You can describe matter based on composition. This means you describe collections of matter based on what makes it up. The first kind of description refers to whether or not you want to describe a pure substance or mixture.
The components of a pure substance can not be separated based on their different physical properties.
Mixtures on the other hand, contain more than one pure substance. Each substance within a mixture can be identified by separate physical properties.
In Figure 1 the left column shows pure substances. In the right column you see pure substances when added together and no chemical change happens, they form a mixture.
The first mixture is an example of two compounds: water and carbon dioxide. When they add together, both still remain the same pure substance with the same physical properties. Carbon dioxide mixes with water in order to make carbonated water. You can separate the carbon dioxide from the water and find they remain unchanged.
The second example shows two elements mixed to together to form a mixture. In this case, when copper is mixed with tin in a three to one mixture, we obtain the alloy bronze.
The key feature of a pure substance is that it possesses a fixed proportion of elements. This proportion is the same no matter what the size of the sample. The fixed proportion remains identical whether you find it in your back yard or on the dark side of the moon. It is a pure substance because it has the same proportion of elements as every other sample of the same pure substance.
Elements are made of one and only one kind of atom. If there is only one kind of atom, then it has a one hundred percent proportion of only one element. An element cannot be broken down further by any chemical means. This also includes elements which are made of more than one atom like: diatomic elements, molecular elements, network solids, and metals.
To qualify as a compound that is a pure substance, the compound contains more than one element. A compound also has a fixed proportion of elements. The compound is a specific compound because of its specific proportion of elements, (formula).
The fixed proportion of elements (sometimes called fixed composition) makes H2O water, and H2O2 hydrogen peroxide. They make separate pure compounds because they have a different proportion of elements.
Each compound has a unique set of physical properties: boiling point, melting point, density, and refractive index. When you test and record the physical properties of a sample, you characterize a compound.
Because each pure compound has unique physical properties, you use this to separate each pure compound when more than one pure substance resides in a mixture.
You can divide compounds into two types: ionic or molecular. The distinction between these is important. Ionic and molecular compounds differ in their physical properties because they are either molecular or ionic.
On the all important microscopic level, you find ionic compounds are stacks of ions attracted to each other by plus being attracted to minus charges. This attraction goes in all directions and repeats without stop.
In contrast, a molecular compound like carbon dioxide holds together in the bulk phase by attraction between the molecules. The bonds between the atoms: carbon to oxygen hold only the pieces of each individual molecule together.
Figure 4 shows the contrast between sodium chloride, NaCl, and carbon dioxide, CO2. The ions pack together in a hungry mass of opposite charges pressed together. Carbon dioxide has independent molecules loosely held together with weak forces between the molecules.
Sodium chloride has a boiling point of 1465°C, compared to carbon dioxide which has a boiling point (sublimation point) of -78ºC.
A mixture of more than one substance forms a mixture because it has a variable composition. This means instead of one fixed proportion of elements, you can have a sample of air which has twenty percent oxygen or fifteen percent oxygen. It still remains a mixture of air.
A salt solution of water provides another common example where you might have a ten percent salt solution or a one percent salt solution. In both case you are referring to a salt solution though the examples do not have identical compositions.
Mixtures themselves can have two possible compositions: homogeneous and heterogeneous. Homogeneous mixtures mean all the components of the mixture are evenly distributed throughout the body of the material. Heterogeneous mixtures where the components are thrown together without the requirement the mixture has uniform components.
The distinction between these two states of mixture plays into how a mixture can be separated into two or more pure substances.
To visualize the difference between types of mixtures on the molecular scale, Figure 5 shows how two typical mixtures might appear.
The part of Figure 5 on the left shows an aqueous solution of sodium chloride. The chloride anions are green and the sodium cations ions are yellow. The important point to notice is that these ions are uniformly distributed through the entire solution. Every part of the mixture has an identical percentage of sodium chloride in the water.
The right side of Figure 5 shows a mixture of water and octane, (a liquid hydrocarbon). The lower layer has a clear separation between itself and the upper level. The mixture of octane and water do not have a uniform distribution.
When a mixture has a uniform distribution of every component substance, the homogeneous mixture is often a solution.
Air makes a great example where all the gases in air are equally present throughout a room. Very few people choose their seat in a class room because they are afraid part of the room has less air than any other part of the room.
You label a mixture as heterogeneous if the components jumble together and are not uniformly distributed. This means the material consists of zones and regions. This includes materials like wood, concrete, and composite materials.
When you follow a chain of questions about any substance, you will find it easy to correctly classify any lump of matter into its correct category.
Figure 6 gives a flowchart where you serially ask questions and find the correct composition of matter.
If the materiel does not have a uniform distribution, then you have a heterogeneous mixture.
If you can separate the components of a sample by physical means: distillation, filtration, chromatography; then the sample is an homogeneous mixture.
If the sample can be decomposed to other pure substances, the your substance is a compound.
If all these questions result in "no" your final remaining option is that your sample is an element.