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.