Scientist have examined the particles that make up a substance and have concluded that in some cases all the particles that make the substance are alike; in other cases, all the particles are different. When all the particles are alike the substance is pure and is called an element. Elements are made of tiny particles called atoms, which we will discuss later. Just remember that atoms of the same element are alike, and atoms of different elements are different. There are just over a hundred elements and as a shorthand way of writing their names we use chemical symbols. Usually the symbol is represented by the first letter of the element's name. The symbol for Hydrogen is H, for oxygen it's O. Sometimes the first and second letters are used, for Aluminum the symbol is Al. Notice that the first letter is capitalized, but the second letter is lowercase. Some symbols use the element's Latin name. The symbol for Sodium is Na, taken from its Latin name Natrium.

A compound is formed when you chemically combine two of more different elements. The properties of the elements that make up a compound are often quite different from the properties of the compound itself. For instance, sodium, a silvery metal that explodes in water(H₂0), will chemically combine with the poisonous gas Chlorine(Cl₂) to form the harmless compound-sodium chloride(NaCl)-which we know as common table salt. Most compounds are made of molecules. A molecule is made of two or more atoms chemically bonded together. Water is a compound because it is the combination of the elements Hydrogen and Oxygen. It is also a molecule because two hydrogen atoms bond with two Oxygen atoms. To show this combination science uses a chemical formula. A chemical formula is a shorthand way of representing chemical substances. H₂0 is the chemical formula for water. C₆H₁₂O₆ is the chemical formula for sugar. When writing a chemical formula, you use the symbol of each element in the compound along with a subscript(the number that appears to the right of and below the symbol) to indicate the number of atoms of the element being used---O₃ [the subscript 3 indicates that there are three atoms being used by oxygen]. When there is only 1 atom of an element, the subscript 1 is not written. It is understood to be 1. The number that appears in front of the formula is called the coefficient and it indicates the number of molecules present in the formula. If no coefficient is given, 1 is understood.

Look at the picture to the left, what does the picture tell you about tap water? That's right, tap water is not pure. It is actually the combination of substances, or for us a mixture. Mixtures can be classified as heterogeneous or homogeneous, depending on how "well mixed" they are. Heterogeneous mixtures can be thought of as the least mixed of mixture. The particles in a heterogeneous mixture are large enough to be seen and can settle to the bottom of the mixture over time, indicating that you might have to shake the mixture to distribute the particles evenly. Itallian dressing, concrete, peanut butter and jelly mixtures and even a hamburger can be classified as heterogeneous mixtures. A mixture that appears to be the same throughout is said to be homogeneous. The particles that make up the mixture are very small and not easily recognizable. The particles do not settle when the mixture is allowed to stand.

A homogeneous mixture in which the particles are mixed together but no dissolving takes place is known as a colloid. The Tyndall Effect refers to the colloids ability to scatter light. You see the Tyndall Effect when you see a spotlight flashing through the nighttime sky. Another phenomena of colloids is Brownian motion, which indicates that colloid particles are in constant motion.In the picture to the left you can see smoke exiting the chimney of the house. Smoke is an example of solid ash particles mixing with the heated air that rises out of the chimney. Other examples of colloids are fog(liquid mixed with a gas), foam(gas in a liquid), emulsion(liquid in a liquid), sol(solid in a liquid), and a gel(liquid in a solid).

A solution is a type of homogeneous mixture formed when one substance dissolves in another. The substance that dissolves is called the solute,and the substance that does the dissolving is called the solvent. Because water can dissolve many substances, it is called the "universal solvent." A substance that dissolves in another substance is said to be soluble, if a substance cannot dissolve it is said to be insoluble. Temperature plays a significant role as to how much solute will dissolve in a solvent. In general, as the temperature of a solvent increases, the solubility of the solute increases. This is the case when you attempt to dissolve more sugar into water. By heating the water more sugar will dissolve into the mixture. The same cannot be said of gaseous solutes. Look at the picture to the right. When temperature increase more of the gas that is dissolved into the solution will escape. That is why soda looses its fizz and taste flat.

Not all solutions are liquids. Solutions can exist in any of the three phases-gas, liquid, or solid. Metal solutions called alloys are examples of solids dissolved in solids.

Another way in which matter can be classified is based upon its physical or chemical properties. A property is a characteristic that a substance demonstrates. Physical properties can be observed such as a material's color, odor, texture, shape, melting point of boiling point. All of these characteristics can be observed without conducting any type of chemical test. Think of an apple, in your mind what can you observe about the apple with your eyes closed? You can observe it's smell, color, shape, and it's texture (feel). Physical properties are useful in helping us identify matter.

Sometimes its necessary for us to change a substance, for example we can't drink ice. Whenever a substance changes, yet its identity remains the same a physical change has occurred. If the change results in a completely new substance being created, a chemical change has occurred. If you leave a pizza in the stove too long, you'll notice an unpleasant smell that alerts you something is burning. This burnt odor is a clue telling you that a new, less desirable substance has formed. A change of one substance to another is a chemical change and this process of change is described as a chemical property. All substances have chemical properties that help us identify them. Iron rusts, silver tarnishes, wood burns, all of these are chemical characteristics that help us identify an unknown substance.

Ask yourself this question, when a substance changes is any mass loss. Such as wood burning, is there any mass lost in this process? The answer is no. Quite simply, the mass of the wood changes into vapor (gas) and ashes. We need to go back to Einstein and the Law of Conservation of Mass that states mass cannot be created or destroyed; mass merely changes from one form to another.