can be classified according to its uniformity.  A phase is a region of matter with a uniform set of properties.  Matter that consists of a single phase is said to be homogeneous.  If more than one phase is present in a piece of matter, it is classified as a heterogeneous mixture.  Sand, rocks, and wood are examples of heterogeneous mixtures because their composition and properties vary throughout the mixture.  It is easy to test a piece of matter to determine whether it is homogeneous or heterogeneous.  Just take the piece of matter and subdivide it into portions.  If the properties of each portion are the same, the matter is homogeneous.  If the properties of the various portions are different, the matter should be classified as a heterogeneous mixture.

Once we have determined that a piece of matter is homogeneous, the next step in the classification depends on whether the piece of matter is a homogeneous mixture or a pure substance.  Pure substances cannot be separated into simpler components by physical changes.  During physical changes a substance changes its physical appearance but NOT its composition.  Common examples of physical changes are changes in state like boiling or melting.  Such physical changes will alter the physical properties of a substance like density but will not alter chemical properties.  Adding salt to water will alter the physical properties of the water.  For example, salt water has a higher boiling point than pure water.  However, the chemical composition of the water is not changed as salt is dissolved in the water.  Salt water is an example of a homogeneous mixture because each drop of salt water has exactly the same properties.  Notice that there is a single phase in this case but there are two substances present!  Because a single phase is present, filtration CANNOT be used to separate salt from water. Homogeneous mixtures are commonly called solutions.  Salt water is a solution in which the salt is referred to as the solute and the water is referred to as the solvent. An aqueous solution is one in which water is the solvent.  The salt water can easily be separated by heating the mixture.  The water will boil off leaving the salt behind.  The water vapor can be captured and cooled to yield pure water.  The diagram below shows a simple distillation apparatus used to separate the components of salt water. 

If we are unable to separate a homogenous piece of matter by using physical changes, we can classify the matter as a substance.  A pure substance has a fixed composition (formula) and distinct physical and chemical properties.  Physical properties like mass, volume, density, color, etc. can be measured without changing the composition of a substance.  Chemical properties describe the way a substance may change or react to form other substances. Flammability is a chemical property that measures how easily a substance combines with oxygen (burns) to form new substances. 

Physical properties may be intensive or extensive.  Extensive are those which depend upon the amount of substance like mass and volume.   Intensive properties do NOT depend on the amount of substance.  The density (density = mass/volume) does not depend on the mass or the volume of the substance!  This ratio of mass to volume is constant for a substance.  Intensive properties are used to identify substances.  All chemical properties are intensive.

Once we have determined that we have a substance, the next step in the classification deals with our ability to separate the substance using chemical changes.  If we are able to separate the substance by chemical changes into simpler substances, the substance is classified as a compound. Remember that chemical changes always produce new substances!  If we pass electricity through pure water, we can separate the water into two component gases, oxygen and hydrogen.  Water is an example of a compound.  If chemical changes do NOT result in the formation of new substances, the substance is classified as an element.  Elements are the basic building blocks of matter.  

At present there are about 120 elements.  Ninety two of the elements occur naturally and the others are man-made. The elements vary widely in their abundance and occurrence as is shown in the diagram below.

 The elements can be classified as either metals or nonmetals. Metals are generally characterized by a shiny luster, high electrical and thermal conductivities, malleability (can be pounded into thin sheets), and ductility (can be drawn into a wire).  Most metals are solids at room temperature. Mercury is a notable exception being a liquid at room temperature.  Non-metals vary greatly in appearance.  They are not lustrous and are not good conductors.  Their melting points are generally lower than that of metals. In the periodic table, the metals are found on the left hand side of the table and the nonmetals are found on the right hand side of the table.  The metals greatly outnumber the nonmetals. Some elements are classified as metalloids.  Metalloids have properties that are intermediate between those of metals and nonmetals. Silicon is a notable metalloid that is a semiconductor used in integrated circuits and computer chips. The periodic table below shows the positions of the metals, nonmetals, and metalloids in the table.