Qualitative chemical tests gather information about observable, but not measurable, properties of a sample. They can be applied to the testing of unknown chemical compounds, products that need to be checked for quality control, and research into the applications of various chemicals. A lab setting is usually required, to provide access to equipment and a clean working environment to note observations. Chemistry students typically learn some qualitative techniques alongside quantitative ones, where they actually measure a property.
The purpose of such tests is to collect information that can help to rule possibilities in or out. If a sample is unknown, certain characteristics will immediately eliminate possible identities. In the case of quality control, it can help with purity assessments, and checks to make sure a compound performs as intended, and as advertised. At the same time technicians perform qualitative chemical tests, they can also do quantitative testing to pick up measurable properties.
One simple example is an observation on the appearance of a sample. Whether it’s a gas, liquid, or solid, the appearance can provide important information about what it is. The observer performing qualitative chemical tests can also note the color, and whether the color changes in different environments or in response to particular chemical agents. Spectroscopy evaluations can provide more information about the performance of the sample, which adds to the body of knowledge about what is inside.
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Solubility testing is another aspect of qualitative chemical tests. Determining whether a compound is soluble can provide important information about what it contains, as can the behavior of the compound when it is mixed with a solvent. Melting and boiling points can also provide further insight, adding to the observations made by the technician. These tests are more quantitative in nature because they result in absolute measurements, but they may be considered qualitative for the purpose of evaluating an unknown sample. A high melting point, for example, rules out compounds which melt at lower temperatures.
In the course of performing such tests, the technician takes careful notes on the equipment used and the behavior of the sample. This can be important if qualitative chemical tests are questioned later, as the technician wants to be able to point to the records to show they were performed appropriately. Technicians also exercise caution with unknown samples, which may be toxic or dangerous, making it important to wear appropriate skin and face protection until the chemicals in the sample are identified.