Oxygen Test: Relighting a Glowing Splint Proves Oxygen
The Science of Fire: What is Combustion?
To understand why a glowing splint reignites in oxygen, we must first understand combustion. Combustion, more commonly known as burning, is a high-temperature exothermic (heat-releasing) chemical reaction between a substance (the fuel) and an oxidizer, usually atmospheric oxygen. For combustion to occur, three components are needed, often called the Fire Triangle:
- Fuel: A material that can burn, like wood, paper, or gasoline.
- Heat: Enough energy to raise the fuel to its ignition temperature.
- Oxidizer: A substance that allows the fuel to burn. The most common oxidizer is oxygen gas ($O_2$).
When you light a wooden splint, the heat from the match provides the initial energy. The wood (fuel) reacts with the oxygen in the air (oxidizer) to produce carbon dioxide ($CO_2$), water vapor ($H_2O$), heat, and light. When you blow out the splint, you remove the visible flame by cooling the fuel below its ignition temperature. However, the tip of the splint often remains hot enough to glow. This glowing ember is still a source of heat, and the wood is still a fuel. The only thing missing is a high enough concentration of the oxidizer—oxygen—to sustain a full flame again. Introducing this glowing ember to a pure or oxygen-rich environment provides an abundance of the missing reactant, allowing the combustion reaction to restart vigorously.
Oxygen: The Vital Supporter of Life and Combustion
Oxygen is a colorless, odorless, and tasteless gas that makes up about 21\% of Earth's atmosphere. It is essential for the respiration of most living organisms and for combustion. Its atomic number is 8, and its molecules are diatomic, meaning two oxygen atoms are bonded together, represented as $O_2$. Oxygen is highly reactive; it readily forms compounds with many other elements through a process called oxidation. Rusting iron is a slow form of oxidation, while burning is a very fast one. This high reactivity is what makes oxygen such a good oxidizer in the Fire Triangle. In a pure oxygen environment, combustion reactions happen much more rapidly and intensely than in air because the reactant ($O_2$) is more concentrated, leading to more frequent and energetic collisions between oxygen molecules and the fuel.
Performing the Oxygen Test: A Step-by-Step Guide
Conducting the glowing splint test is straightforward but requires care and proper safety equipment. Always wear safety goggles and work under adult supervision if you are a student.
Materials Needed:
- A source of oxygen (e.g., from a chemical reaction like hydrogen peroxide decomposition, or a commercial oxygen cylinder)
- A test tube or gas jar with a lid to collect the gas
- Wooden splints (like long matchsticks without the head)
- Bunsen burner or candle to light the splint
- Safety goggles
- Tongs or a clamp to hold the splint
Procedure:
- Collect the Gas: First, you need to generate and collect the gas you want to test into an upright test tube or gas jar. The method of collection is important; for oxygen, it is typically collected by displacing water or air because it is only slightly soluble in water and has a similar density to air.
- Light the Splint: Light the tip of a wooden splint using a Bunsen burner or candle until it is burning steadily.
- Create the Glowing Ember: After the splint has been burning for a few seconds, gently blow out the flame. The tip should glow red-hot. This is your test probe.
- Perform the Test: Quickly, while the splint is still glowing, use tongs to lower it into the mouth of the test tube containing the unknown gas. Do not drop the splint into the tube.
- Observe the Result:
- Positive Test for Oxygen: If the gas is oxygen, the glowing splint will instantly and brightly reignite, often with a small "pop" sound.
- Negative Test: If the gas is not oxygen (like nitrogen or carbon dioxide), the glowing splint will simply go out.
Comparing Common Gas Tests
The glowing splint test is specific to oxygen. Chemists use different tests to identify other common gases. The table below provides a clear comparison.
| Gas | Test Method | Observation | Interpretation |
|---|---|---|---|
| Oxygen ($O_2$) | Glowing splint | Splint reignites | Oxygen is a supporter of combustion. |
| Hydrogen ($H_2$) | Lighted splint | Burns with a "squeaky pop" sound | Hydrogen is flammable. The pop is a small explosion. |
| Carbon Dioxide ($CO_2$) | Bubble through limewater (calcium hydroxide solution) | Limewater turns from clear to cloudy/milky | $CO_2$ reacts to form a white precipitate of calcium carbonate. |
| Carbon Dioxide ($CO_2$) | Glowing splint | Splint is extinguished | $CO_2$ is denser than air and does not support combustion. |
Practical Applications: From Labs to Life Support
The principle behind the glowing splint test extends far beyond the school laboratory. Understanding and detecting oxygen is crucial in many fields.
- Chemical Industry: When producing gases on an industrial scale, simple tests like this are used for quick quality control to check the purity of the oxygen being produced.
- Spacecraft and Submarines: In enclosed environments like spacecraft and submarines, life support systems must carefully monitor oxygen levels. While they use sophisticated electronic sensors, the fundamental principle—that oxygen supports combustion—is the same. Some emergency kits may contain chemical oxygen candles that produce oxygen through a reaction, and a similar test could conceptually verify their function.
- Welding and Metal Cutting: Oxy-acetylene torches use pure oxygen to support the combustion of acetylene gas, creating an extremely hot flame capable of melting metal. The presence of high-purity oxygen is essential for this process to work efficiently.
- Medical Oxygen: While not tested with a glowing splint for obvious safety reasons, medical oxygen tanks used in hospitals rely on the same life-supporting property of oxygen that makes the splint test work. The oxygen helps patients who have difficulty breathing.
Common Mistakes and Important Questions
A: Using a burning splint is a test for a flammable gas like hydrogen, not oxygen. If you put a burning splint into a tube of pure oxygen, it will burn more brightly, but this is not a unique or dramatic enough test to be definitive. The reignition of a *glowing* splint is a much more specific and dramatic positive result for oxygen. If you put a burning splint into a tube of air (which is only 21\% oxygen), it will keep burning. If you put it into pure oxygen, it will also keep burning, just faster. The reignition test provides a clear "yes" or "no" answer.
A: It depends on the concentration. The test is very sensitive and can detect oxygen-rich environments. Normal air (21\% $O_2$) will not cause a glowing splint to reignite. However, if the oxygen concentration is significantly higher than in air (e.g., above 30-40\%), the splint may reignite. The test is most reliable for pure or nearly pure oxygen.
A: The two most common mistakes are: 1) Using a splint that is not hot enough. If the ember has died out completely, it will not reignite. You must act quickly after blowing out the flame. 2) Dropping the splint into the test tube. This can be dangerous and also contaminate the gas sample. The splint should only be lowered into the mouth of the tube.
Footnote
1 Exothermic: A chemical reaction that releases energy, usually in the form of heat.
2 Diatomic: A molecule that consists of two atoms. Other common diatomic gases include hydrogen ($H_2$) and nitrogen ($N_2$).
3 Oxidation: A chemical reaction in which a substance loses electrons. In simpler terms, it often involves the combination of a substance with oxygen.
4 Precipitate: An insoluble solid that emerges from a liquid solution as the result of a chemical reaction.