Impure Zinc Makes Hydrogen Faster Than Pure Zinc Does
Set up the laboratory preparation of hydrogen from granulated zinc and dilute sulphuric acid, learn why that metal and that acid are chosen, collect the gas over water, and follow the industrial route.
Why is impure zinc the better choice for making hydrogen?
Almost everywhere else in chemistry, a purer reagent is a better reagent. Not here.
Drop a piece of very pure zinc into dilute sulphuric acid and the reaction is disappointingly slow — a few bubbles, and not much else. Drop in a granule of ordinary commercial zinc, which carries traces of other metals, and a steady stream of gas comes off at once.
The impurities are doing the work, and they are the reason every school laboratory keeps a jar of granulated zinc rather than the pure metal.
That is one of four choices in this preparation that all have reasons behind them. The metal is zinc rather than sodium or lead, the acid is dilute sulphuric rather than nitric or concentrated sulphuric, the gas is collected over water rather than over air, and the metal is granulated rather than powdered — and an examiner can ask about any of them.
The previous part of this chapter supplied the rule those choices obey: a usable metal must lie above hydrogen in the activity series, react at a controllable rate, and give a soluble salt. Zinc is the metal that satisfies all three.
This page covers the second part of the ICSE Class 9 Chemistry chapter on hydrogen: the laboratory preparation with its equation and precautions, why zinc and why granulated, how the gas is purified and collected, and the industrial route.
Drop a piece of very pure zinc into dilute sulphuric acid and the reaction is disappointingly slow — a few bubbles, and not much else. Drop in a granule of ordinary commercial zinc, which carries traces of other metals, and a steady stream of gas comes off at once.
The impurities are doing the work, and they are the reason every school laboratory keeps a jar of granulated zinc rather than the pure metal.
That is one of four choices in this preparation that all have reasons behind them. The metal is zinc rather than sodium or lead, the acid is dilute sulphuric rather than nitric or concentrated sulphuric, the gas is collected over water rather than over air, and the metal is granulated rather than powdered — and an examiner can ask about any of them.
The previous part of this chapter supplied the rule those choices obey: a usable metal must lie above hydrogen in the activity series, react at a controllable rate, and give a soluble salt. Zinc is the metal that satisfies all three.
This page covers the second part of the ICSE Class 9 Chemistry chapter on hydrogen: the laboratory preparation with its equation and precautions, why zinc and why granulated, how the gas is purified and collected, and the industrial route.
How is hydrogen prepared in the laboratory, and what are the precautions?
Hydrogen is prepared by the action of dilute sulphuric acid on granulated zinc.
The apparatus. Granulated zinc is placed at the bottom of a Woulfe's bottle or a flat-bottomed flask. The bottle carries two fittings: a thistle funnel through which the acid is added, and a delivery tube that carries the gas away. The delivery tube leads under water in a trough, to a beehive shelf over which an inverted gas jar full of water is placed.
Dilute sulphuric acid is poured in through the thistle funnel. Effervescence begins at once, hydrogen travels along the delivery tube, and it collects in the gas jar by pushing the water out of it.
The precautions, each with its reason.
- Keep every flame well away from the apparatus. A mixture of hydrogen and air explodes when ignited, and there is air in the apparatus at the start
- The lower end of the thistle funnel must dip below the surface of the acid. If it does not, the hydrogen takes the easy route and escapes up the funnel instead of going down the delivery tube
- Discard the gas collected in the first jar. The apparatus was full of air when the acid went in, so the first jar is mostly air — and an explosive mixture at that
- Test the gas for purity before igniting it. A small sample is collected in a test tube and a flame brought to its mouth; pure hydrogen burns quietly, while a mixture with air gives a sharp pop
- Add the acid gradually. The reaction is exothermic, and adding acid all at once makes it violent and difficult to control
- Do not use concentrated sulphuric acid or nitric acid. Both are oxidising agents and neither gives hydrogen, for the reasons set out in the previous part
One precaution is more often misunderstood than the others. The thistle funnel dipping below the acid is not about keeping air out — it is about making the delivery tube the only exit. A gas will always leave by the path of least resistance, and a funnel open to the atmosphere above the liquid is a much easier path than a narrow tube leading under water. Sealing that path with liquid is what forces the gas to go where you want it, and the same trick appears in every gas preparation in the practical syllabus.
