Two Different Gases Both Relight a Glowing Splint
Identify nine gases from colour, odour and one confirmatory test, predict what happens when carbonates, hydrated salts and nitrates are heated, and tell true sublimation from the kind that only looks like it.
Why is one test never enough to identify a gas?
Bring a glowing splint to the mouth of a test tube and it bursts into flame. The gas supports combustion, so it is oxygen.
Except that it might be nitrogen dioxide, which relights a glowing splint just as convincingly.
The two are easy to tell apart the moment you look at them — oxygen is colourless and nitrogen dioxide is reddish-brown — but the splint test alone cannot separate them, and a student who runs one test and stops has guessed rather than identified.
That is the discipline this chapter is really teaching. An identification needs the colour, the odour and a confirmatory test, and the confirmatory test has to be one that only the suspected gas would pass.
The same care applies to heating a solid. Three different nitrates give exactly the same two gases when heated, so the gas tells you nothing about which nitrate you had — the colour of the residue is what identifies it. And a white solid that vanishes on heating and re-forms higher up the tube has not necessarily sublimed; it may have broken into two gases that recombined on cooling.
Observation and inference are separate steps, and most marks lost in the practical paper are lost by running them together.
This page covers the first part of the ICSE Class 9 Chemistry practical syllabus: identifying nine gases, the action of heat on carbonates and hydrated salts, the action of heat on nitrates, and distinguishing true sublimation from apparent sublimation.
Except that it might be nitrogen dioxide, which relights a glowing splint just as convincingly.
The two are easy to tell apart the moment you look at them — oxygen is colourless and nitrogen dioxide is reddish-brown — but the splint test alone cannot separate them, and a student who runs one test and stops has guessed rather than identified.
That is the discipline this chapter is really teaching. An identification needs the colour, the odour and a confirmatory test, and the confirmatory test has to be one that only the suspected gas would pass.
The same care applies to heating a solid. Three different nitrates give exactly the same two gases when heated, so the gas tells you nothing about which nitrate you had — the colour of the residue is what identifies it. And a white solid that vanishes on heating and re-forms higher up the tube has not necessarily sublimed; it may have broken into two gases that recombined on cooling.
Observation and inference are separate steps, and most marks lost in the practical paper are lost by running them together.
This page covers the first part of the ICSE Class 9 Chemistry practical syllabus: identifying nine gases, the action of heat on carbonates and hydrated salts, the action of heat on nitrates, and distinguishing true sublimation from apparent sublimation.
How do you identify hydrogen, chlorine, ammonia and the rest?
Use three things for every gas: its colour, its odour, and one confirmatory test that nothing else would pass.
Hydrogen — colourless, odourless. A lighted splint at the mouth of the tube makes it burn with a pale blue flame and a sharp pop. It does not support combustion, so the splint itself goes out.
Oxygen — colourless, odourless. It relights a glowing splint. It does not burn itself.
Carbon dioxide — colourless, odourless. It turns lime water milky, and if the gas is passed in excess the milkiness disappears again. It extinguishes a burning splint, and turns moist blue litmus faintly red.
Chlorine — greenish-yellow, with a pungent, suffocating smell. It turns moist blue litmus red and then bleaches it, and it turns moist starch-iodide paper blue-black.
Hydrogen chloride — colourless, with a pungent smell. It gives dense white fumes when a rod moistened with ammonia solution is held near it, and a white precipitate with silver nitrate solution. It turns moist blue litmus red, and it is extremely soluble in water.
Sulphur dioxide — colourless, with the pungent smell of burning sulphur. It turns acidified potassium dichromate paper from orange to green, and decolourises acidified potassium permanganate solution. It turns moist blue litmus red.
Hydrogen sulphide — colourless, with the smell of rotten eggs. It turns lead acetate paper black, and gives a black precipitate with lead nitrate solution.
Ammonia — colourless, with a pungent characteristic smell. It turns moist red litmus blue — it is the only alkaline gas in this list — and gives dense white fumes with hydrogen chloride.
Nitrogen dioxide — reddish-brown, with a pungent smell. It turns moist blue litmus red, and it supports combustion, relighting a glowing splint.
Now three traps that come directly out of that list.
Both oxygen and nitrogen dioxide relight a glowing splint. The splint test cannot separate them, and the colour must. A confirmatory test is only confirmatory if no other gas in the syllabus passes it, and this one is not.
Ammonia and hydrogen chloride each give dense white fumes with the other. So "dense white fumes" identifies a gas only if you know which reagent you brought to it. Bringing an acid-moistened rod and getting fumes means the gas was ammonia; bringing an alkali-moistened rod and getting fumes means the gas was hydrogen chloride.
Four of the nine turn moist blue litmus red — carbon dioxide, hydrogen chloride, sulphur dioxide and nitrogen dioxide — so a litmus test narrows the field and never finishes the job. Only ammonia turns red litmus blue, which makes that one test genuinely decisive.
