Acid-Base Reactions: Ammonia () is a basic gas that reacts with water on damp litmus paper to produce hydroxide ions, which change red litmus to blue. Conversely, chlorine reacts with water to form acidic species, causing an initial color change to red before bleaching occurs.
Combustion and Oxidation: Oxygen acts as an oxidizing agent that supports combustion, allowing a glowing splint to relight. Hydrogen, being highly flammable, reacts explosively with atmospheric oxygen in a small-scale combustion reaction, producing the characteristic 'squeaky pop' and forming water vapor:
Precipitation with Limewater: Carbon dioxide reacts with limewater, which is an aqueous solution of calcium hydroxide (). This reaction produces insoluble calcium carbonate (), which suspends in the liquid and makes it appear cloudy or milky:
| Gas | Initial Tool | Result | Definitive? |
|---|---|---|---|
| Hydrogen | Lighted Splint | Squeaky Pop | Yes |
| Oxygen | Glowing Splint | Relights | Yes |
| Carbon Dioxide | Limewater | Cloudy White | Yes |
| Chlorine | Blue Litmus | Bleached White | Yes |
| Ammonia | Red Litmus | Turns Blue | Yes |
Ammonia vs. Ammonium: Ammonia refers to the gaseous molecule , while ammonium refers to the polyatomic cation found in salts or aqueous solutions. Testing for the cation requires adding sodium hydroxide and warming to release the ammonia gas.
Lighted vs. Glowing Splints: Hydrogen requires a lighted splint to provide the activation energy for its explosive combustion. Oxygen requires only a glowing splint because the gas itself provides the necessary environment to restart a flame that is already smoldering.
Precision in Terminology: Always specify that litmus paper must be damp. Dry litmus paper will not show a reaction because the gas molecules cannot ionize or react with the indicator dye without a solvent layer.
Avoid Non-Definitive Answers: Do not suggest that 'extinguishing a flame' is a test for carbon dioxide. While does extinguish flames, many other gases (like nitrogen) do the same, so examiners will only accept the limewater test for identification.
Observation vs. Inference: When asked to describe a test, provide both the action (e.g., 'bubble through limewater') and the specific observation (e.g., 'turns cloudy'). Missing the observation or the procedure often results in lost marks.
The Chlorine 'Smell' Trap: While chlorine has a distinct 'swimming pool' odor, this is a physical characteristic and not a valid chemical test. Identification must rely on the bleaching of litmus paper for a scientific result.
Splint Placement: In the hydrogen test, students often place the splint too deep into the tube. This can extinguish the flame before the pop occurs because there is insufficient oxygen inside the tube to support the rapid combustion needed.
Contamination in Ammonia Tests: When testing ammonia produced by adding to a solution, avoid letting the litmus paper touch the sides of the test tube. Traces of liquid are also alkaline and will turn the paper blue, creating a false positive for the gas.