The apparatus. Granulated zinc is placed at the bottom of a Woulfe's bottle or a flat-bottomed flask. The bottle carries two fittings: a thistle funnel through which the acid is added, and a delivery tube that carries the gas away. The delivery tube leads under water in a trough, to a beehive shelf over which an inverted gas jar full of water is placed.
Dilute sulphuric acid is poured in through the thistle funnel. Effervescence begins at once, hydrogen travels along the delivery tube, and it collects in the gas jar by pushing the water out of it.
The precautions, each with its reason.
- Keep every flame well away from the apparatus. A mixture of hydrogen and air explodes when ignited, and there is air in the apparatus at the start
- The lower end of the thistle funnel must dip below the surface of the acid. If it does not, the hydrogen takes the easy route and escapes up the funnel instead of going down the delivery tube
- Discard the gas collected in the first jar. The apparatus was full of air when the acid went in, so the first jar is mostly air — and an explosive mixture at that
- Test the gas for purity before igniting it. A small sample is collected in a test tube and a flame brought to its mouth; pure hydrogen burns quietly, while a mixture with air gives a sharp pop
- Add the acid gradually. The reaction is exothermic, and adding acid all at once makes it violent and difficult to control
- Do not use concentrated sulphuric acid or nitric acid. Both are oxidising agents and neither gives hydrogen, for the reasons set out in the previous part
One precaution is more often misunderstood than the others. The thistle funnel dipping below the acid is not about keeping air out — it is about making the delivery tube the only exit. A gas will always leave by the path of least resistance, and a funnel open to the atmosphere above the liquid is a much easier path than a narrow tube leading under water. Sealing that path with liquid is what forces the gas to go where you want it, and the same trick appears in every gas preparation in the practical syllabus.
Why granulated zinc rather than any other metal, and how is the gas purified?
Zinc is chosen because it satisfies all three conditions at once, and it is cheap.
- It lies above hydrogen in the activity series, so it can displace hydrogen from a dilute acid
- It reacts at a moderate, controllable rate. Potassium, sodium and calcium react far too violently to be handled safely; iron reacts too slowly to be practical
- Zinc sulphate is soluble, so it dissolves away and leaves fresh metal exposed. Lead fails exactly here — the insoluble lead sulphate it forms coats the metal and stops the reaction
- It is inexpensive and easily available
Why granulated and not powdered or in one lump. Granules give a large surface area, so the gas comes off in a steady usable stream. A single lump has too little surface and the reaction is slow. A powder has so much surface that the reaction becomes violent, froths up and carries acid spray into the delivery tube.
Why the impure metal beats the pure metal. Very pure zinc reacts with dilute sulphuric acid slowly. Ordinary commercial granulated zinc carries traces of other metals such as copper and iron, and each trace particle in contact with the zinc forms a tiny electrochemical couple that speeds the reaction up considerably.
So the impurity is not tolerated — it is wanted. This is the one place in the syllabus where a purer reagent gives the worse result, and a question asking why pure zinc is not used is testing whether you know the reason rather than merely the fact. The traces work because they are different metals in electrical contact with the zinc, which is the same effect that makes iron rust faster when it touches copper.
The hydrogen collected is not pure either. It carries:
- Acid spray and water vapour, thrown up by the effervescence
- Hydrogen sulphide, phosphine and arsine, produced from sulphide, phosphide and arsenide impurities in the zinc
- Traces of sulphur dioxide
The purification train removes them in order.
- First through water, which dissolves out the acid spray
- Then through lead nitrate solution, which removes hydrogen sulphide, arsine and phosphine. Potassium permanganate solution serves the same purpose
- Finally through concentrated sulphuric acid, which dries the gas. Anhydrous calcium chloride can be used instead
The order cannot be rearranged. Drying must come last, because the gas passes through two aqueous solutions before it and would pick up fresh moisture from each. A purification train is a sequence, not a set, and putting the drying agent first wastes it entirely.
