Introduction to Matter

Chemistry is the microscopic (atomic and molecular) understanding of the world around us and how it transforms. Chemists desire to know nature and work within natural laws. Humankind's knowledge of Chemistry has a profound effect upon civilization. Chemical knowledge is required to live in modern technology.
Everyone has an example of how 'chemical' have been used to improve or degrade the quality of their lives. Some of the most important issues regarding the survival of the Human race are a result of, and will be corrected by, the manipulation and control of chemical reactions. Chemistry itself is neither Good nor Evil. The use of chemical knowledge is no longer in debate. The undestanding of chemistry is needed to solve the Earth's problems and help protect you as an individual from errors of ignorance.

It is easy to 'see' macroscopic properties of matter. It has been found that the properties of materials are a consequence of their microscopic structure --- the structure of the atoms and molecules that compose it. It is not possible to see an atom. We must therefore go beyond the senses that are our birthright and learn about what we cannot see, hear, touch or smell.
We learn about things we cannot see all the time. I have never seen a gravity field, but I know how it affects my life. Precise measurements and predictive theory of the unseen gravity field are made everytime we throw a ball.
Measurement allows us to 'see' beyond our senses and develop new ones.
Science is an institution which refines, explains and communicates measurements to predict future behavior.
Chemistry is the science of molecules (or perhaps the electrons in molecules).

Matter is a term to describe all the stuff around us that has mass an occupies space. Everything else is Energy. Matter, as we might have guessed already, is composed of very small 'particles' called molecules, which themselves are composed of atoms. The way the molecules in a substance move determines the phase of the matter, eg. molecules move freely in a gas. There are many kinds of matter, but only a few different phases of matter (what are they?) That means there are many different kinds of molecules. A substance that only has one kind of molecule in it is called a pure compound. A substance that has only one kind of atom in it is called an element. A substance that has more than one kind of molecule in it is called a mixture. Here is a cartoon of the molecules in a few example gases:

The classification of the type of matter that you have, involves knowing something about the physical (macroscopic) structure as well as the molecular (microscopic) structure of the the substance. This can sometimes be tricky, and is even sometimes open to a certain amount of interpretation. Often, a classification scheme like the following is used:

Lets discuss the questions in the blue boxes. The first question, 'Is it uniform?', really questions the macroscopic uniformity. If treated microscopically, nothing is uniform because even atoms themselves are lumpy. So, in answering this question we must examine the uniformity of the material over a distance that corresponds to many molecular diameters. So, a glass of water is uniform, but a piece of wood is not.
The second question, 'can it be separated?', also referes to processes that can be made on the bulk material, but by methods that can discriminate between individual molecules by some property. Examples of physical separation include crystallization, distillation, extraction, centrifugation, and all chromatographic methods.

The third question, 'can it be decomposed', now refers to chemical processes that can rearrange and perhaps separate the atoms within molecules. This is usually thought of as thermal decomposion, but remember that reactions with atmospheric gases (Oxygen, in particular) can occur even for a pure element and does not count as a decomposition. A pure (elemental) metal may oxidize in air at high temperature, but in a vacuum it just melts and then vaporizes (no chemical change)

In short, the classification of a substance as to mixture, compound, or element requires knowledge of what moleules are present and how they are arrange. To get that 'picture', you need to use your brain, not a flowchart.

The elemental composition of the matter around us is complicated, but does not involve an equal contribution from all the elements. Depending on what you are looking at, the abundance of the elements in a material will vary. Here are the elemental abundances, BY MASS, in the earths crust and in the Human Body:

If you cound the abundance of the elements in the Human Body by number of atoms and not by weight, then the most abundant element is by far HYDROGEN. In fact, if you consider the composition of the entire universe by number of atoms of each element, the universe is 91% H, 8.75% He, and 0.25% everything else. How can we convert between the percent by weight and percent by number of atoms? We need to know how much each atom weighs!

Here, with a total of 8 marbles, the number percent of yellow marbles is 3/8=37.5%. But the mass of the yellow marbles is 1.0 g/marble which is less than the average weight of the marbles in the box. The mass percent of yellow marbles is 3/16=18.75%. Can you find the number and mass percent of the other color marbles? answer

Measurement: The Eyes of Science

Because the science of chemistry needs to quantify very large and very small properties, we need a convenient way of expressing these properties in an undestandable and standard fashion. We desire to have convenient units for many different kinds of measurements, but allow these units to be interconcertable. We therefore choose a standard set of units as a 'base' for commonly measured things:

You are familiar with some of these units and they are used by most modern countries even by non-scientists. One of the problems with discussing the properties of molecules (like we do in Chemistry) is that moleules are very tiny and there properties are very small and their numbers (count) are very large. It is for this reason that we invent the unit MOLE, which is like a 'bakers dozen' of atoms. So, instead of saying that 18 grams of water has 6.0 x 10²³ molecules in it, we say it has 1.0 mole of molecules, where
1.00 mole of objects = 6.02 x 10²³ objects just like 1.00 dozen objects = 1.2 x 10¹ objects

Unfortunately, we also have to 'scale' all the other units for very small and very large measurements. We do this by putting a prefix on the unit base that conveys how many powers of ten we wish to multiply or divide the base unit by. Common prefixes are:

The units for length, volume, and mass for the SI system are quite cleverly interconverted. The unit of volume is defined to be 1 cubic decimeter. The unit of mass is 1 liter of WATER at 277 K. (water is common, and it has a maximum density at 4 ^oC. How the meter itself is defined is a long and historically boring story.

In general, the mass of a given volume of some substance is defined as the Density (or more precisely Mass Density) of that substance. So, we know the value of density for at least one sustance, water, in SI units; the density of water is 1.00 kg/l (or 1.00 g/cm³) at 277K.

Temperature, on the other hand, can be a bit of a mess. Even though the KELVIN temperature scale is supposed to be the standard, many scientists still report temperature in Celsius, and even worse, the weather channel report temperatures in degrees Fahrenheit. Luckily, the Kelvin and Celcius scales have the same size unit (a change of 1 K results in a change of 1 ^oC) and only differ by the ZERO of the temperature scale. BUT the zero of the Kelvin temperature scale is actually ABSOLUTE ZERO, so it is called an absolute temperature scale. Rational beings use only absolute temperature scales. If you always convert ALL of your temperatures to Kelvin at the beginning of every calculation, you cannot go wrong. Here is how the three most common temperature scales stack up.

What is the absolute zero of temperature on the Celsius scale? The Fahrenheit scale? answer

The Scientific Method
A complete philosophical dicussion of how science evaluates and understands nature through the test of hypothesis is clearly beyond the scope of this course. Remember, however, that science is an iterative process, and that occasionally, in the face of new evidence, we must abandon theories and concepts which seemed to serve us well...

Here are some interesting Lengths (a) Volumes (b) and Masses (c). Note the non-linear scale

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PJ Brucat // University of Florida