Identification of Gases
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge AS Level Physics 9702
Why Identify Gases? Importance: In chemistry, many reactions produce gases as products. Being able to identify these gases helps us: Confirm which reaction has taken place Identify unknown substances Ensure safety in the laboratory Understand chemical processes Summary of Gas Tests Gas Test Method Positive Result Hydrogen H₂ Lit splint held at mouth of test tube "Squeaky pop" sound Oxygen O₂ Glowing splint inserted into test tube Splint relights (bursts into flame) Carbon dioxide CO₂ Bubble through limewater Limewater turns milky/cloudy Chlorine Cl₂ Observe color; damp litmus paper Yellow-green gas; bleaches damp litmus (turns white) Ammonia NH₃ Smell (carefully); damp red litmus paper Pungent smell; turns damp red litmus blue 1.
Test for Hydrogen (H₂) Test Method: Collect the gas in a test tube Light a wooden splint Hold the burning splint to the mouth of the test tube Listen for the sound Observation What It Indicates "Squeaky pop" or "pop" sound Hydrogen gas is present No sound or different sound Hydrogen is NOT present Chemical Equation: Hydrogen burns in oxygen (from the air) to form water: 2H₂(g) + O₂(g) → 2H₂O(l) The rapid combustion produces the characteristic "pop" sound.
Common Sources of Hydrogen: Reaction of acids with metals (e.g., Zn + HCl) Reaction of metals with water (e.g., Na + H₂O) Electrolysis of water (at the cathode) Safety Note: Hydrogen is flammable and can be explosive when mixed with air.
Only use small amounts and perform the test carefully. 2. Test for Oxygen (O₂) Test Method: Collect the gas in a test tube Light a wooden splint and then blow it out so it is glowing (not flaming) Insert the glowing splint into the test tube Observe what happens to the splint Observation What It Indicates Glowing splint relights (bursts into flames) Oxygen gas is present Glowing splint goes out or stays glowing Oxygen is NOT present (or concentration too low) Why This Works: Oxygen supports combustion.
The glowing splint contains hot carbon that reacts rapidly with oxygen: C(s) + O₂(g) → CO₂(g) This rapid oxidation causes the splint to burst into flames. Common Sources of Oxygen: Thermal decomposition of metal oxides (e.g., heating mercury oxide) Decomposition of hydrogen peroxide with a catalyst (MnO₂) Electrolysis of water (at the anode) Heating potassium chlorate with MnO₂ catalyst Important: The splint must be glowing , not flaming.
If it's flaming, it already has enough oxygen from the air and won't show a difference when inserted into pure oxygen. 3. Test for Carbon Dioxide (CO₂) Test Method: Collect or produce the gas Bubble the gas through limewater (calcium hydroxide solution) Observe any changes to the limewater Observation What It Indicates Limewater turns milky/cloudy (white precipitate forms) Carbon dioxide is present If excess CO₂ is bubbled through, the milky solution becomes clear again Calcium carbonate dissolves to form soluble calcium hydrogencarbonate Limewater stays clear Carbon dioxide is NOT present Chemical Equations: Formation of milky precipitate: CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l) Calcium carbonate is insoluble - appears as white milky precipitate With excess carbon dioxide (precipitate dissolves): CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq) Calcium hydrogencarbonate is soluble - solution becomes clear again Common Sources of Carbon Dioxide: Combustion of carbon-containing compounds (burning) Reaction of carbonates with acids (e.g., CaCO₃ + HCl) Respiration (breathing out) Thermal decomposition of carbonates (e.g., heating limestone) Fermentation of sugars Interactive Diagram: Visual showing limewater test apparatus with gas being bubbled through limewater in a test tube or beaker.
Should show: (1) clear limewater before test, (2) milky/cloudy limewater after CO₂ bubbled through, (3) optional - clear solution again after excess CO₂. Include delivery tube and labels. 4. Test for Chlorine (Cl₂) Test Method: Observe the color of the gas Hold damp blue litmus paper in the gas Observe what happens to the litmus paper Observation What It Indicates Yellow-green colored gas Likely chlorine (distinctive color) Damp blue litmus turns red, then white (bleached) Chlorine gas is present Pungent, choking smell Also characteristic of chlorine (but DON'T deliberately smell it!) Why This Works: Chlorine is acidic in water (turns litmus red): Cl₂(g) + H₂O(l) → HCl(aq) + HClO(aq) Chlorine is also a strong bleach (removes color - turns litmus white): The hypochlorous acid (HClO) formed is a powerful oxidizing agent that destroys dyes.
Common Sources of Chlorine: Electrolysis of concentrated sodium chloride solution (at the anode) Reaction of concentrated HCl with oxidizing agents (e.g., MnO₂) Reaction of bleach with acid Safety Warning: Chlorine is toxic and corrosive.
It irritates the respiratory system. Only produce small amounts and work in a well-ventilated area or fume cupboard. Never deliberately inhale chlorine gas. Key Point: The litmus paper must be DAMP. Dry litmus paper w…
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