One point about odour and safety. Never put your nose over the mouth of a test tube. The correct method is to fan a little of the gas towards you with your hand from a safe distance, and this applies especially to chlorine, hydrogen sulphide and sulphur dioxide, all of which are poisonous. A question asking how you would smell a gas expects the method, not just the smell.
Hydrogen — colourless, odourless. A lighted splint at the mouth of the tube makes it burn with a pale blue flame and a sharp pop. It does not support combustion, so the splint itself goes out.
Oxygen — colourless, odourless. It relights a glowing splint. It does not burn itself.
Carbon dioxide — colourless, odourless. It turns lime water milky, and if the gas is passed in excess the milkiness disappears again. It extinguishes a burning splint, and turns moist blue litmus faintly red.
Chlorine — greenish-yellow, with a pungent, suffocating smell. It turns moist blue litmus red and then bleaches it, and it turns moist starch-iodide paper blue-black.
Hydrogen chloride — colourless, with a pungent smell. It gives dense white fumes when a rod moistened with ammonia solution is held near it, and a white precipitate with silver nitrate solution. It turns moist blue litmus red, and it is extremely soluble in water.
Sulphur dioxide — colourless, with the pungent smell of burning sulphur. It turns acidified potassium dichromate paper from orange to green, and decolourises acidified potassium permanganate solution. It turns moist blue litmus red.
Hydrogen sulphide — colourless, with the smell of rotten eggs. It turns lead acetate paper black, and gives a black precipitate with lead nitrate solution.
Ammonia — colourless, with a pungent characteristic smell. It turns moist red litmus blue — it is the only alkaline gas in this list — and gives dense white fumes with hydrogen chloride.
Nitrogen dioxide — reddish-brown, with a pungent smell. It turns moist blue litmus red, and it supports combustion, relighting a glowing splint.
Now three traps that come directly out of that list.
Both oxygen and nitrogen dioxide relight a glowing splint. The splint test cannot separate them, and the colour must. A confirmatory test is only confirmatory if no other gas in the syllabus passes it, and this one is not.
Ammonia and hydrogen chloride each give dense white fumes with the other. So "dense white fumes" identifies a gas only if you know which reagent you brought to it. Bringing an acid-moistened rod and getting fumes means the gas was ammonia; bringing an alkali-moistened rod and getting fumes means the gas was hydrogen chloride.
Four of the nine turn moist blue litmus red — carbon dioxide, hydrogen chloride, sulphur dioxide and nitrogen dioxide — so a litmus test narrows the field and never finishes the job. Only ammonia turns red litmus blue, which makes that one test genuinely decisive.
One point about odour and safety. Never put your nose over the mouth of a test tube. The correct method is to fan a little of the gas towards you with your hand from a safe distance, and this applies especially to chlorine, hydrogen sulphide and sulphur dioxide, all of which are poisonous. A question asking how you would smell a gas expects the method, not just the smell.
What happens when you heat copper carbonate, washing soda and blue vitriol?
Some of these decompose and some only lose their water of crystallisation, and knowing which is which is the point of the exercise.
Copper carbonate — a green powder.
- Observations: the green solid turns black, and a gas is given off that turns lime water milky
- Inference: the carbonate has decomposed to the oxide and carbon dioxide
Zinc carbonate — a white powder.
- Observations: the residue is yellow while hot and white again when cold, and a gas is given off that turns lime water milky
- Inference: decomposition to zinc oxide and carbon dioxide
The colour change on cooling is a property of zinc oxide itself, not evidence of a second reaction — so "yellow when hot, white when cold" is the signature of zinc oxide wherever it turns up, including in the nitrate experiment below.
Washing soda — , colourless transparent crystals.
- Observations: the crystals first appear to melt, dissolving in their own water of crystallisation, then dry to a white powder. Water condenses on the cooler upper part of the tube. No gas turns lime water milky
- Inference: only the water of crystallisation has been driven off. Sodium carbonate itself is stable to heat and does not decompose
Copper sulphate crystals — , blue.
- Observations: the blue crystals turn white, and water condenses on the cooler part of the tube. Adding a few drops of water to the white residue restores the blue colour, with warmth
- Inference: the water of crystallisation has been lost, giving anhydrous copper sulphate; the change is reversible
Now the general rule, and it is the examinable idea in this section. Whether a carbonate decomposes on heating depends on how reactive its metal is.
- The carbonates of the less reactive metals — copper, zinc, lead, magnesium, calcium — decompose to the oxide and carbon dioxide
- The carbonates of the very reactive metals — sodium and potassium — are stable and do not decompose at the temperature of a bunsen burner. Heating them only drives off water of crystallisation
So "heating a carbonate gives the oxide and carbon dioxide" is not a universal rule, and washing soda is the counter-example the syllabus uses to make the point. The test that separates the two cases is the lime water: if no gas turns lime water milky, no carbonate has decomposed.