- It lies above hydrogen in the activity series, so it can displace hydrogen from a dilute acid
- It reacts at a moderate, controllable rate. Potassium, sodium and calcium react far too violently to be handled safely; iron reacts too slowly to be practical
- Zinc sulphate is soluble, so it dissolves away and leaves fresh metal exposed. Lead fails exactly here — the insoluble lead sulphate it forms coats the metal and stops the reaction
- It is inexpensive and easily available
Why granulated and not powdered or in one lump. Granules give a large surface area, so the gas comes off in a steady usable stream. A single lump has too little surface and the reaction is slow. A powder has so much surface that the reaction becomes violent, froths up and carries acid spray into the delivery tube.
Why the impure metal beats the pure metal. Very pure zinc reacts with dilute sulphuric acid slowly. Ordinary commercial granulated zinc carries traces of other metals such as copper and iron, and each trace particle in contact with the zinc forms a tiny electrochemical couple that speeds the reaction up considerably.
So the impurity is not tolerated — it is wanted. This is the one place in the syllabus where a purer reagent gives the worse result, and a question asking why pure zinc is not used is testing whether you know the reason rather than merely the fact. The traces work because they are different metals in electrical contact with the zinc, which is the same effect that makes iron rust faster when it touches copper.
The hydrogen collected is not pure either. It carries:
- Acid spray and water vapour, thrown up by the effervescence
- Hydrogen sulphide, phosphine and arsine, produced from sulphide, phosphide and arsenide impurities in the zinc
- Traces of sulphur dioxide
The purification train removes them in order.
- First through water, which dissolves out the acid spray
- Then through lead nitrate solution, which removes hydrogen sulphide, arsine and phosphine. Potassium permanganate solution serves the same purpose
- Finally through concentrated sulphuric acid, which dries the gas. Anhydrous calcium chloride can be used instead
The order cannot be rearranged. Drying must come last, because the gas passes through two aqueous solutions before it and would pick up fresh moisture from each. A purification train is a sequence, not a set, and putting the drying agent first wastes it entirely.
Why is hydrogen collected over water instead of over air?
Hydrogen is collected by downward displacement of water, because it is almost insoluble in water and much lighter than air.
Both properties are needed, and each rules out a different alternative.
Insolubility permits water to be used at all. Hydrogen dissolves in water only to a negligible extent, so a gas jar of water can be filled with it without losing any to the liquid. A gas that dissolves readily — ammonia, or hydrogen chloride — would simply disappear into the water and none would collect.
Its low density rules out collecting it over air. Hydrogen is the lightest of all gases. It cannot be collected by displacement of air, because it would mix with the air in the jar and diffuse away rather than displacing it cleanly. There is nothing to hold it in place.
How the method works. An inverted gas jar is filled with water and stood on a beehive shelf in a trough of water, with the delivery tube leading up underneath it. The hydrogen bubbles up into the jar, and because it cannot escape past the water seal it collects in the top of the jar and pushes the water down and out. Hence the name: the water is displaced downwards.
The jar has to stay inverted afterwards, because the moment it is turned the right way up the hydrogen escapes upwards into the room — a consequence of the same low density that made the collection method necessary.
Match the method to the properties, because that is the examinable skill.
- Insoluble in water — collect over water
- Soluble in water but denser than air — collect by upward displacement of air, in a jar standing upright
- Soluble in water and lighter than air — collect by downward displacement of air, in an inverted jar
- Needed dry — collect in a dry jar and not over water at all
So the collection method is a deduction from two properties rather than a fact to memorise. Given any gas's solubility and density you can work out how to collect it, and a question giving you an unfamiliar gas is asking for exactly that reasoning. The gas that is both soluble and close to air in density is the awkward case — it has to be collected in a dry jar by displacement, and hydrogen's combination of properties is by comparison the easiest there is.
Both properties are needed, and each rules out a different alternative.
Insolubility permits water to be used at all. Hydrogen dissolves in water only to a negligible extent, so a gas jar of water can be filled with it without losing any to the liquid. A gas that dissolves readily — ammonia, or hydrogen chloride — would simply disappear into the water and none would collect.
Its low density rules out collecting it over air. Hydrogen is the lightest of all gases. It cannot be collected by displacement of air, because it would mix with the air in the jar and diffuse away rather than displacing it cleanly. There is nothing to hold it in place.
