Definitions of Chemistry

• Chemistry deals with the properties of materials and the changes these materials undergo. (DLB, 7th Ed., Prentice-Hall)

• Chemistry is a physical science that deals with the composition, structure and properties of substances and the reactions they undergo. As a science, chemistry involves the intuitive and creative application of knowledge gained from experiment observation and theoretical descriptions.

• Anything that bites or scratches is biology

  Anything that stinks or pops is chemistry

  Anything that does not work is physics

Definition of Matter

The two distinguishing characteristics of matter is that matter is classified as anything that has mass and occupies space.

Composition of Matter

How do chemists classify matter? Since there is a lot of matter and chemists/scientists have had a long time to think about this classification scheme, matter has been classified in the broadest sense as consisting of mixtures or pure substances.

Pure substances are probably the first things to consider. Pure substances are either elements or compounds.

An element is an example of matter which consists of only one kind of atom, either individually or combined into larger units. Atoms are the smallest particles of an element that retain the chemical properties of the element. So elements are composed of atoms. An element is composed of only one kind of atom. For example, carbon is an example of an element, because when it is a pure substance it is composed of only carbon atoms. Diamond for example is composed of only carbon atoms. It is a pure substances and diamond, which consists of only carbon atoms is an element. The atmosphere is contains the element oxygen. Pure oxygen is composed of oxygen molecules. Molecules are units of matter consisting of two or more atoms combined in a definite ratio. So an element may consist of only atoms, all of the same kind, as in the case of carbon, or an element may consist of molecules. The molecules are made up of two or more atoms, but all the atoms must be identical to be an element. Oxygen consist of two atoms combined together to make a molecule. Phosphorus is another element. Phosphorus is composed of four phosphorus atoms combined together to make a molecule. But in the case of phosphorus the molecules are composed of four atoms! So what about these elements, which are composed of atoms and which are composed of molecules? The table below summarizes that for us;

Element Name	Molecules/atoms	Symbol	Formula
Hydrogen, oxygen, nitrogen, fluorine, chlorine, bromine, iodine	molecules (2 atoms)	H, O, N, F Cl, Br, I	H2, O2, N2, F2 Cl2, Br2, I2
phosphorus	molecules (4 atoms)	P	P4
sulfur	molecules (8 atoms)	S	S8
all the remaining elements in the periodic table	atoms	C, Si, Mo, Pb etc.	C, Si, Mo, Pb etc.

Compounds are the other example of pure substances. Compounds are also composed of atoms, but now there are more than one kind of atom. Water is an example of a compound because it consists of hydrogen and oxygen atoms in a definite ratio. Water is exists as molecules, because water is made up of more than one atom. Water is also a compound because its molecules are made up of two different kinds of atoms. The formula for water is H₂O. It consists of two atoms of hydrogen and one atom of oxygen. Elements that can exist in different forms are called allotropic. An allotrope is a different form of the same element which exists in the same physical state. Two examples are the elements oxygen and carbon. Oxygen and ozone are allotropic forms of oxygen. Graphite and diamond are allotropic forms of carbon.

Mixtures are composed of two or more pure substances. Mixtures come in two varieties, heterogeneous mixtures by far the largest group and homogeneous mixtures. In class I showed you one example of a heterogeneous mixture - the mineral sample. Heterogeneous mixtures contain two or more substances combined such that the composition, appearance and properties of the mixture is nonuniform. In the mineral sample it was possible to see different substances on the sample. On the other hand, for homogeneous mixtures the properties, composition and appearance are uniform. We can have mixtures of elements or mixtures of compounds or even mixtures of elemetns and compounds. These can be either homogeneous or heterogeneous.

So would I expect you to be able to look at an example of matter and know whether it is an element or a compound, or a mixture. Probably not without some practice. In class I had some examples of some elements. I showed you the elements; chlorine, bromine, mercury, lead, helium and nitrogen. I also showed you some examples of compounds; carbon dioxide, water, and toluene. I showed you an example of a heterogeneous mixture of water and toluene. Could you tell which of the elements exists as atoms or molecules? And if an element exists as molecules, how many atoms of the element are in the molecule? Could you tell me what atoms are in the three compounds I showed you? Could you tell me how many atoms of each element are in each compound? For right now I'll only expect you to be able to recognize elements and compounds that I have showed you or that we have talked about, or that you have encountered in laboratory.

