why is qualitative analysis important in chemistry

Qualitative analysis is the process by which a chemist determines what
chemical elements are present in a given sample of material. For instance,
many people are now concerned about the presence of lead in our
environment. Lead is a highly toxic element that can cause both mental and
physical problems and, in high doses, even death. Suppose that a parent
wants to know if the paint used on his or her house contains lead. That
parent can take a sample of the paint to a chemist for qualitative
analysis. That analysis will tell whether or not lead is present in the
paint. The science of qualitative analysis is based on the fact that every
element and compound has distinctive properties such as melting point,
boiling point, color, texture, density, and so on. Every element also
reacts with other chemicals in very distinctive ways. For example, the
lead used in house paints reacts with a compound known as hydrogen sulfide
to form a distinctive black precipitate. In many instances, the question facing a chemist is similar to the lead
problem described above: what element or elements are present in some
unknown mixture. Chemists have developed a systematic method for answering
that question. The method divides the most common chemical elements into
about six groups. The elements are grouped in each case according to the
way they react with some specific chemical. The three elements in Group I
of the system, for example, are silver, lead, and mercury. These elements
are grouped together because, when treated with hydrochloric acid, they
all form a solid and precipitate out of solution. The seven elements in Group II are grouped together because they all form
precipitates with hydrogen sulfide. And so on. Qualitative analysis is a process-of-elimination procedure. A chemist
tests first for one group of elements (Group I) and makes a note of any
elements found in this group. He or she then tests for a second group of
elements (Group II) and makes a note of any additional elements found in
this group. Eventually, every element is either identified or eliminated
from consideration. A number of mechanical devices have been invented to identify elements and
compounds on the basis of certain physical properties.


The mass
spectrometer is one such instrument. In a mass spectrometer, an unknown
material is first vaporized (converted into a gas) and then accelerated
into the middle of a large magnet. The material travels along a curved
path within the magnet and emerges onto a photographic plate. The shape of
the path taken by the particles that make up the material and the point of
impact they make on the photographic plate are determined by the mass and
the velocity of the particles. (The term velocity refers both to the speed
with which an object is moving and to the direction in which it is
moving. ) For example, helium atoms, hydrogen molecules, and chlorine
molecules all travel through the magnet in distinctive pathways that can
be recorded on the photographic plate. Chemists can study the photographic
images made in a mass spectrograph and identify the particles that made
those images. Chromatography is another system for identifying the components of a
mixture. In a chromatography column, a mixture of substances is allowed to
pass through a long column containing some sticky material. Each component
of the mixture has a different tendency to stick to the material. Chemists
can look at the pattern of materials attached to the chromatography column
and determine which substances were present in the original mixture. One area in which qualitative analysis has become very important is the
matching of human DNA tissue by law enforcement agencies to prove the
presence or absence of a person at a crime scene.
, branch of chemistry that deals with the identification of elements or grouping of elements present in a sample. The techniques employed in qualitative analysis vary in complexity, depending on the nature of the sample. In some cases it is necessary only to verify the presence of certain elements or groups for which specific tests applicable directly to the sample ( e. g. , flame tests, spot tests) may be available. More often the sample is a complex mixture, and a systematic analysis must be made in order that all the constituents may be identified.


It is customary to classify the methods into two classes: qualitative inorganic analysis and qualitative organic analysis. The classical procedure for the complete systematic analysis of an inorganic sample consists of several parts. First, a preliminary dry test may be performed, which may consist of heating the sample to detect the presence of such constituents as carbon (marked by the appearance of smoke or char) or water (marked by the appearance of moisture) or introducing the sample into a flame and noting the colour produced. Certain elements may be identified by means of their characteristic flame colours. After preliminary tests have been performed, the sample is commonly dissolved in water for later determination of anionic constituents ( i. e. , negatively charged elements or groupings of elements) and constituents ( i. e. , positively charged elements or groupings of elements). The procedure followed is based on the principle of treating the solution with a succession of reagents so that each separates a group of constituents. The groups are then treated successively with reagents that divide a large group into subgroups or separate the constituents singly. When a constituent has been separated it is further examined to confirm its presence and to establish the amount present (quantitative analysis). Portions of the material are dissolved separately, and different procedures are used for each to detect the cationic and anionic constituents. A typical analytical scheme for the separation of the cations into groups is summarized in the table. The analysis for anions is more difficult and less systematic than that for cations. The organic nature of a compound is generally indicated by its behaviour on being heated in air; solids usually melt, then burn with either a smoky or nonsmoky flame, in some instances leaving a black residue of carbon. The elements usually present in these compounds are carbon, hydrogen, oxygen, nitrogen, sulfur, and, occasionally, phosphorus, halogens, and some metals. Specific tests are available for each of the individual elements.

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