How the method works. An inverted gas jar is filled with water and stood on a beehive shelf in a trough of water, with the delivery tube leading up underneath it. The hydrogen bubbles up into the jar, and because it cannot escape past the water seal it collects in the top of the jar and pushes the water down and out. Hence the name: the water is displaced downwards.
The jar has to stay inverted afterwards, because the moment it is turned the right way up the hydrogen escapes upwards into the room — a consequence of the same low density that made the collection method necessary.
Match the method to the properties, because that is the examinable skill.
- Insoluble in water — collect over water
- Soluble in water but denser than air — collect by upward displacement of air, in a jar standing upright
- Soluble in water and lighter than air — collect by downward displacement of air, in an inverted jar
- Needed dry — collect in a dry jar and not over water at all
So the collection method is a deduction from two properties rather than a fact to memorise. Given any gas's solubility and density you can work out how to collect it, and a question giving you an unfamiliar gas is asking for exactly that reasoning. The gas that is both soluble and close to air in density is the awkward case — it has to be collected in a dry jar by displacement, and hydrogen's combination of properties is by comparison the easiest there is.
How is hydrogen manufactured on an industrial scale?
In the Bosch process, steam is passed over white-hot coke to give water gas; the carbon monoxide in it is then converted to carbon dioxide with more steam over a catalyst, and the two oxides are washed out, leaving hydrogen.
Stage 1 — making water gas. Steam is passed over coke heated to white heat:
The product is a mixture of carbon monoxide and hydrogen in equal volumes, called water gas.
Stage 2 — the shift reaction. The water gas is mixed with about twice its volume of steam and passed over a catalyst of ferric oxide with chromium oxide as a promoter, at a moderate temperature:
Every molecule of carbon monoxide becomes one of carbon dioxide and yields a further molecule of hydrogen, so this stage both removes a problem and increases the product.
Stage 3 — removing the carbon dioxide. The mixture is passed through water under pressure, in which carbon dioxide dissolves readily and hydrogen does not. Caustic potash solution can be used instead.
Stage 4 — removing the last of the carbon monoxide. Any carbon monoxide that survived stage 2 is absorbed by passing the gas through ammoniacal cuprous chloride solution.
What leaves the plant is hydrogen.
Now the question worth asking: why is stage 2 there at all? Water gas already contains the hydrogen. Why not simply separate the carbon monoxide out and be done with it?
Because you cannot. Carbon monoxide and hydrogen are both insoluble in water, both colourless, and close enough in behaviour that no simple washing step will separate them. Carbon dioxide, on the other hand, dissolves in water easily.
So stage 2 is not about making more hydrogen — it is about converting an inseparable impurity into a separable one. The extra hydrogen is a bonus. That is the single most important idea in this preparation, and it is the reason the process has a catalytic stage in the middle rather than going straight from water gas to purification.
The same logic explains stage 3's use of pressure. Carbon dioxide is more soluble in water at higher pressure, so squeezing the mixture into water under pressure removes far more of it than washing at ordinary pressure would. Each stage is chosen for a property of the substance it is trying to remove — solubility for the dioxide, absorption by a specific reagent for the monoxide — and reciting the stages without those reasons is what loses marks on this question.
Stage 1 — making water gas. Steam is passed over coke heated to white heat:
The product is a mixture of carbon monoxide and hydrogen in equal volumes, called water gas.
Stage 2 — the shift reaction. The water gas is mixed with about twice its volume of steam and passed over a catalyst of ferric oxide with chromium oxide as a promoter, at a moderate temperature:
Every molecule of carbon monoxide becomes one of carbon dioxide and yields a further molecule of hydrogen, so this stage both removes a problem and increases the product.
Stage 3 — removing the carbon dioxide. The mixture is passed through water under pressure, in which carbon dioxide dissolves readily and hydrogen does not. Caustic potash solution can be used instead.
Stage 4 — removing the last of the carbon monoxide. Any carbon monoxide that survived stage 2 is absorbed by passing the gas through ammoniacal cuprous chloride solution.
What leaves the plant is hydrogen.
Now the question worth asking: why is stage 2 there at all? Water gas already contains the hydrogen. Why not simply separate the carbon monoxide out and be done with it?