And notice which two experiments have no gas at all. Washing soda and copper sulphate crystals both give only water, because both are hydrated salts losing water of crystallisation — the efflorescence and hydration chemistry of the earlier chapter, seen as a laboratory observation. The condensation on the cool part of the tube is the observation to report, and it is what tells you the change was a loss of water rather than a decomposition.
Copper carbonate — a green powder.
- Observations: the green solid turns black, and a gas is given off that turns lime water milky
- Inference: the carbonate has decomposed to the oxide and carbon dioxide
Zinc carbonate — a white powder.
- Observations: the residue is yellow while hot and white again when cold, and a gas is given off that turns lime water milky
- Inference: decomposition to zinc oxide and carbon dioxide
The colour change on cooling is a property of zinc oxide itself, not evidence of a second reaction — so "yellow when hot, white when cold" is the signature of zinc oxide wherever it turns up, including in the nitrate experiment below.
Washing soda — , colourless transparent crystals.
- Observations: the crystals first appear to melt, dissolving in their own water of crystallisation, then dry to a white powder. Water condenses on the cooler upper part of the tube. No gas turns lime water milky
- Inference: only the water of crystallisation has been driven off. Sodium carbonate itself is stable to heat and does not decompose
Copper sulphate crystals — , blue.
- Observations: the blue crystals turn white, and water condenses on the cooler part of the tube. Adding a few drops of water to the white residue restores the blue colour, with warmth
- Inference: the water of crystallisation has been lost, giving anhydrous copper sulphate; the change is reversible
Now the general rule, and it is the examinable idea in this section. Whether a carbonate decomposes on heating depends on how reactive its metal is.
- The carbonates of the less reactive metals — copper, zinc, lead, magnesium, calcium — decompose to the oxide and carbon dioxide
- The carbonates of the very reactive metals — sodium and potassium — are stable and do not decompose at the temperature of a bunsen burner. Heating them only drives off water of crystallisation
So "heating a carbonate gives the oxide and carbon dioxide" is not a universal rule, and washing soda is the counter-example the syllabus uses to make the point. The test that separates the two cases is the lime water: if no gas turns lime water milky, no carbonate has decomposed.
And notice which two experiments have no gas at all. Washing soda and copper sulphate crystals both give only water, because both are hydrated salts losing water of crystallisation — the efflorescence and hydration chemistry of the earlier chapter, seen as a laboratory observation. The condensation on the cool part of the tube is the observation to report, and it is what tells you the change was a loss of water rather than a decomposition.
Why do three different nitrates all give the same brown gas?
All three nitrates decompose to the metal oxide, nitrogen dioxide and oxygen — so the gases are identical and the residue is what identifies the salt.
Zinc nitrate.
- Observations: a reddish-brown gas is given off; the residue is yellow while hot and white when cold
Copper nitrate.
- Observations: a reddish-brown gas is given off; the residue is black
Lead nitrate.
- Observations: a reddish-brown gas is given off; the crystals crackle loudly, which is called decrepitation; the residue is yellow while hot and buff-yellow when cold
Check the balancing on any one of them, because these are the equations most often written wrongly. For lead nitrate: lead ; nitrogen ; oxygen on the left against on the right. **The coefficient of on the nitrogen dioxide and the single are what make it balance**, and writing instead of leaves the oxygen short.
So how do you tell the three apart? Not by the gas. Use the residue:
- Black residue — copper nitrate
- Residue yellow hot, white cold — zinc nitrate
- Residue yellow hot, buff-yellow cold, with decrepitation — lead nitrate
The decrepitation is a useful extra clue. It is a physical effect: water trapped inside the crystals turns to steam and bursts them apart with a crackling noise. Only lead nitrate does this among the three, so hearing it settles the identification before the residue has even cooled.
Now a trap worth knowing. The mixture of gases leaving the tube contains oxygen, so it will relight a glowing splint. A student who tests the gas and reports "oxygen" has found something true and missed the point — the gas is a mixture, and the reddish-brown colour proves nitrogen dioxide is present too. Reporting a mixture as one of its components is a standard way to lose a mark in the practical paper.
And the boundary case, which parallels the carbonates exactly. The nitrates of the very reactive metals behave differently. Sodium nitrate and potassium nitrate do not go all the way to the oxide — they decompose only as far as the nitrite, giving oxygen and no brown gas at all:
So the absence of a brown gas is itself an observation. A nitrate that gives a colourless gas relighting a splint, with no brown fumes, belongs to a very reactive metal. The pattern is the same one the carbonates showed — the more reactive the metal, the more stable its salt is to heat — and recognising that single rule handles both sets of experiments.
Zinc nitrate.
- Observations: a reddish-brown gas is given off; the residue is yellow while hot and white when cold
Copper nitrate.
- Observations: a reddish-brown gas is given off; the residue is black
Lead nitrate.
- Observations: a reddish-brown gas is given off; the crystals crackle loudly, which is called decrepitation; the residue is yellow while hot and buff-yellow when cold
Check the balancing on any one of them, because these are the equations most often written wrongly. For lead nitrate: lead ; nitrogen ; oxygen on the left against on the right. **The coefficient of on the nitrogen dioxide and the single are what make it balance**, and writing instead of leaves the oxygen short.
