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Posted by on Aug 1, 2011 in General Chemistry | 0 comments

Characterization of Matter

Introduction to physical chemistry

Properties of Matter:

At the energy we mostly deal with in chemistry matter is composed of atoms, molecules and ions.  From a chemical point of view, these particles interact though the electromagnetic force.

Atoms Molecules and Ions

In contrast physics is interested in energies all the way up to the Big Bang Theory whence matter is composed of elementary particles.  A major part of understanding matter in physics is the creation and destruction of particles.  In chemistry we do not concern ourselves much with that, although photons are regularly created and destroyed in chemical processes and studied as spectroscopy.

Let us look at some physical characteristics of matter.  Generally matter exists in three different forms: solids, liquids and gases.  You can read about exceptions in the article “Fluids and Physical Chemistry – Cool Interesting Facts.”

Transformations between the phases are physical changes called phase transitions. Ice changing to liquid and then to gas is an example.  Such solids have a sharp or definite temperature at which a substance melts or boils.  In contrast amorphous materials, like glass, do not have a fixed transition temperature, but start to flow as the temperature rises.

Here you can see a movie that shows different phases of matter:

Homogeneous and Heterogeneous

Homogeneous substance: A pure substance in which the properties remain the same throughout the sample and which cannot be separated into two or more distinct substances.  Think of water or any pure substance.

Homogeneous mixture:  the properties remain the same throughout the substance but it can be separated into two or more pure substances.  Think of ethanol in water.

Heterogeneous properties:  A mixture of two or more substances which can be separated by physical methods.  Think of oil and water.

There are three major methods for the physical separation of heterogeneous substanes:

  • Filtration
  • Separation
  • Distillation

Physical Properties

Physical Properties of Gold

These are properties that can be detected by means that do NOT cause chemical changes.  How a substance appears, feels, smells, boils and melts are examples of physical properties.

Chemical Characterization

All of matter is made up of atoms (the elements) and atoms form molecules.  An atom cannot be divided into smaller units by chemical means. The elements are denoted by a distinct symbol, for example C for carbon.

The elements are arranged into the periodic table which is discussed in more detail later.  The following is a table you can put on your computer desktop):

Periodic Table PC Periodic Table Mac

The period table organizes the elements into categories based upon electron configuration.  There 118 elements known, but only the first 94 are natural to Earth.  The first 80 are stable while many are radioactive.

Chemical Reactions

The most important part of chemistry is the understanding of the chemical properties of matter, their reactions and producing new chemicals with desired properties. This is based upon bonding.  In the following  you can see animations of covalent, ionic and partially covalent bonds,

Chemical reaction change matter from one form to another forms.  Burning gasoline changes hydrocarbons into products such as carbon dioxide and water.  In the vast majority of chemical processes, electrons move from one bond, or element, and participate in the making and breaking of chemical bonds.  We represent chemical reactions by equations.  The following movie shows how atoms and ions are depicted.

Isotopes

An isotope of an element has the same atomic number (the number of protons) but different atomic masses.  The mass of an element is basically the sum of the number of protons and the number of neutrons.  When the number of neutrons change, the mass changes even though the number of protons is the same.  Hence many elements have different isotopes.

For example, hydrogen has three isotopes:

1H which is normal hydrogen with one proton only

2H of D which stands for deuterium.  It must have only one proton, but it has two neutrons.

3H or T which stands for tritium, has three protons and is radioactive.

H2O (mass of 18) is water and D2O (mass of 20) is called heavy water.

 

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