Because you cannot. Carbon monoxide and hydrogen are both insoluble in water, both colourless, and close enough in behaviour that no simple washing step will separate them. Carbon dioxide, on the other hand, dissolves in water easily.
So stage 2 is not about making more hydrogen — it is about converting an inseparable impurity into a separable one. The extra hydrogen is a bonus. That is the single most important idea in this preparation, and it is the reason the process has a catalytic stage in the middle rather than going straight from water gas to purification.
The same logic explains stage 3's use of pressure. Carbon dioxide is more soluble in water at higher pressure, so squeezing the mixture into water under pressure removes far more of it than washing at ordinary pressure would. Each stage is chosen for a property of the substance it is trying to remove — solubility for the dioxide, absorption by a specific reagent for the monoxide — and reciting the stages without those reasons is what loses marks on this question.
Exam tip
Exam tip: every choice in this preparation has a reason — give it
Write the equation with the state of the acid stated: (dilute) .
Name the apparatus: a Woulfe's bottle or flat-bottomed flask, a thistle funnel, a delivery tube, a beehive shelf and an inverted gas jar over water.
The thistle funnel must dip below the acid, so that the delivery tube is the only exit for the gas.
Discard the first jar — it holds the air that was in the apparatus.
Justify zinc with all three conditions: above hydrogen in the activity series, a controllable rate, and a soluble salt. Then add that it is cheap.
Pure zinc reacts slowly. Traces of other metals in commercial zinc form tiny electrochemical couples that speed the reaction up — so the impure metal is preferred.
Granules, not powder: powder gives too much surface, froths and carries acid spray over.
Name the three purification stages in order — water for acid spray, lead nitrate solution for , arsine and phosphine, and concentrated sulphuric acid to dry. Drying is always last.
Justify the collection with BOTH properties: almost insoluble in water, so water can be used; lighter than air, so displacement of air will not work.
For the Bosch process give four stages: water gas from coke and steam; the shift over ferric oxide with a chromium oxide promoter; removed in water under pressure; residual absorbed in ammoniacal cuprous chloride.
And say why the shift stage exists — carbon monoxide cannot be washed out of hydrogen, and carbon dioxide can.
Name the apparatus: a Woulfe's bottle or flat-bottomed flask, a thistle funnel, a delivery tube, a beehive shelf and an inverted gas jar over water.
The thistle funnel must dip below the acid, so that the delivery tube is the only exit for the gas.
Discard the first jar — it holds the air that was in the apparatus.
Justify zinc with all three conditions: above hydrogen in the activity series, a controllable rate, and a soluble salt. Then add that it is cheap.
Pure zinc reacts slowly. Traces of other metals in commercial zinc form tiny electrochemical couples that speed the reaction up — so the impure metal is preferred.
Granules, not powder: powder gives too much surface, froths and carries acid spray over.
Name the three purification stages in order — water for acid spray, lead nitrate solution for , arsine and phosphine, and concentrated sulphuric acid to dry. Drying is always last.
Justify the collection with BOTH properties: almost insoluble in water, so water can be used; lighter than air, so displacement of air will not work.
For the Bosch process give four stages: water gas from coke and steam; the shift over ferric oxide with a chromium oxide promoter; removed in water under pressure; residual absorbed in ammoniacal cuprous chloride.
And say why the shift stage exists — carbon monoxide cannot be washed out of hydrogen, and carbon dioxide can.
Did you know
Why the lightest gas is the hardest one to keep
Every property that makes hydrogen convenient to collect also makes it awkward to keep.
It is collected over water because it is almost insoluble and very light. Those are helpful properties during the preparation. Ten minutes later they are the reason the jar has to stay upside down on the bench, because the moment it is righted the gas is gone.
The trouble runs deeper than a gas jar. Hydrogen molecules are the smallest molecules there are, and small fast molecules find their way through gaps that larger ones cannot. A rubber tube that holds carbon dioxide comfortably will lose hydrogen through its walls. A balloon filled with hydrogen goes slack faster than the same balloon filled with air, and not because of a leak you could find.
There is a neat piece of evidence for this in the practical syllabus, in the behaviour of a porous pot. Fill one with hydrogen and stand it in air, and the hydrogen rushes out faster than the air can come in — the pressure inside falls. The lighter the gas, the faster it diffuses, and hydrogen is the lightest of all.