So how do you tell the three apart? Not by the gas. Use the residue:
- Black residue — copper nitrate
- Residue yellow hot, white cold — zinc nitrate
- Residue yellow hot, buff-yellow cold, with decrepitation — lead nitrate
The decrepitation is a useful extra clue. It is a physical effect: water trapped inside the crystals turns to steam and bursts them apart with a crackling noise. Only lead nitrate does this among the three, so hearing it settles the identification before the residue has even cooled.
Now a trap worth knowing. The mixture of gases leaving the tube contains oxygen, so it will relight a glowing splint. A student who tests the gas and reports "oxygen" has found something true and missed the point — the gas is a mixture, and the reddish-brown colour proves nitrogen dioxide is present too. Reporting a mixture as one of its components is a standard way to lose a mark in the practical paper.
And the boundary case, which parallels the carbonates exactly. The nitrates of the very reactive metals behave differently. Sodium nitrate and potassium nitrate do not go all the way to the oxide — they decompose only as far as the nitrite, giving oxygen and no brown gas at all:
So the absence of a brown gas is itself an observation. A nitrate that gives a colourless gas relighting a splint, with no brown fumes, belongs to a very reactive metal. The pattern is the same one the carbonates showed — the more reactive the metal, the more stable its salt is to heat — and recognising that single rule handles both sets of experiments.
Which of these really sublime, and which only appear to?
Iodine truly sublimes, ammonium chloride only appears to, and ammonium dichromate does not sublime at all.
Iodine — shiny greyish-black scales.
- Observations: violet vapours fill the tube, and shiny dark crystals re-form on the cooler upper part
- Inference: true sublimation — the solid has turned directly to vapour and back to solid with no chemical change. It is a physical change, and the substance that reappears is the same substance that left
Ammonium chloride — a white solid.
- Observations: the solid disappears from the bottom of the tube and a white solid re-forms on the cooler upper part, which looks exactly like sublimation
- Inference: it has actually dissociated into two gases, which have recombined on cooling:
This is often called dissociative sublimation or apparent sublimation, and it is a chemical change followed by its reverse.
Here is how you prove there are two gases and not one. Hold a moistened red litmus paper near the upper part of the tube and a moistened blue litmus paper lower down. The red one turns blue, showing ammonia; the blue one turns red, showing hydrogen chloride.
The two gases separate because ammonia is lighter and diffuses faster, reaching the top of the tube ahead of the hydrogen chloride. A single substance subliming could not possibly turn red litmus blue in one place and blue litmus red in another, so that one observation distinguishes true from apparent sublimation — and it is exactly what the practical question is looking for.
Ammonium dichromate — orange crystals.
- Observations: the solid decomposes vigorously, throwing out sparks; the volume increases enormously; and a green fluffy residue is left behind
- Inference: it has decomposed irreversibly to chromium oxide, nitrogen and water. It does not sublime
Check the balance: nitrogen ; hydrogen ; chromium ; oxygen on the left against on the right.
Why the volume increases so dramatically. A small quantity of solid produces nitrogen gas and steam — two gases in quantity — and the green residue is fluffy and far bulkier than the crystals were. Nothing reappears on the cool part of the tube, which is the observation that rules out sublimation completely.
So the three experiments form a deliberate sequence, and the sequence is the lesson.
- Iodine — the same substance goes up and comes back. Physical change, true sublimation
- Ammonium chloride — two different substances go up and recombine on the way down. Chemical change, apparent sublimation
- Ammonium dichromate — nothing comes back at all. Chemical change, no sublimation
A solid re-forming on the cool part of a tube is therefore not proof of sublimation. It is proof only that something condensed there, and the question you must answer next is whether it is the same substance you started with. That distinction between an observation and the inference drawn from it is what the practical paper is testing throughout this chapter.
Iodine — shiny greyish-black scales.
- Observations: violet vapours fill the tube, and shiny dark crystals re-form on the cooler upper part
- Inference: true sublimation — the solid has turned directly to vapour and back to solid with no chemical change. It is a physical change, and the substance that reappears is the same substance that left
Ammonium chloride — a white solid.
- Observations: the solid disappears from the bottom of the tube and a white solid re-forms on the cooler upper part, which looks exactly like sublimation
- Inference: it has actually dissociated into two gases, which have recombined on cooling:
This is often called dissociative sublimation or apparent sublimation, and it is a chemical change followed by its reverse.
Here is how you prove there are two gases and not one. Hold a moistened red litmus paper near the upper part of the tube and a moistened blue litmus paper lower down. The red one turns blue, showing ammonia; the blue one turns red, showing hydrogen chloride.
The two gases separate because ammonia is lighter and diffuses faster, reaching the top of the tube ahead of the hydrogen chloride. A single substance subliming could not possibly turn red litmus blue in one place and blue litmus red in another, so that one observation distinguishes true from apparent sublimation — and it is exactly what the practical question is looking for.
