compounds, elements, and mixtures

John Dalton (1766-1844) provided us with the Atomic Theory. His theory says that all matter is composed of small particles called atoms.

• All matter is composed of indivisible atoms. An atom is an extremely small particle of matter that retains its identity during chemical reactions.
• An element is a type of matter composed of only one kind of atom, each atom of a given kind having the same properties.
• A compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions.

We now know that atoms are not indivisible , since they contain protons, neutrons, electrons and other subatomic particles. However, other than this mistake Dalton's ideas are essentially correct.

An element is a substance that is composed of only one kind of atom like aluminum, iron, or neon. Today, 109 elements are known and listed on the periodic table.

A compound is a substance of more than one element, chemically combined. A more scientific definition is that a compound is a type of matter composed of atoms of two or more elements chemically combined in fixed proportions. An example of a compound would be water. It is a compound that contains the elements hydrogen and oxygen fixed in the ratio 2 to 1. A compound has new properties unlike the elements which make it up. A compound has a chemical formula such as H₂O.

A mixture is a material that can be separated by physical means into two or more substances. A classic example of a mixture lab would be one in which you were presented a mixture of sand, iron filings, and salt. You are told to separate these materials. How do you do that? Well, think about the various physical properties of each material. You use a magnet to separate out the iron filings. You then mix water with the sand and salt mixture. You swish the water, salt and sand around for a while and then filter it. The salt dissolved into the water, so the salt water solution passes through the filter while the sand gets left in the filter. Now we slowly heat up the salt water solution, and evaporate the water, and we are left with salt. In a mixture, the compounds are not in a definite proportion. For example, a teaspoon of salt in a liter of water is salt water, but so is a cup of salt in a liter of water.