So the same property that makes hydrogen rise to the top of an inverted gas jar and sit there obligingly also means it will not stay anywhere for long.
There is a practical consequence that follows directly from the precaution list. Hydrogen leaking into a room does not pool on the floor the way a heavy gas does — it rises to the ceiling and spreads. That is why the warning in the laboratory is about keeping flames away from the whole room rather than from the bench, and why the gas is tested for purity in a small test tube before any larger quantity is lit.
A gas that is hard to contain and easy to ignite is a gas whose handling rules are about where it goes rather than where you put it — which is exactly why this preparation has more precautions attached to it than any other in the chapter.
It is collected over water because it is almost insoluble and very light. Those are helpful properties during the preparation. Ten minutes later they are the reason the jar has to stay upside down on the bench, because the moment it is righted the gas is gone.
The trouble runs deeper than a gas jar. Hydrogen molecules are the smallest molecules there are, and small fast molecules find their way through gaps that larger ones cannot. A rubber tube that holds carbon dioxide comfortably will lose hydrogen through its walls. A balloon filled with hydrogen goes slack faster than the same balloon filled with air, and not because of a leak you could find.
There is a neat piece of evidence for this in the practical syllabus, in the behaviour of a porous pot. Fill one with hydrogen and stand it in air, and the hydrogen rushes out faster than the air can come in — the pressure inside falls. The lighter the gas, the faster it diffuses, and hydrogen is the lightest of all.
So the same property that makes hydrogen rise to the top of an inverted gas jar and sit there obligingly also means it will not stay anywhere for long.
There is a practical consequence that follows directly from the precaution list. Hydrogen leaking into a room does not pool on the floor the way a heavy gas does — it rises to the ceiling and spreads. That is why the warning in the laboratory is about keeping flames away from the whole room rather than from the bench, and why the gas is tested for purity in a small test tube before any larger quantity is lit.
A gas that is hard to contain and easy to ignite is a gas whose handling rules are about where it goes rather than where you put it — which is exactly why this preparation has more precautions attached to it than any other in the chapter.
Exam relevance
Why does JEE Main keep returning to catalysts and gas purification?
Because the industrial route on this page is one of the standard worked examples of catalysis and equilibrium control in Class 11 and 12.
This is the foundation for Class 11 Chemistry Hydrogen and Equilibrium, and Class 12 The p-Block Elements and Surface Chemistry, examined in both JEE Main and NEET. The shift reaction used here is one of the syllabus's reference reactions for a heterogeneous catalyst with a promoter — and Surface Chemistry asks exactly that: name the catalyst, name the promoter, and say what a promoter does. The ferric oxide and chromium oxide pair is worth memorising in those terms.
The shift reaction becomes an equilibrium problem. Class 11 Equilibrium treats it as reversible and applies Le Chatelier's principle — why excess steam is used, how temperature shifts the position, and how removing a product drives the reaction forward. The detail that about twice the volume of steam is used is the practical face of the principle, and questions asking which change increases the yield of a given industrial reaction are a recurring JEE Main type.
**Class 11 Hydrogen covers the preparation and the uses directly. It treats the laboratory and commercial methods, then the hydrides — ionic, covalent and metallic — heavy water, and hydrogen peroxide with its structure and reactions. Hydrogen as a fuel and the hydrogen economy are examined as short-answer topics, and they follow from the manufacture described here.
The purification logic reappears as separation technique.** Class 11 Some Basic Concepts and the practical syllabus both use the principle that a mixture is separated by exploiting a property in which the components differ — here, solubility in water. Converting an inseparable impurity into a separable one is a standard idea in process chemistry, and it is the reason the shift stage exists.
Gas collection and identification stay examinable through the practical papers. The solubility-and-density reasoning used here to justify collection over water is applied in Class 11 and 12 practicals to every gas prepared, and NEET asks it as recall — match a gas to its correct method of collection.
The activity-series reasoning behind the choice of zinc becomes electrochemistry. Class 12 Electrochemistry replaces "above hydrogen" with a standard electrode potential, and the tiny electrochemical couples that make impure zinc react faster are the same galvanic cells that chapter builds deliberately. The corrosion section explains the impure-zinc effect explicitly, using the contact of two different metals in an electrolyte.
