Hydrogen burns with a squeaky pop
- Collect the gas in a test tube and hold a lit splint at the mouth of the tube.
- Hydrogen gives a squeaky pop, which is the sound of hydrogen and oxygen from the air burning very rapidly.
- The reaction forms water: 2H2(g)+O2(g)→2H2O(l)2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)}2H2(g)+O2(g)→2H2O(l)
- Hydrogen is produced when a metal reacts with a dilute acid, or with water in the case of very reactive metals.
- Hydrogen has a low density, so a tube of hydrogen is held with its mouth pointing down to stop the gas escaping.
Common Mistake
Do not describe the hydrogen test with a glowing splint, because a glowing splint is the test for oxygen.
Oxygen relights a glowing splint
Definition
Glowing splint
A wooden splint that has been lit and blown out, so that its tip glows without a flame.
- Light a wooden splint, blow it out so that only the tip glows, then put it into the gas.
- Oxygen relights the splint, because it lets the hot wood burn much faster than it does in air.
- Air contains only about 21% oxygen, which is not enough to relight a glowing splint.
- Oxygen can be made by adding manganese(IV) oxide, a catalyst, to hydrogen peroxide solution.
- The hydrogen peroxide decomposes into water and oxygen: 2H2O2(aq)→2H2O(l)+O2(g)2\text{H}_2\text{O}_2\text{(aq)} \rightarrow 2\text{H}_2\text{O(l)} + \text{O}_2\text{(g)}2H2O2(aq)→2H2O(l)+O2(g)
Practical
Collecting a gas to test it
- Aim: collect a sample of gas so that it can be tested.
- Apparatus: a conical flask with a bung and delivery tube, a trough of water, test tubes with bungs, a test tube rack, the reactants that make the gas, wooden splints and a lighter.
Method
- Fill a test tube with water and turn it upside down in the trough, keeping its mouth under the water.
- Set up the reaction in the conical flask and fit the bung and delivery tube.
- Hold the test tube over the end of the delivery tube.
- Let the first few bubbles escape, because they are mostly air pushed out of the apparatus.
- When the tube is full of gas, put a bung in it while its mouth is still under water.
- Lift out the tube, remove the bung and test the gas straight away, before it escapes or mixes with air.
Accuracy and safety
- Collect a gas that dissolves well in water, such as chlorine, in a dry tube instead, because it would dissolve in the water in the trough.
- If the flask is heated, lift the delivery tube out of the water before you stop heating, so that water is not sucked back into the hot flask.
- Test chlorine in a fume cupboard, because it is toxic.
- Wear eye protection throughout.
Carbon dioxide turns limewater milky
Definition
Limewater
A colourless solution of calcium hydroxide in water, used to test for carbon dioxide.
Definition
Precipitate
An insoluble solid that forms when two solutions are mixed and react.
- Bubble the gas through limewater using a delivery tube.
- Carbon dioxide turns the limewater milky, or cloudy.
- The cloudiness is a white precipitate of calcium carbonate: CO2(g)+Ca(OH)2(aq)→CaCO3(s)+H2O(l)\text{CO}_2\text{(g)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}CO2(g)+Ca(OH)2(aq)→CaCO3(s)+H2O(l)
- Carbon dioxide is released when a carbonate reacts with an acid and when fuels containing carbon burn.
- A lit splint cannot confirm carbon dioxide, because other gases, such as nitrogen, also put out a flame.
Common Mistake
Do not describe the limewater result as fizzing or bubbling, because the observation that identifies carbon dioxide is the milky colour.
Chlorine bleaches damp litmus paper
Definition
Litmus paper
Paper containing an indicator that is red in acidic conditions and blue in alkaline conditions.
- Hold a piece of damp blue litmus paper in the gas, using tongs.
- Chlorine turns the paper red and then bleaches it white.
- The paper must be damp, because chlorine dissolves in the water on the paper to form an acidic solution that bleaches.
- Chlorine is a pale green gas with a sharp smell, and it is toxic, so the test is done in a fume cupboard.
- Chlorine is produced at the positive electrode during the electrolysis of a solution containing chloride ions.
Common Mistake
Do not call the gas chloride, because chloride is the name of the ion and chlorine is the name of the element and the gas.
How a gas was made suggests its test
- A metal reacting with an acid suggests hydrogen, so test the gas with a lit splint.
- A carbonate reacting with an acid suggests carbon dioxide, so bubble the gas through limewater.
- Hydrogen peroxide decomposing, or pondweed photosynthesising, suggests oxygen, so test the gas with a glowing splint.
- The electrolysis of a chloride solution can give chlorine, so test the gas with damp litmus paper.
- A negative result rules a gas out, so a gas that leaves limewater clear is not carbon dioxide.
Exam technique
- When a question asks you to describe a gas test, give both what you do and what you see, such as putting a glowing splint into the gas and seeing it relight.
- Name the observation precisely, because 'relights', 'squeaky pop', 'milky' and 'bleached' identify a gas, while 'goes out' or 'changes colour' do not.
Self review
- How is hydrogen identified?
- Why does a glowing splint relight in oxygen but not in air?
- What forms when carbon dioxide reacts with limewater?
- Why must the litmus paper be damp in the test for chlorine?
- Which test would you use on the gas given off when marble chips react with hydrochloric acid?
