Classification of Matter Matter can be classified into two main categories, pure substances and mixtures

Pure Substances Pure substances are made up of only one component. A pure substance may be further classified as either an element or a compound. –An element is a material that has only one type of atom in it. Some examples are gold, silver, lead, and mercury. –Molecules and compounds are materials that are made of two or more types of atoms joined chemically. Some examples are water (H 2 O), carbon dioxide (CO 2 ) and sodium chloride (NaCl).

Pure compounds can only be separated by chemical means - a process that changes the identify of the substance.

Mixtures Mixtures are loose arrangements of materials. A mixture contains two or more substances that are not chemically joined, but loosely joined and easily separated. In a mixture, each component retains its own identity, however, the ratio of the components that make up the mixture can change.

A regular bag of M& M's Chocolate Candies contains 20% orange and 13% brown candies, with the remaining 67% being split (unevenly) between green, blue, red and yellow. When M & M's sell for Halloween, the ratios of the colors change such that only orange and brown M & M's appear in the bag. This ability to change ratios is a characteristic of a mixture.

Mixtures can be further classified into two categories homogeneous and heterogeneous.

Homogeneous mixtures are uniform - the composition of the mixture is the same throughout the mixture on a microscopic level. –One good example is salt water. Homogeneous mixtures may look like pure substances do to their uniformity, but they are not. One property of a homogenous mixture is transparency.

In a heterogeneous mixture, the composition isn't uniform, and you can sometimes see the various components with your naked eye. One example of a heterogeneous mixture is potting soil. Another is oil and vinegar salad dressing.

A mixture of either sort can often be separated into its various components by physical means, such as filtration, distillation (boiling) or extraction. Mixtures make up most of the matter in the world around us. Solutions are one kind of mixture.

Solutions Solutions are all transparent but are very different in composition. Solutions contain a solute (the substance that dissolves) and a solvent (the substance that does the dissolving). –When the solvent is able to dissolve a solute, the solute is soluble. –When the solvent in unable to dissolve the solute, the solute is insoluble.

Alloys Alloys are homogenous mixtures of one or more metals. Alloys are tremendously important metal solutions. Adding small amounts of other substances dramatically changes the properties of the pure metal. Iron is relatively soft and easily rusts, but the addition of small amounts of carbon makes steel – which is much stronger. If nickel and chromium are also added, the result is stainless steel! Gold is too soft to be pure in jewelry so an alloy of gold and copper is often used. Gold is beautiful and lustrous, and the copper adds strength.

Mechanical Mixtures Mechanical mixtures are heterogeneous, which means that you can see the different particles. Mechanical mixtures can be classified further, into three categories, based on the size of the particles. Ordinary mechanical mixtures have the different parts large enough to be seen but can stay mixed. A sand and salt mixture for example.

Suspensions In a suspension, the particles may be seen with the unaided eye or through a low powered microscope. If a suspension is left undisturbed, gravity will eventually cause the suspended particles to separate. An example of a suspension is dirty river water - after awhile, the dirt settles.

Keeping things suspended Emulsifying agents are one way of preventing suspensions from separating. Adding flour to fat from a turkey makes gravy. The flour prevents the fat from separating from the rest of the mixture. Milk straight from a cow is a natural suspension. It quickly separates into two layers. Homogenization uses high speed agitation to break the particles of fat into tiny droplets that can remain in suspension.

Colloids If the suspended particles are small enough, gravity will not cause them to separate. This type of mechanical mixture is called a colloid.

Tyndall Effect Colloids are heterogeneous but appear homogenous. The particles in a colloid are too small to be seen with a microscope, but they are larger than the particles in a solution. Shining a beam of light through the mixture is one way to know that the mixture is a colloid. The light from the beam will be scattered or bent. This is called the Tyndall Effect.