Identification of Common Gases (AQA GCSE Combined Science: Trilogy: Chemistry): Flashcards
Exam code: 8464
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What is the test for hydrogen gas?
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What is the test for hydrogen gas?
Hold a burning (lighted) splint at the open end (mouth) of a test tube containing the gas. If hydrogen is present, it burns with a loud squeaky pop.
What chemical reaction causes the squeaky pop in the gas test?
The hydrogen reacts rapidly with oxygen in the air in a combustion reaction:
H2 + O2 → H2O
The rapid release of energy produces the characteristic pop sound.
True or False?
A glowing splint is used to test for hydrogen.
False.
A burning (lighted) splint is used for hydrogen.
A glowing splint is used for oxygen.
A useful memory aid: ligHted splint has an H for Hydrogen; glOwing splint has an O for Oxygen.
For the hydrogen gas test, why must the burning splint be held at the mouth of the test tube, not inserted inside?
Hydrogen needs air (oxygen) to combust. If the splint is inserted deep into the tube, there is insufficient oxygen for the reaction, and the test will not produce the squeaky pop result.
To test for hydrogen: hold a ________ splint at the ________ of the tube. If hydrogen is present, the gas burns with a ________.
To test for hydrogen: hold a burning (lighted) splint at the mouth of the tube. If hydrogen is present, the gas burns with a squeaky pop.
What is the test for oxygen gas?
Insert a glowing splint into the test tube of gas. If oxygen is present, the splint relights.
A splint that glows very brightly without fully relighting is also a positive result.
Why must the splint be glowing rather than burning when testing for oxygen?
Mark schemes explicitly do not accept a burning (lit) splint for the oxygen test.
A burning splint is the test for hydrogen.
Only a glowing splint correctly distinguishes between the two gases.
True or False?
A burning splint is used to test for oxygen.
False.
A glowing splint is used for oxygen; a burning (lit) splint is used for hydrogen.
Stating "burning splint" for oxygen is explicitly rejected by mark schemes.
In the oxygen test, the glowing splint glows very brightly but does not fully relight. Is this a positive result?
Yes. A splint that glows very brightly is also accepted as a positive result for oxygen.
In an exam, however, state that the splint relights, as this is the expected wording.
To test for oxygen: place a ________ splint ________ the test tube. If oxygen is present, the splint ________.
To test for oxygen: place a glowing splint inside the test tube. If oxygen is present, the splint relights.
What is the test for carbon dioxide gas?
Bubble the gas through limewater (calcium hydroxide solution). If carbon dioxide is present, the limewater turns milky or cloudy (a white precipitate forms).
Why does limewater turn milky when carbon dioxide is bubbled through it?
Carbon dioxide reacts with calcium hydroxide (limewater) to form calcium carbonate, which is an insoluble white solid:
Ca(OH)2 + CO2 → CaCO3 + H2O.
The white precipitate makes the solution appear milky.
True or False?
Extinguishing a burning splint is a definitive test for carbon dioxide.
False.
Extinguishing a burning splint is a property of carbon dioxide, but it is not definitive because other gases (such as nitrogen) will also extinguish a splint.
The definitive test is bubbling the gas through limewater.
In a mark scheme, the result mark (limewater turns milky) is dependent on which step being correct first?
The result mark is dependent on correctly naming limewater (or calcium hydroxide solution) as the reagent.
If the reagent is wrong, the result mark cannot be awarded, even if the colour change is correct.
To test for carbon dioxide: bubble the gas through ________. If CO2 is present, the solution turns ________.
To test for carbon dioxide: bubble the gas through limewater (calcium hydroxide solution). If CO2 is present, the solution turns milky / cloudy (a white precipitate forms).
What is the test for chlorine gas?
Hold damp litmus paper in the gas. If chlorine is present, the litmus paper is bleached white. (It may first turn red before bleaching.)
Why must the litmus paper be damp when testing for chlorine?
The word "damp" is essential in exam answers.
Dry litmus paper does not react effectively with chlorine gas.
Examiners report that students who know the test often lose marks because they miss the word omit "damp" from their answer.
True or False?
Dry blue litmus paper is used to test for chlorine.
False.
The litmus paper must be damp.
Mark schemes require "damp" explicitly. This is the most common reason students score zero on the chlorine test.
What is the full sequence of colour changes when damp blue litmus paper is held in chlorine gas?
The litmus paper first turns red (because acids form when chlorine contacts water), then is bleached white.
A student says that chlorine smells like swimming pools and uses this to identify it. Is this an acceptable answer in an exam?
No. Smell is a characteristic of chlorine but is not an acceptable identification method in an exam.
The definitive test is damp litmus paper turning bleached white. Always describe the chemical test, not a sensory observation.
To test for chlorine: hold ________ litmus paper in the gas. The result is the paper is ________.
To test for chlorine: hold damp litmus paper in the gas. The result is the paper is bleached (white).
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