Chemistry is the study of matter, its properties, and how it changes or transforms. It includes the different classifications of matter, and how materials interact with energy and with other materials.
The terms matter and substance are often used as if they are the same thing. In science, matter is anything that has mass and volume (takes up space). Examples of matter include rocks, water, air, wood, iron, etc. Note that just because you can’t see it, doesn’t mean it isn’t matter. Oxygen is invisible, but it does have mass and it does take up space. (Think of a tire filled with air...it
has a greater mass full than empty, and when filled with air, it expands.)
Not everything is matter. Electricity, heat, light, sound, etc. are forms of energy; they don’t take up space or have a mass.
Not all matter is considered a substance...the scientific definition of a substance is a specific type of matter that has a uniform and definite composition. All samples of a pure substance will have identical physical properties. What are some examples of physical properties? Examples are color, solubility, density, odor, hardness, electrical conductivity, magnetism, melting point, boiling point, etc.
At the particle level, a substance is composed entirely of identical particles. There are two classifications of substances, based on the particles that compose them: elements and compounds .
Elements are the simplest forms of matter that can exist under normal conditions. They cannot be broken down into simpler substances. Elements are composed entirely of the same type of atom. An atom is the simplest particle of matter that retains its properties. We will look more closely at the structure of atoms later.
Compounds are also pure substances, but they are composed entirely of groups of atoms, combined in a specific ratio. In other words, elements are composed of one type of atom, and compounds are formed by the chemical combination of two or more elements. Compounds have different properties from the elements from which they are made.
The difference between elements and compounds is that compounds can be broken down into simpler substances by chemical reactions, by separating the elements that make up the compound. The elements which make up a compound will always be present in the same ratio.
A simple example of this can be seen with water:
Water Hydrogen
(compound) (chemical change) (element)
+ Oxygen
(element)

The formula for water is H O. This means that in water is formed by a chemical combination of
2 hydrogen (H) and oxygen (O) atoms, and that there are always two hydrogen atoms for every one oxygen atom. This ratio never changes. (This doesn’t mean that hydrogen and oxygen cannot combine in a different ratio; but, if they do, the resulting substance will not be water, and will have different properties.)
Every pure substance, whether it is an element or a compound, has a unique set of properties that can be used to identify it.
Mixtures
Not all matter is a pure substance. Mixtures consist of a physical blend of two or more substances. Most materials found in nature are mixtures. It is not always easy to tell the difference.
Some properties of mixtures:
They are composed of more than one pure substance. Mixtures will contain at least two or more different elements and/or compounds.
Variable composition. Unlike substances, mixtures can have variable composition, in other words the substances which make up a mixture can be put together in different ratios. This is because they are only combined physically; the particles do not join together chemically.
Mixtures, because their parts are not joined chemically, can be separated into their individual components using physical means.
Mixtures can be divided into two categories.
Heterogeneous mixtures. These are not uniform in composition. A sample of one portion of the mixture can be different from a sample from another portion of the mixture. Examples of this could be beef stew, soil, a bag of mixed nuts, etc. The makeup of any one part of the mixture may be different from other parts.
Homogeneous mixtures. These are completely uniform in composition. All components are evenly distributed throughout the mixture. In other words, the parts have diffused until they are all spread evenly, with equal concentrations of all components in all parts of the mixture. Homogeneous mixtures are also called solutions . Salt water, air, and steel are all solutions.
Note that uniform composition is different from definite composition . Solutions still have variable (not definite) composition, in that you can alter the relative amounts of each substance, but they will still be evenly distributed .

Physical and Chemical Properties
A physical property is a characteristic of a substance. Examples of physical properties include melting and boiling point, hardness, smell, colour, texture, density, solubility (how easily it dissolves and in what materials), whether it is brittle or malleable, conductivity, etc. Physical properties can be measured or observed without changing the substance chemically. In other words, you can test them, and still have the substance as it originally was. For example, you could heat a piece of metal to find its melting point. Once the metal cooled, it would solidify again, and still be the same substance.
A chemical property describes how a substance changes when it reacts with another substance.
Chemical properties describe the behaviours of substances as they are changed to a new substance, either by combining to form a more complex substance, or by breaking down into simpler substances. During a chemical change, the materials that were present to begin with are called the reactants , and the materials present after the chemical change occurs are called the products .
A physical change involves altering the state or shape of a substance, but not changing it chemically. Bending, melting, freezing, breaking, etc. are all examples of physical changes. In general, physical changes are reversible. (Breaking something may not technically be reversible, but you could put the pieces of the object back in the same position.) No new materials are formed.
During a chemical change , new substances are formed, and they will have different physical and chemical properties from the original substances. Chemical changes are generally not reversible.
There are five main signs to look for that indicate a chemical change has taken place.
•
•
• Colour change :
Energy change:
Gas is formed:
A new color appears that wasn’t there before
Heat or light is given off
Usually indicated by bubbles in a liquid
•
• Precipitate formed: A solid material that forms in a liquid
Odor Change: There is a change in the smell of the material
Note that none of these indicators are absolute proof of a chemical change (for example, heating a solid chemical containing water can produce a color change, but it is not a true chemical reaction, and it can be reversed by adding water back to the solid.) However, they are all strong evidence of a reaction, especially when two or more of them occur together.
States of Matter and Kinetic Theory
Kinetic theory is the belief that all matter is composed of small particles (atoms) and that these particles are in constant motion. The behavior of the three main physical states of matter (solid, liquid, and gas) can be explained using kinetic theory.