Ammonium dichromate — orange crystals.
- Observations: the solid decomposes vigorously, throwing out sparks; the volume increases enormously; and a green fluffy residue is left behind
- Inference: it has decomposed irreversibly to chromium oxide, nitrogen and water. It does not sublime
Check the balance: nitrogen ; hydrogen ; chromium ; oxygen on the left against on the right.
Why the volume increases so dramatically. A small quantity of solid produces nitrogen gas and steam — two gases in quantity — and the green residue is fluffy and far bulkier than the crystals were. Nothing reappears on the cool part of the tube, which is the observation that rules out sublimation completely.
So the three experiments form a deliberate sequence, and the sequence is the lesson.
- Iodine — the same substance goes up and comes back. Physical change, true sublimation
- Ammonium chloride — two different substances go up and recombine on the way down. Chemical change, apparent sublimation
- Ammonium dichromate — nothing comes back at all. Chemical change, no sublimation
A solid re-forming on the cool part of a tube is therefore not proof of sublimation. It is proof only that something condensed there, and the question you must answer next is whether it is the same substance you started with. That distinction between an observation and the inference drawn from it is what the practical paper is testing throughout this chapter.
Exam tip
Exam tip: report the observation and the inference as separate lines
Write observations and inferences in two columns or two separate lines. "Gas turns lime water milky" is the observation; "gas is carbon dioxide" is the inference. Merging them loses marks.
For every gas give colour, odour and one confirmatory test. Hydrogen — burns with a pop. Oxygen — relights a glowing splint. Carbon dioxide — lime water milky, clearing in excess. Chlorine — greenish-yellow, bleaches litmus, starch-iodide blue-black. Hydrogen chloride — white precipitate with silver nitrate. Sulphur dioxide — dichromate orange to green. Hydrogen sulphide — rotten eggs, lead acetate paper black. Ammonia — red litmus blue. Nitrogen dioxide — reddish-brown.
Oxygen and nitrogen dioxide both relight a splint — use the colour to decide.
Only ammonia turns red litmus blue. Four of the nine turn blue litmus red, so that test never finishes the job.
Say how you would smell a gas — fan a little towards you from a distance, never put your nose over the tube.
Copper carbonate gives a black residue; zinc carbonate gives one yellow hot, white cold. Both release carbon dioxide.
Washing soda and copper sulphate crystals give only water — the carbonate and the sulphate themselves are unchanged. No gas turns lime water milky, and that absence is the observation.
Sodium and potassium carbonates do not decompose on heating. The more reactive the metal, the more stable the salt.
**All three nitrates give and — identify by the residue: black for copper, yellow-hot-white-cold for zinc, yellow-hot-buff-cold with decrepitation for lead.
Write the nitrate equations with and one , and check the oxygen count.
Sodium and potassium nitrates give the nitrite and oxygen, with no brown gas.
And for sublimation: iodine is true, ammonium chloride only apparent (it dissociates and recombines — prove it with litmus at two heights), ammonium dichromate does not sublime** at all.
For every gas give colour, odour and one confirmatory test. Hydrogen — burns with a pop. Oxygen — relights a glowing splint. Carbon dioxide — lime water milky, clearing in excess. Chlorine — greenish-yellow, bleaches litmus, starch-iodide blue-black. Hydrogen chloride — white precipitate with silver nitrate. Sulphur dioxide — dichromate orange to green. Hydrogen sulphide — rotten eggs, lead acetate paper black. Ammonia — red litmus blue. Nitrogen dioxide — reddish-brown.
Oxygen and nitrogen dioxide both relight a splint — use the colour to decide.
Only ammonia turns red litmus blue. Four of the nine turn blue litmus red, so that test never finishes the job.
Say how you would smell a gas — fan a little towards you from a distance, never put your nose over the tube.
Copper carbonate gives a black residue; zinc carbonate gives one yellow hot, white cold. Both release carbon dioxide.
Washing soda and copper sulphate crystals give only water — the carbonate and the sulphate themselves are unchanged. No gas turns lime water milky, and that absence is the observation.
Sodium and potassium carbonates do not decompose on heating. The more reactive the metal, the more stable the salt.
**All three nitrates give and — identify by the residue: black for copper, yellow-hot-white-cold for zinc, yellow-hot-buff-cold with decrepitation for lead.
Write the nitrate equations with and one , and check the oxygen count.
Sodium and potassium nitrates give the nitrite and oxygen, with no brown gas.
And for sublimation: iodine is true, ammonium chloride only apparent (it dissociates and recombines — prove it with litmus at two heights), ammonium dichromate does not sublime** at all.
Did you know
Why a white solid reappearing higher up the tube proves nothing
Heat ammonium chloride in a dry test tube and watch. The white powder at the bottom vanishes, and a white powder appears on the cool glass near the top.