For NEET, this material is examined as recall and reasoning: the reagents for a named preparation, the drying agent used, the catalyst in an industrial process, or the correct collection method for a given gas. Chemistry questions of this kind reward precise names rather than description.
What the questions look like. For board work, expect describe the laboratory preparation with a labelled diagram, equation and precautions, give three reasons for choosing granulated zinc, explain why pure zinc is not used, name the purifying agents in order, justify the method of collection, and describe the Bosch process with its reactions and conditions. Reasons carry as many marks as the equations. For JEE Main and NEET, expect catalyst and promoter identification, Le Chatelier applications, hydride classification and collection matching.
How board and competitive emphasis differ. A board paper rewards the named apparatus, the stated precaution and the reason behind each choice. A competitive paper assumes the procedure and asks which condition maximises the yield, or what role the promoter plays.
The single trap that costs the most marks. Putting the drying agent anywhere but last in the purification train. The gas passes through two aqueous solutions before it, so drying first achieves nothing at all. The defence is to ask what each stage adds as well as what it removes — a wash bottle removes an impurity and adds water vapour, which is precisely why the drying stage has to come after every wet stage.
This is the foundation for Class 11 Chemistry Hydrogen and Equilibrium, and Class 12 The p-Block Elements and Surface Chemistry, examined in both JEE Main and NEET. The shift reaction used here is one of the syllabus's reference reactions for a heterogeneous catalyst with a promoter — and Surface Chemistry asks exactly that: name the catalyst, name the promoter, and say what a promoter does. The ferric oxide and chromium oxide pair is worth memorising in those terms.
The shift reaction becomes an equilibrium problem. Class 11 Equilibrium treats it as reversible and applies Le Chatelier's principle — why excess steam is used, how temperature shifts the position, and how removing a product drives the reaction forward. The detail that about twice the volume of steam is used is the practical face of the principle, and questions asking which change increases the yield of a given industrial reaction are a recurring JEE Main type.
**Class 11 Hydrogen covers the preparation and the uses directly. It treats the laboratory and commercial methods, then the hydrides — ionic, covalent and metallic — heavy water, and hydrogen peroxide with its structure and reactions. Hydrogen as a fuel and the hydrogen economy are examined as short-answer topics, and they follow from the manufacture described here.
The purification logic reappears as separation technique.** Class 11 Some Basic Concepts and the practical syllabus both use the principle that a mixture is separated by exploiting a property in which the components differ — here, solubility in water. Converting an inseparable impurity into a separable one is a standard idea in process chemistry, and it is the reason the shift stage exists.
Gas collection and identification stay examinable through the practical papers. The solubility-and-density reasoning used here to justify collection over water is applied in Class 11 and 12 practicals to every gas prepared, and NEET asks it as recall — match a gas to its correct method of collection.
The activity-series reasoning behind the choice of zinc becomes electrochemistry. Class 12 Electrochemistry replaces "above hydrogen" with a standard electrode potential, and the tiny electrochemical couples that make impure zinc react faster are the same galvanic cells that chapter builds deliberately. The corrosion section explains the impure-zinc effect explicitly, using the contact of two different metals in an electrolyte.
For NEET, this material is examined as recall and reasoning: the reagents for a named preparation, the drying agent used, the catalyst in an industrial process, or the correct collection method for a given gas. Chemistry questions of this kind reward precise names rather than description.
What the questions look like. For board work, expect describe the laboratory preparation with a labelled diagram, equation and precautions, give three reasons for choosing granulated zinc, explain why pure zinc is not used, name the purifying agents in order, justify the method of collection, and describe the Bosch process with its reactions and conditions. Reasons carry as many marks as the equations. For JEE Main and NEET, expect catalyst and promoter identification, Le Chatelier applications, hydride classification and collection matching.
How board and competitive emphasis differ. A board paper rewards the named apparatus, the stated precaution and the reason behind each choice. A competitive paper assumes the procedure and asks which condition maximises the yield, or what role the promoter plays.