In a solid , the particles are tightly packed, and while they are in constant motion, the motion consists mostly of vibration in place...the particles are not able to move around one another.
Because of this, matter in the solid state maintains its shape, and has a fixed volume (it cannot be compressed). A crystal is a solid in which the particles are not only tightly packed, but arranged in a specific shape. For example, table salt particles arrange themselves in a cube shape, and ice crystals form six-sided shapes.
In a liquid , the particles are still fairly close together, but not so tightly packed as in a solid...the
particles are moving more rapidly than in a solid, bouncing farther off of each other when they collide. Because of this, there is room for the particles to move freely around one another, or flow . This allows liquids to take the shape of their container from the bottom up. However, liquids still have a fixed volume, and cannot be compressed.
The particles in a gas are moving very rapidly, spread out widely from one another, and have very little interaction at all aside from occasional collisions. This gives gases a variable shape and volume...they take the shape of their container, they can be compressed into smaller spaces under pressure, and will expand to fill whatever space their container allows. A vapor is the gas state of a substance that is normally solid or liquid at room temperature (about 20°C).
These links show simple animations of the differences in particle movement in each state.
http://www.harcourtschool.com/activity/states_of_matter/ http://www.educationusingpowerpoint.org.uk/Animations/states%20of%20matter.html
Changes of state can occur from solid to liquid ( melting ); liquid to gas ( vaporization or boiling ); gas to liquid ( condensation ); liquid to solid ( freezing or solidification ); or even directly from solid to gas or gas to solid, without becoming liquid in between ( sublimation ).
Examples of sublimation are steam (water vapor) rising from ice cubes taken out of the freezer, or frost (solid) forming when moisture in the air (water vapor) contacts a cold surface, such as a car windshield.
Diffusion
Because particles of matter are in constant motion, they are also constantly bumping into one another and rebounding. Particles will normally spread out, going from areas of high density, where they are tightly packed together to areas of lower density. This tendency is called diffusion . The faster the particles are moving, the more rapidly diffusion will occur. So, it will happen most rapidly in gases, and very slowly, if at all, in solids.
The following links contain simple animations to demonstrate how diffusion occurs.
http://www.biosci.ohiou.edu/introbioslab/Bios170/diffusion/Diffusion.html
http://lessons.harveyproject.org/development/general/diffusion/diffnomemb/diffnomemb.html

Atomic Structure
Atoms are the simplest particles of matter that retain their properties. This does not mean that atoms cannot be broken down into simpler particles. Atoms are composed of smaller, subatomic particles called protons , electrons , and neutrons . (There are other, even smaller particles. For example, protons are composed of more basic particles called quarks . However, for this course, we will only be looking at the protons, neutrons, and electrons)
Protons carry an electric charge of 1+ (positive one). The mass of a proton is 1 AMU . (AMU stands for atomic mass unit ...and is used as a unit for measuring masses of subatomic particles.)
Electrons carry an electric charge of 1- (negative one). The charges of a proton and an electron are opposite, and will cancel one another exactly. However, electrons have a much smaller mass than protons, being about 2000 times smaller. (The mass of an electron is about 1/1840 AMU.)
Neutrons have about the same mass as protons, approximately 1 AMU. However, they are electrically neutral , and carry neither a positive or negative charge. They add mass to the atom, but do not affect the atoms charge.
There are two major areas within an atom. The nucleus is the central part of the atom, which contains the protons and neutrons. Almost all of the mass of an atom is located in the nucleus, but compared to the entire size of an atom, the nucleus is very small. The second area is the electron cloud , an area surrounding the nucleus where the electrons are found.

Because atoms in their natural state contain the same number of protons and electrons , an atom has an overall charge of 0 (neutral) . This is because the positive and negative charges cancel each other. For example, an atom which contains 5 protons would have a total positive charge of 5+ . The same atom has 5 electrons , for a total negative charge of 5. When these charges are added together, the overall charge of the atom is 0. º (5+) + (5-) = 0.
Neutrons, since they carry no electric charge, do not affect the atom’s overall charge.
The mass number of an atom is its total mass, measured in AMU (Atomic Mass Units). To find the mass number, add the number of protons and the number of neutrons. Since both protons and neutrons have a mass of about 1 AMU, this gives you the total mass number for the atom. We can ignore the electrons, since it would take almost 2000 electrons to equal the mass of a single proton.
For example, an atom which contains 5 protons , 5 electrons , and 5 neutrons would have a mass number of 10 . (5 p + 5 n = 10). An atom which contains 5 protons , 5 electrons , and 7 neutrons would have a mass number of 12 . (5 p + 7 n = 12). Remember, electrons are not counted when finding the mass number of an atom.

Isotopes
Because neutrons carry no charge, varying the number of neutrons in an atom changes only the atoms mass number . Two atoms which contain the same number of protons and electrons, but different numbers of neutrons, are called isotopes . Isotopes are identified by their mass numbers .
For example, oxygen-16 is the isotope of oxygen with a mass number of 16; since oxygen always has 8 protons, oxygen-16 would have 8 neutrons as well. Another isotope, oxygen-17, would have 9 neutrons.
The most common isotope of potassium is potassium-39, with 19 protons and 20 neutrons.
Another isotope, potassium-41, has 22 neutrons.
Most elements occur as several isotopes. The link below shows the various isotopes of most elements: http://ie.lbl.gov/education/isotopes.htm