The obvious conclusion is that the solid turned into vapour and then back into solid — sublimation, a physical change, the same substance throughout.
It is the wrong conclusion, and what makes this experiment worth doing is how convincingly wrong it looks. The solid at the top really is ammonium chloride. It really did come from the solid at the bottom. Nothing in the appearance of the experiment is misleading.
What happened in between is the part you cannot see. The solid broke into two separate gases, they travelled up the tube independently, and they recombined when they reached the cool glass. Two chemical changes, exactly reversing one another, with a moment in the middle when the substance did not exist at all.
And the proof is beautifully simple: hold red litmus high in the tube and blue litmus lower down. The red turns blue and the blue turns red. One substance cannot be alkaline in one place and acidic in another — so there must be two, and they must have separated on the way up, the lighter ammonia running ahead of the heavier hydrogen chloride.
A single test tube, two pieces of paper, and a conclusion that overturns what the experiment appears to show.
This is worth carrying beyond the chemistry. The observation "a white solid reappeared at the top" is compatible with two completely different explanations, and no amount of staring at it will choose between them. It took a second, different kind of test — one that asked about chemical character rather than appearance — to settle the question.
That is why the practical syllabus insists on separating observations from inferences. The observation is what you saw; the inference is a claim about what it means, and the same observation will often support more than one claim.
The obvious conclusion is that the solid turned into vapour and then back into solid — sublimation, a physical change, the same substance throughout.
It is the wrong conclusion, and what makes this experiment worth doing is how convincingly wrong it looks. The solid at the top really is ammonium chloride. It really did come from the solid at the bottom. Nothing in the appearance of the experiment is misleading.
What happened in between is the part you cannot see. The solid broke into two separate gases, they travelled up the tube independently, and they recombined when they reached the cool glass. Two chemical changes, exactly reversing one another, with a moment in the middle when the substance did not exist at all.
And the proof is beautifully simple: hold red litmus high in the tube and blue litmus lower down. The red turns blue and the blue turns red. One substance cannot be alkaline in one place and acidic in another — so there must be two, and they must have separated on the way up, the lighter ammonia running ahead of the heavier hydrogen chloride.
A single test tube, two pieces of paper, and a conclusion that overturns what the experiment appears to show.
This is worth carrying beyond the chemistry. The observation "a white solid reappeared at the top" is compatible with two completely different explanations, and no amount of staring at it will choose between them. It took a second, different kind of test — one that asked about chemical character rather than appearance — to settle the question.
That is why the practical syllabus insists on separating observations from inferences. The observation is what you saw; the inference is a claim about what it means, and the same observation will often support more than one claim.
Exam relevance
How does practical chemistry help in JEE Main and NEET?
Because qualitative analysis becomes a formal Class 11 and 12 topic, and the confirmatory tests learnt here are the ones those chapters build on.
This is the foundation for the Class 11 and 12 Chemistry practical syllabus and for Qualitative Analysis, examined in both JEE Main and NEET. Class 12 systematises the identification of acidic and basic radicals in salts, and the gas tests on this page are the first stage of it — a carbonate identified by carbon dioxide turning lime water milky, a sulphide by hydrogen sulphide blackening lead acetate paper, a sulphite by sulphur dioxide turning dichromate green, and a nitrate by the brown fumes. The gas evolved in a preliminary test is what tells you which radical to look for.
The reagents become named reactions. Class 12 The p-Block Elements and The d- and f-Block Elements cover the chemistry behind each test: why acidified potassium dichromate turns green (chromium is reduced from the higher to the lower oxidation state), why potassium permanganate is decolourised, and why lead sulphide is black. Explaining a colour change by an oxidation-state change is a recurring JEE Main type, and it is the redox chapter applied to these very tests.
Thermal stability becomes a periodic trend. Class 11 The s-Block Elements explains why the carbonates and nitrates of sodium and potassium are stable to heat while those of the less reactive metals decompose, using lattice energy and the polarising power of the cation. The rule stated here — the more reactive the metal, the more stable the salt — is what that theory justifies, and questions asking you to arrange carbonates or nitrates in order of thermal stability are common.
The nitrate decomposition pattern is examined directly. Class 12 covers the three routes by which nitrates decompose — to the nitrite for the most reactive metals, to the oxide for the middle group, and to the metal itself for the least reactive. The boundary case given here is the first two of those three, and a question asking what a named nitrate gives on heating expects the correct route.
Sublimation reappears as a purification technique. Class 11 Organic Chemistry — Some Basic Principles and Techniques lists sublimation among the methods for purifying an organic solid, and the distinction between a true physical sublimation and a dissociation is what decides whether the method can be used at all. A substance that dissociates cannot be purified by sublimation, and that is the practical consequence of this page's ammonium chloride experiment.
For NEET, this material appears as recall: match a gas to its test, match a salt to its residue colour, or identify the gas evolved when a named solid is heated. Colour is heavily examined — the colours of the oxides, of the hydrated and anhydrous salts, and of the gases. For NEET Biology, lime water as a test for carbon dioxide is used in the respiration experiments of Respiration in Plants.
