Mixtures and Pure Substances

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A Classification Scheme for Matter

The word "matter" describes everything that has physical existence.

We can successively separate matter into categories by asking a sequence of "yes/no" questions. The classification scheme that follows is only one possible sequence. There are other ways to ask questions and in different orders. For example, a possible first question might be "Is it uniform throughout?" in which case, the initial division is into heterogeneous and homogeneous categories.

I. Matter

Question #1: All matter can be separated into two categories by asking the question "Is only one chemical substance present in the sample being considered?"

YES - Pure Substance
NO - Mixture

A mixture is one in which two or more pure substances retain their chemical identity. For example, if you dissolve some sugar into water, the sugar molecules and water molecules remain as sugar and water, it is just that the two are now dispersed in each other. Another definition of mixture: a dispersion of two or more pure substances that can be separated using physical means only.

All mixtures have two parts, the "dispersing medium" and the "dispersed phase." Generally speaking, the dispersed phase is in the smaller amount and is spread throughout the dispersing medium. In most cases, the dispersed phase is quite small in amount compared to the amount of the dispersing medium. Only sometimes, in our studies in this class, will the two amounts become near to equal.

IIa. Pure Substances

Question #2: All pure substances can be separated into two categories by asking the question "Can the sample be further decomposed by chemical means?"

YES - Compound
NO - Element

The definitions of element and compound, as well as examples, are found in Elements and Compounds.

First Historical Note: our concept of an element is due to Robert Boyle (1627-1691). His definition was experimentally-based: an element could not be broken down into simpler substances. This meant that all element identifications were tenative, since better techniques meant that a compound, mistakenly thought to be an element, might be shown to be an element.

Misindentifications of compounds as elements continued to be problems through the 1800's. Not only problems associated with methods, but with getting everybody to agree to use the same definitions. In our era, these problems have been resolved.

One story: hydrogen is the lightest element known. Up until 1913, it was a possibility that there could be lighter elements and there was even an occasional announcement of such a discovery. However, in 1913, work done by Henry Moseley (which you will learn about later) forever shut the door to the possibilty of elements lighter than hydrogen.

Second Historical Note: another important idea at that time was the immutability of atoms. An atom of copper has always been copper and always will be copper. Nothing can change it. This idea has been shown to be wrong by the modern discoveries of radioactivity, fission and fusion. These topics overlap between physics and chemistry and will be studied later in the school year.

IIb. Mixtures

Question #3: All mixtures can be separated into two categories based on the question "Is the sample of constant composition?"

YES - Homogeneous mixture
NO - Heterogeneous mixture

Constant composition means that all parts of the mixture are the same. For example, dissolve sugar in water and mix it completely. Now take several samples from random areas. They will be the same, therefore this is a homogeneous mixture. Take some sand and some water and mix it up well. Take some samples and MAYBE they are the same. Allow the water to stand undisturbed and then sample it. One portion will be more sand than water and another will be more water than sand. This is heterogeneous.

Generally speaking, heterogeneous mixtures can be separated by allowing them to stand undisturbed, letting the "formed portion" (the solids) to settle out. However, filtering or centrifuging may be required. For example, not all the solid components of blood will settle out simply by standing. The blood sample must be placed in a centrifuge and spun at several times the force of gravity. The ribosomes in a cell can be separated from the cell in an ultracentrifuge, a device which can produce 50,000 to 100,000 times the force of gravity.

The technical name for a heterogeneous mixture is 'suspension.' The solid pieces which are dispersed in the suspension are sometimes able to be seen with the naked eye and can definitly be seen under a light microscope. (Homogeneous mixtures will be divided into two types -- solution and colloid. In both cases the dispersed phase cannot be seen under a microscope.)

In chemistry, homogeneous mixtures are much more common than heterogeneous, so we will pretty much end our heterogeneous mixture discussion at this point.

III. Homogeneous Mixtures

Homogeneous mixtures do not settle out upon standing undisturbed and they cannot be separated by filtering or centrifuging. There are two broad categories of homogeneous mixtures.

Question #4: All homogeneous mixtures can be separated into two categories based on the question "Are the constituents of the sample at a molecular or ionic level?"

YES - Solution
NO - Colloid

Solutions: these are, by far, the most important homogeneous mixture in chemistry. Only in more advanced classes will you start to study the characteristics of colloids.

Solutions are made up of a solute and a solvent. The solvent (usually liquid water) is the dispersing medium (component present in greater amount) and the solute (usually, but not always, a solid) is the dispersed phase (component present in the lesser amount). In solutions, the solute is present either as individual ions or individual molecules. There is no "clumping" into pieces made of many ions or molecules.

The word homogeneous is important: the solue is dispersed in an equal manner throughout the solvent. If you sampled two equal-sized regions of the solution, they would contain identical amounts of solute.

There is no standard way to classify solutions. Here are some possible ways solutions could be sub-divided into different types:

acid, basic, neutral (based on pH of the solution)
saturated, unsaturated, supersaturated (based on amount of solute)
solid, liquid, gas (based on physical state of the solution)

These types of categories are usually discussed during a specific teaching unit. For example, acid base chemistry is a teaching unit found in most chemistry classes. The classification of acid, base, and neutral solutions would most often be discussed during that unit.

Colloids: this is a state intermediate between solutions and suspensions. The dispersed phase IS NOT at the molecular level nor is it of such a size to be visible under the microscope. Generally speaking, the dispersed colloidal particles are on the order of nanometers (10¯9 meters), anywhere from about 1 nm to about 100 nm. They are sometimes called colloidal suspensions.

Types of Colloids

 Dispersing Medium
Gas FoamFoam
SolidAerosolSolSolid Sol

There is no gas-gas colloid. Why? Go to the answer.

Colloids which are transparent are characterized by something called the "Tyndall Effect." When the sun sometimes rises or sets with all the brilliant reds and oranges; the colors come about due to the Tyndall Effect. When you see "rays of sunlight," like on a misty day or in the forest, this is caused by the Tyndall Effect. Mist is tiny drops of water suspended in air and, in the forest, dust plays the some role as the mist.

Examples of Colloids

 Dispersing Medium
Gas shaving cream,
whipped cream
foam rubber,
Liquidfogs, clouds,
aerosol can spray
milk, face cream,
hair gel
jelly, cheese,
car exhaust,
airborne viruses
Gold in water,
milk of magnesia,
river silt
alloys of metals
(steel, brass)

The "gold in water" refers to a famous colloid made by Michael Faraday. It still exists (in the collection of the Royal Institution in London) and it HAS NOT stttled out. By the way, colloids as a group were first recognized to exist by Thomas Graham around 1860. He found that substances like starch or gelatin diffused very slowly though water as compared to sugar or salt. Also, the former DID NOT diffuse through membranes that sugar and salt could. Graham also found he could make crystals of salt or sugar, but not of starch or gelatin. He coined the word "colloid" (from the Greek for "glue") to describe this new category of mixtures so different from suspensions or solutions.

Some colloidal substances have now been crystallized, but only with great difficulty. This points out the difficulty of drawing a sharp dividing line between solutions and colloids. However, it remains true that the particles of a colloidal suspension are relatively large compared to the molecules making up a solution. Since colloids are made up of finely divided particles, possessing LOTS of surface area, many of the properties of colloids are based on the properties of surfaces. This point will not be explored further.

Colloids with water as the dispersing medium can be classified as "hydrophobic" or "hydrophilic," but we will leave that discussion for another chemistry class.

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