All matter is made up of atoms. Whether matter is found in a mixture or as elements or compounds, it is still comprised of atoms.

A substance composed of a single kind of atoms is called an element. These atoms may be bonded together to form molecules. An example is oxygen that exists as molecules of O₂. (Many people erroneously think that only compounds are made of molecules.)

A compound is formed when two or more different kinds of atoms bind together chemically. Water is a compound because it is formed from two different kinds of atoms—atoms of hydrogen and oxygen.

Any material that is composed of only one kind of “particle” is considered to be a substance. Elements and compounds are both substances because they both consist of one type of “particle.” Iron, an element, is a substance because it is made of only iron atoms. Water, a compound, is a substance because it is made only of molecules of H₂O.

A mixture is made of two or more different kinds of particles. Sugar water is a mixture because it is composed of two different kinds of particles—sugar molecules and water molecules. A mixture is NOT a substance. (It is actually two or more different substances, each of which retains its own characteristic properties.)

When a mixture is made, the properties of the mixture will not be exactly like the properties of the pure substances that comprise the mixture. The substance in the largest quantity in the mixture will dictate, to a large extent, the properties of the mixture. The pure substances that make up the mixture still retain their characteristic properties as no new substances are produced in making a mixture. For example, in the mixture of gases that makes up earth’s atmosphere. Oxygen still has the same density, solubility in water at a given temperature, boiling point, ect. as pure oxygen. However, in order to measure those properties of oxygen, it must first be separated from the other gases (nitrogen, hydrogen, etc.) that make up our atmospheric mixture. Conversely, if one measured the density of the mixture it would be different from the density of any of the individual gases that comprise the mixture.

3. Mixtures can be classified as heterogeneous or homogeneous. Solutions are homogenous mixtures at the particle level.

A heterogeneous mixture is usually apparent at the macroscopic level. It is obviously two different substances mixed (salt and pepper, oil and vinegar, etc.) that may or may not be in different physical states: liquid and gas).

Homogenous mixtures appear to be one substance at the macroscopic level. This means that only one physical state is apparent (i.e. only solid, only liquid, etc.) and the two or more substances are mixed in such a way that it is impossible with the naked eye to distinguish the individual substances. An example is margarine. Margarine is usually made up of at least three substances. However, all the substances mix to produce what looks like “one” substance.

Many, but not all, homogenous mixtures are solutions. A solution is a homogenous mixture that is also homogenous at the particle level. A solution is formed when tiny individual particles (<1 mm in diameter) of one substance are uniformly dispersed among the individual particles of the other substance. An example of a solution is sugar water. Individual molecules of sugar are uniformly distributed among molecules of the water.

Some homogeneous mixtures are NOT solutions. Examples are margarine and milk. At the microscopic level, the particles that comprise margarine or milk are not randomly scattered but, instead, clump together. A homogeneous mixture like margarine or milk is called a colloid. In a colloid, the dispersed particles, or clumps of particles, are greater than 1 mm in diameter.

The Tyndall Effect is often used to determine if a homogeneous mixture is a solution or a colloid. The path of a beam of light can be seen passing through a colloid because the dispersed particles are large enough to scatter light. The path of a light bean cannot be seen in a true solution because the dispersed particles (such as the sugar molecules in a glass of sugar water) are too small to intercept and scatter the light.

When two substances combine to form a solution, the substance that is dissolved is called the solute and the substance that does the dissolving is called the solvent. The substance present in greater amounts is generally considered the solvent and the physical characteristics of the solution (including physical state) are typically more like the solvent than the solute.

While most people think of a solution as being solid dissolved in a liquid, solutions (and colloids) may be formed from different combinations of physical states. Drinking water is a liquid that often contains dissolved gases, such as oxygen and carbon dioxide, as well as dissolved solid minerals. Air, a mixture of several different gases, is a solution. Brass is a solution of solids.

4. A mixture can be separated into pure substances by making use of the differences in the characteristic properties of the substances contained in the mixture.

Because the pure substances the make up a mixture still retain their characteristic properties, those properties can be used to separate the components of a mixture from one another. For example, brass is a mixture of metals that have different melting points; therefore, heat could be used to separate these metals from one another. Or iron can be separated from a mixture of carious scrap metals because of a difference in magnetic properties. Chromatography may be used to separate a mixture of pigments that have differences in solubility in a particular solvent such as water or alcohol.

5. Sometimes when substances are mixed, they react chemically to form new substances with different characteristic properties. Mass is conserved in a chemical reaction.