What the questions look like. For board work, expect give the colour, odour and confirmatory test for a named gas, state the observations and write the equation when a named solid is heated, identify a salt from the colour of its residue, and state which of a given set of substances sublime, with reasons. Marks come from the observation-and-inference pairing and from correctly balanced equations. For JEE Main and NEET, expect radical identification, colour matching, thermal-stability ordering and oxidation-state explanations of the test reagents.
How board and competitive emphasis differ. A board paper rewards the observation stated separately from the inference, with the equation. A competitive paper assumes the test and asks why the reagent changes colour, or which of four salts is thermally the most stable.
The single trap that costs the most marks. Reporting a mixture of gases as one of its components. Heating any of the three nitrates gives nitrogen dioxide and oxygen together, so the gas relights a glowing splint and is reddish-brown — and calling it oxygen is only half an observation. The defence is to record every observation before drawing any inference, because the colour and the splint result together identify the mixture while either one alone misidentifies it.
This is the foundation for the Class 11 and 12 Chemistry practical syllabus and for Qualitative Analysis, examined in both JEE Main and NEET. Class 12 systematises the identification of acidic and basic radicals in salts, and the gas tests on this page are the first stage of it — a carbonate identified by carbon dioxide turning lime water milky, a sulphide by hydrogen sulphide blackening lead acetate paper, a sulphite by sulphur dioxide turning dichromate green, and a nitrate by the brown fumes. The gas evolved in a preliminary test is what tells you which radical to look for.
The reagents become named reactions. Class 12 The p-Block Elements and The d- and f-Block Elements cover the chemistry behind each test: why acidified potassium dichromate turns green (chromium is reduced from the higher to the lower oxidation state), why potassium permanganate is decolourised, and why lead sulphide is black. Explaining a colour change by an oxidation-state change is a recurring JEE Main type, and it is the redox chapter applied to these very tests.
Thermal stability becomes a periodic trend. Class 11 The s-Block Elements explains why the carbonates and nitrates of sodium and potassium are stable to heat while those of the less reactive metals decompose, using lattice energy and the polarising power of the cation. The rule stated here — the more reactive the metal, the more stable the salt — is what that theory justifies, and questions asking you to arrange carbonates or nitrates in order of thermal stability are common.
The nitrate decomposition pattern is examined directly. Class 12 covers the three routes by which nitrates decompose — to the nitrite for the most reactive metals, to the oxide for the middle group, and to the metal itself for the least reactive. The boundary case given here is the first two of those three, and a question asking what a named nitrate gives on heating expects the correct route.
Sublimation reappears as a purification technique. Class 11 Organic Chemistry — Some Basic Principles and Techniques lists sublimation among the methods for purifying an organic solid, and the distinction between a true physical sublimation and a dissociation is what decides whether the method can be used at all. A substance that dissociates cannot be purified by sublimation, and that is the practical consequence of this page's ammonium chloride experiment.
For NEET, this material appears as recall: match a gas to its test, match a salt to its residue colour, or identify the gas evolved when a named solid is heated. Colour is heavily examined — the colours of the oxides, of the hydrated and anhydrous salts, and of the gases. For NEET Biology, lime water as a test for carbon dioxide is used in the respiration experiments of Respiration in Plants.
What the questions look like. For board work, expect give the colour, odour and confirmatory test for a named gas, state the observations and write the equation when a named solid is heated, identify a salt from the colour of its residue, and state which of a given set of substances sublime, with reasons. Marks come from the observation-and-inference pairing and from correctly balanced equations. For JEE Main and NEET, expect radical identification, colour matching, thermal-stability ordering and oxidation-state explanations of the test reagents.
How board and competitive emphasis differ. A board paper rewards the observation stated separately from the inference, with the equation. A competitive paper assumes the test and asks why the reagent changes colour, or which of four salts is thermally the most stable.
The single trap that costs the most marks. Reporting a mixture of gases as one of its components. Heating any of the three nitrates gives nitrogen dioxide and oxygen together, so the gas relights a glowing splint and is reddish-brown — and calling it oxygen is only half an observation. The defence is to record every observation before drawing any inference, because the colour and the splint result together identify the mixture while either one alone misidentifies it.
Key takeaways
Gas tests, action of heat and sublimation: quick revision
- Every identification needs colour, odour and one confirmatory test.
- Hydrogen — colourless, odourless; burns with a pale blue flame and a pop; does not support combustion.
- Oxygen — colourless, odourless; relights a glowing splint.
- Carbon dioxide — colourless, odourless; turns lime water milky, and the milkiness clears in excess; extinguishes a burning splint.
- Chlorine — greenish-yellow, pungent and suffocating; turns moist blue litmus red then bleaches it; turns starch-iodide paper blue-black.
- Hydrogen chloride — colourless, pungent; dense white fumes with ammonia; white precipitate with silver nitrate; very soluble in water.
