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Matter Terminology    Classifying Matter  Phases of Matter  Physical and Chemical Changes  Separation Techniques  Vapor Pressure   Phase Changes  Heating Curve  Phase Diagrams

 

Matter Terminology

 

Matter-The word "matter" describes everything that has physical existence. Therefore it has mass and takes up space.

Pure Substance-A form of matter that has both definite composition and distinct properties. This is broken down into elements and compounds.

Element-A pure substance which cannot be broken down into simpler substances by chemical means. Set of stable atoms from which all known molecules are made. These atoms are organized as a function of their chemical properties in the so-called `Periodic Table of the Elements'.

Compound-A pure substance which contains two or more different elements chemically bonded together in stoichiometric proportions.

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.

Heterogeneous Mixture-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 ribosome 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 a suspension. The solid pieces which are dispersed in the suspension are sometimes able to be seen with the naked eye and can definitely 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 more often found, so we will pretty much end our heterogeneous mixture discussion at this point.

Homogeneous Mixture-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.

Solution-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 solute 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.

SUPPLEMENTAL INFORMATION

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 (109 meters), anywhere from about 1 nm to about 100 nm. They are sometimes called colloidal suspensions.

Types of Colloids
  Dispersing Medium
Gas Liquid Solid
Gas   Foam Foam
Liquid Aerosol Emulsion Gel
Solid Aerosol Sol Solid Sol

There is no gas-gas colloid. Why?

Answer: the dispersed phase (gas #1) is already at the molecular level and disperses evenly throughout the dispersing medium (gas #2). It is completely homogeneous at the molecular level. It is a solution.

An example of this is the atmosphere. Using only the two major gases, we can consider the atmosphere to be oxygen dissolved into nitrogen.

Another point: there are two colloid types called foam, where a gas is the dispersed phase. Since gases exist at the molecular level, why are not these two called solutions?

Answer: consider what the gases do. They form bubbles within the dispersing medium. The bubbles are not of uniform size and they are not distributed evenly over the entire sample. It is the bubbles of gas that is considered, not the fact that each bubble contains lots of gas molecules.

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 Liquid Solid
Gas   shaving cream,
whipped cream
foam rubber,
sponge,
pumice
Liquid fogs, clouds,
aerosol can spray
mayonnaise,
milk, face cream
jelly, cheese,
butter
Solid smoke,
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 settled 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.

Matter Terminology    Classifying Matter  Phases of Matter  Physical and Chemical Changes  Separation Techniques  Vapor Pressure   Phase Changes  Heating Curve  Phase Diagrams

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