A chemical reaction always involves the breaking and/or making of bonds between atoms. The same atoms are rearranged into new compounds in chemical reactions. This rearrangement produces new substances with different characteristic properties like density, melting point, boiling point, etc.  In a chemical reaction, no atoms are created or destroyed. Thus mass is conserved.  For example, when iron rusts, iron atoms are rearranged with oxygen atoms to produce iron oxide or rust. We can represent this chemical reaction with an equation:

Iron + Oxygen  --- rust

4Fe + 3O₂ --- 2Fe₂O₃

In this example, the total number if iron atoms, 4, and the total number of oxygen atoms, 6, are the same before and after the reaction. There are the same kinds of atoms and the same number of atoms before and after the reaction. However, the rearrangement of the atoms results in the formation of a new substance, rust, that has very different properties from the original substances. Oxygen is a colorless, odorless gas and iron is a gray metal. Rust is a reddish-brown powdery substance that does not even physically resemble the substances that were rearranged to produce it.

6. At the macroscopic level, a chemical reaction can often be recognized by the absorption or release of energy, changes of physical state (such as formation of a solid or gas), and color changes. At the microscopic level, atoms are rearranged in new combinations.

The study of chemical reactions, for chemists and students, begins by making observations. Chemists mix substances together and look for evidence (such as formation of a solid or gas, color change, or energy change) to indicate that a reaction has occurred. Additional experiments are conducted to identify the products. It is only through extensive experimentation that enough information is accumulated to enable us to predict the products of some reactions. Therefore, observations are necessary, as is speculation.

Breaking chemical bonds requires energy and making them produces energy, so chemical reactions always involve energy changes. It is the balance between bond breaking and bond formation in chemical reactions that dictates whether there will be a net consumption of energy (endothermic) or a net production of energy (exothermic).

When substances react chemically, the products of the reaction may have a different color than the reactants. Chemical reactions involve the reshuffling of atoms to form new kinds of particles that, compared to the original particles, may differ in size, spacing, orientation to one another, etc. These differences may cause the new substance to absorb a different set of wavelengths of light than the original substance. And hence, to have a different color.

Sometimes when substances react chemically, the product of the reaction may have a different physical state than either of the reactants. Consider, for example, when a solid is added to a liquid and, without heating, bubbles of a gas are produced (such as when an Alka-Seltzer^® is dropped in a glass of water). This gas is a new substance since neither of the original reactants had the preferred state of gas under the conditions of temperature and pressure present. Here is another example: Suppose you dissolve Substance A in a glass of water, and similarly dissolve Substance B in another glass of water. You then pour the two clear liquid solutions together, and almost instantaneously, particles of a white solid appear in the liquid mixture. After a few minutes, these solid particles settle to the bottom of the glass. The appearance of a solid when two liquid solutions are mixed indicates that a chemical change has occurred. Why? Since both of the original substances, A and B, were soluble in water, the white solid must be a different substance because it is insoluble in water. (A solid produced in this way is called a precipitate.) Of course, many times the appearance of a different physical state of matter is NOT the result of a chemical reaction. An example is the boiling or evaporation of a liquid. Consequently, any observed changes in physical state must be carefully analyzed before deciding that a chemical reaction has, indeed, taken place.

7. Chemical reactions occur at different rates, depending on factors such as concentration, temperature, and surface area.

Chemical reactions proceed at different rates just as a rocket travels faster than a car, which travels faster than a turtle. An example of a fast chemical reaction is the combustion of rocket fuel. A slow chemical reaction is iron rusting. The rate at which a chemical reaction occurs depends on many factors, including temperature, concentration, surface area, and the addition of a catalyst.

Have you ever wondered why it is easier to light a fire with small sticks (kindling) than with large logs? (Surface area) Or why does fruit spoil more quickly in an open fruit bowl than in the refrigerator? (Temperature) Or why acetylene burns with a blue flame, hot enough to weld metals together, if given enough oxygen, as in an oxyacetylene torch, yet burns with a yellow, sooty flame in “regular” air? (Concentration) Answers to these and many other interesting and puzzling questions can be found in a study of reaction rates.

8. Chemical reactions are vital to human life.

Chemical reactions occur all around us, for example in health care, cooking, agriculture, cosmetics, transportation, and even in nature. Complex chemical reactions involving carbon-based molecules take place constantly in every cell in our bodies. These chemical reactions change food into simpler molecules (fuel) that our bodies need for maintaining constant body temperature, walking, running, working, and even sleeping.

Photosynthesis is perhaps the world’s most important chemical reaction, providing us with food, oxygen, materials for shelter and clothing, and even vital medicines.

Many other important reactions in the everyday world, such as the combustion of gasoline in the automobile engine, are for the purpose of producing energy rather than for the purpose of producing new materials. The burning of fossil fuels, for example, produces large amounts of heat and light.