- Sulphur dioxide — colourless, smell of burning sulphur; turns acidified potassium dichromate from orange to green; decolourises potassium permanganate.
- Hydrogen sulphide — colourless, rotten eggs; turns lead acetate paper black.
- Ammonia — colourless, pungent; turns red litmus blue — the only alkaline gas; dense white fumes with .
- Nitrogen dioxide — reddish-brown, pungent; supports combustion.
- Traps: oxygen and nitrogen dioxide both relight a splint — use the colour. **Ammonia and each fume with the other — the reagent you brought decides which. Four gases turn blue litmus red, so that test only narrows the field.
- Smell a gas by fanning it towards you, never by leaning over the tube.
- Copper carbonate: green to black**, gas turns lime water milky — .
- Zinc carbonate: white to yellow hot, white cold — . That colour pair is the signature of zinc oxide.
- Washing soda: crystals melt in their own water, then a white powder; water condenses on the cool glass; no gas turns lime water milky — .
- Copper sulphate crystals: blue to white, water condenses, and adding water restores the blue — .
- The rule: carbonates of less reactive metals decompose to the oxide; those of sodium and potassium are stable to heat. The lime water test separates the two cases.
- Nitrates: ; ; .
- All three give the same gases, so identify by the residue: black — copper; yellow hot, white cold — zinc; yellow hot, buff cold with decrepitation — lead.
- Decrepitation is trapped water bursting the crystals — only lead nitrate among the three.
- The gas mixture relights a splint AND is reddish-brown — report both, not just oxygen.
- Sodium and potassium nitrates give the nitrite and oxygen only, with no brown gas: .
- Iodine truly sublimes — violet vapours, shiny crystals re-form, a physical change.
- Ammonium chloride only appears to sublime — it dissociates and recombines: . Proof: red litmus turns blue high up (ammonia diffuses faster) and blue litmus turns red lower down.
- Ammonium dichromate does not sublime — it decomposes with sparks, a great increase in volume and a green fluffy residue: .
- A solid reappearing on the cool glass proves only that something condensed — whether it is the same substance is a separate question.
Cover the gas list and see whether you can name the confirmatory test for all nine, then try to say which two would fool a glowing splint and which two would fool each other.
- Hydrogen — colourless, odourless; burns with a pale blue flame and a pop; does not support combustion.
- Oxygen — colourless, odourless; relights a glowing splint.
- Carbon dioxide — colourless, odourless; turns lime water milky, and the milkiness clears in excess; extinguishes a burning splint.
- Chlorine — greenish-yellow, pungent and suffocating; turns moist blue litmus red then bleaches it; turns starch-iodide paper blue-black.
- Hydrogen chloride — colourless, pungent; dense white fumes with ammonia; white precipitate with silver nitrate; very soluble in water.
- Sulphur dioxide — colourless, smell of burning sulphur; turns acidified potassium dichromate from orange to green; decolourises potassium permanganate.
- Hydrogen sulphide — colourless, rotten eggs; turns lead acetate paper black.
- Ammonia — colourless, pungent; turns red litmus blue — the only alkaline gas; dense white fumes with .
- Nitrogen dioxide — reddish-brown, pungent; supports combustion.
- Traps: oxygen and nitrogen dioxide both relight a splint — use the colour. **Ammonia and each fume with the other — the reagent you brought decides which. Four gases turn blue litmus red, so that test only narrows the field.
- Smell a gas by fanning it towards you, never by leaning over the tube.
- Copper carbonate: green to black**, gas turns lime water milky — .
- Zinc carbonate: white to yellow hot, white cold — . That colour pair is the signature of zinc oxide.
- Washing soda: crystals melt in their own water, then a white powder; water condenses on the cool glass; no gas turns lime water milky — .
- Copper sulphate crystals: blue to white, water condenses, and adding water restores the blue — .
- The rule: carbonates of less reactive metals decompose to the oxide; those of sodium and potassium are stable to heat. The lime water test separates the two cases.
- Nitrates: ; ; .
- All three give the same gases, so identify by the residue: black — copper; yellow hot, white cold — zinc; yellow hot, buff cold with decrepitation — lead.
- Decrepitation is trapped water bursting the crystals — only lead nitrate among the three.
- The gas mixture relights a splint AND is reddish-brown — report both, not just oxygen.
- Sodium and potassium nitrates give the nitrite and oxygen only, with no brown gas: .
- Iodine truly sublimes — violet vapours, shiny crystals re-form, a physical change.
- Ammonium chloride only appears to sublime — it dissociates and recombines: . Proof: red litmus turns blue high up (ammonia diffuses faster) and blue litmus turns red lower down.
- Ammonium dichromate does not sublime — it decomposes with sparks, a great increase in volume and a green fluffy residue: .
- A solid reappearing on the cool glass proves only that something condensed — whether it is the same substance is a separate question.
Cover the gas list and see whether you can name the confirmatory test for all nine, then try to say which two would fool a glowing splint and which two would fool each other.