A test tube is a fundamental piece of laboratory equipment, typically a finger-length cylinder of glass or plastic, open at one end and closed with a rounded or U-shaped bottom. They are designed to safely contain small volumes of liquids, usually between 5 to 50 milliliters, for experimental purposes. This simple, contained form makes the test tube a versatile vessel, allowing scientists to manipulate, observe, and store materials for various chemical and biological procedures.
Containing and Heating Chemical Mixtures
The test tube is used in chemistry laboratories for conducting small-scale reactions that do not require highly precise volume measurements. Glass test tubes are frequently constructed from borosilicate glass, which has a low coefficient of thermal expansion. This composition allows the glass to withstand sudden and significant temperature changes without cracking, a property known as thermal shock resistance. Scientists can safely heat the contents over a Bunsen burner flame or in a heating block to drive chemical processes. The narrow diameter helps slow the rate of evaporation and minimize the loss of volatile compounds. Test tubes are also used for simple qualitative analysis, such as mixing two reagents and observing immediate results like a color change, precipitate formation, or gas release.
Cultivating Biological Samples
In microbiology and cell biology, the test tube, often called a culture tube, provides a contained, sterile environment for nurturing living organisms. These tubes are filled with specific growth media necessary to sustain life, such as a liquid nutrient broth or a solid agar mixture. Broth cultures allow for the rapid, high-density growth of microorganisms like bacteria, yeast, or fungi in a uniform suspension. For long-term storage of microbial strains, test tubes are often prepared as agar slants, where the nutrient agar is solidified at an angle. This technique maximizes the surface area for aerobic growth while minimizing the risk of contamination. A sterile cap or stopper is essential to maintain aseptic conditions, regulate gas exchange, and prevent the entry of airborne contaminants.
Holding Clinical and Analytical Specimens
Specialized test tubes are used in clinical diagnostics as the primary container for collecting, transporting, and preparing patient specimens like blood, urine, or saliva. These tubes frequently utilize a vacuum system, which ensures a predetermined volume of fluid is drawn directly into the container. The stoppers are color-coded, with each color signifying the presence of a specific pre-measured additive required for a particular test. For example, a lavender-topped tube contains the anticoagulant EDTA, necessary for a complete blood count (CBC) test. A light-blue-topped tube contains sodium citrate, used specifically for coagulation studies like Prothrombin Time. These additives ensure the integrity of the sample, stabilizing cellular components or preventing chemical changes until analysis.
Supporting Specialized Procedures
The geometric structure of the test tube supports several mechanical separation and analytical techniques in the laboratory. The most prominent application is centrifugation, where the tube is spun at high speeds to separate components of a mixture based on density. Specialized centrifuge tubes are designed with thick walls to withstand extreme rotational forces, often exceeding 10,000 times the force of gravity. Many centrifuge tubes feature a conical bottom, a shape that concentrates the densest components into a small, tightly packed mass called a pellet. This concentration allows scientists to easily remove the liquid layer, known as the supernatant, for further analysis, a process essential for procedures like DNA and protein isolation. The tubular shape is also utilized in sequential assays, allowing for efficient mixing and sequential reaction monitoring as multiple reagents are added in a specific order.