How do we tell if a substance is an element or a compound? First we must purify the substance so we know we do not have a mixture. Than we see if it is possible to break the substance down into other substances. This is usually done by heating, or passing electricity through the sample, or shining light on the sample. If none of these extreme conditions cause the substance to break down into other substances it is an element, if it does break down into other substances we know we had a compound.

To help us organize substances we record the properties of the pure substance.

Diagram of an STM instrument.

Some STM Images of atoms and or molecules.

Here is another site with STM images.

Characterization of Matter

To characterize matter we use physical and chemical properties.

Physical properties are properties that can be measured and observed without changing the identity of the substance.

1. State(gas, liquid and solid) another example using water

2. Color

3. Melting Point (Important to note that a change in phase, which occurs at the boiling point or melting point of a substance does not necessarily mean a chemical change has occurred.)

4. Boiling Point

5. Density

6. Texture

7. Hardness

   Also important are

8. Mass

9. Volume

   but these properties have less to do with distinguishing between substances.

Examples of physical properties of matter.

	The obvious physical properties are that sulfur is a yellow solid (rhombic sulfur). Less obvious but which we can locate in a reference book or textbook is that sulfur has a melting point of 112.8 C and boils at 444.6 C. Several allotropic forms exist. Orthorombic sulfur density is 2.069 g/cm3, monoclinic (at 95 C) has a density of 1.94 - 2.01 g/cm 3 .
	Silvery, heavy, moveable liquid. Its melting point is -38 C, boiling point is 356 C. It has a very high density at 13.5 g/cm3. Mercury is readily enhaled. It is not so much of a hazard when ingested. Spilled mercury should be cleaned up immediately to prevent long term, low-level exposure.
	Iodine is black looking solid at room temperature and readily forms a violet vapor. The solid has a metallic luster and forms plates or scales. The vapor is corrosive with a sharp odor. Its melting point is 114 C, boiling point is 184 C and it has a density of 4.94 g/cm3. Iodine is not very soluble in hot or cold water, but it is soluble in solvents like alcohol, benzene and ether. Iodine is found in seawater. Ingestion of 2 to 4 grams have been known to be fatal.
	Nitrogen dioxide, NO2, is a brownish-red gas at room temperature. It is sealed in the glass tube to confine it and to protect us. Nitrogen dioxide is a poisonous gas. In the Merck Index Nitrogen dioxide is described as an 'insidious gas'. The gas can be fatal at a level of 200 ppm, and dangerous at 100 ppm. Its melting point is -9.3 C and it boils at 21 C. Its density is 3.3 grams/liter.

Chemical properties are also used to characterize matter.

Chemical properties involve the transformation of substances into other substances.

Note: Sample chemical reactions were demonstrated and discussed in class.

Atomic Theory and a Microscopic Model of Matter

It was a gentleman by the name of John Dalton who organized a collection of experimental observations into a theoretical framework. Dalton, who was a meteorologist, had considerable experience with air and wind and the effects of temperature on the volume of a gas. The results of his study of air suggests that his observations could be understood if matter consisted of tiny particles, a sort of submicroscopic billard ball. So Dalton proposed his atomic theory to explain his observations. Each element is made up of tiny, indivisible particles called atoms.

1. All atoms of a given element possess identical properties.

2. Atoms of different elements have different properties.

3. Chemical changes involve the combination, separation or rearrangement of atoms: atoms are neither destroyed, created or changed.

4. When atoms combine they do so in fixed ratios of whole numbers forming particles called molecules.

Click here to view a figure of this microscopic model.

What is the microscopic model of the three phases of matter?

Here is an atomic level exercise from the first Problem Set.

(Note: To run the atomic level exercise the version of Netscape Navigator must be accessorized with the MacroMedia ShockWave Plugin. If Netscape displays a broken icon after clicking on the link above, then you need to download the plugin and install it. To download the plugin go to the Shockwave site and follow the instructions.)