The single trap that costs the most marks. Putting the drying agent anywhere but last in the purification train. The gas passes through two aqueous solutions before it, so drying first achieves nothing at all. The defence is to ask what each stage adds as well as what it removes — a wash bottle removes an impurity and adds water vapour, which is precisely why the drying stage has to come after every wet stage.
Key takeaways
Preparing, purifying and collecting hydrogen: quick revision
- Laboratory preparation: dilute sulphuric acid on granulated zinc — .
- Apparatus: a Woulfe's bottle or flat-bottomed flask, a thistle funnel, a delivery tube, a beehive shelf, and an inverted gas jar of water in a trough.
- Precautions: keep flames away (hydrogen and air explode); the thistle funnel must dip below the acid so the delivery tube is the only exit; discard the first jar of air; test for purity before igniting; add acid gradually because the reaction is exothermic; never use concentrated sulphuric or nitric acid, which are oxidising.
- Why zinc: above hydrogen in the activity series; a controllable rate, unlike sodium, potassium and calcium; a soluble sulphate, unlike lead whose insoluble sulphate coats the metal; and it is cheap.
- Why granulated: enough surface for a steady stream. A lump is too slow; a powder reacts violently and carries acid spray over.
- Why impure zinc: pure zinc reacts slowly. Traces of other metals form tiny electrochemical couples that speed the reaction up.
- Impurities in the gas: acid spray, water vapour, hydrogen sulphide, phosphine, arsine and traces of sulphur dioxide.
- Purification in order: water for acid spray; lead nitrate (or potassium permanganate) solution for , arsine and phosphine; concentrated sulphuric acid (or anhydrous calcium chloride) to dry. Drying is always last.
- Collection by downward displacement of water, because hydrogen is almost insoluble in water and lighter than air — so displacement of air would let it mix and diffuse away.
- Keep the jar inverted afterwards, or the gas escapes upwards.
- Matching method to properties: insoluble — over water; soluble and denser than air — upward displacement of air; soluble and lighter than air — downward displacement of air; needed dry — a dry jar.
- Bosch process, stage 1: over white-hot coke, giving water gas.
- Stage 2, the shift: with about twice the volume of steam, over ferric oxide with a chromium oxide promoter.
- Stage 3: carbon dioxide dissolved out in water under pressure, or in caustic potash solution.
- Stage 4: residual carbon monoxide absorbed in ammoniacal cuprous chloride.
- Why the shift stage exists: carbon monoxide is insoluble and cannot be washed out of hydrogen, while carbon dioxide dissolves easily — so the impurity is converted into a separable one, and extra hydrogen is the bonus.
Sketch the apparatus from memory and label six parts, then write one reason beside each of the four choices — metal, acid, granule size and collection method.
- Apparatus: a Woulfe's bottle or flat-bottomed flask, a thistle funnel, a delivery tube, a beehive shelf, and an inverted gas jar of water in a trough.
- Precautions: keep flames away (hydrogen and air explode); the thistle funnel must dip below the acid so the delivery tube is the only exit; discard the first jar of air; test for purity before igniting; add acid gradually because the reaction is exothermic; never use concentrated sulphuric or nitric acid, which are oxidising.
- Why zinc: above hydrogen in the activity series; a controllable rate, unlike sodium, potassium and calcium; a soluble sulphate, unlike lead whose insoluble sulphate coats the metal; and it is cheap.
- Why granulated: enough surface for a steady stream. A lump is too slow; a powder reacts violently and carries acid spray over.
- Why impure zinc: pure zinc reacts slowly. Traces of other metals form tiny electrochemical couples that speed the reaction up.
- Impurities in the gas: acid spray, water vapour, hydrogen sulphide, phosphine, arsine and traces of sulphur dioxide.
- Purification in order: water for acid spray; lead nitrate (or potassium permanganate) solution for , arsine and phosphine; concentrated sulphuric acid (or anhydrous calcium chloride) to dry. Drying is always last.
- Collection by downward displacement of water, because hydrogen is almost insoluble in water and lighter than air — so displacement of air would let it mix and diffuse away.
- Keep the jar inverted afterwards, or the gas escapes upwards.
- Matching method to properties: insoluble — over water; soluble and denser than air — upward displacement of air; soluble and lighter than air — downward displacement of air; needed dry — a dry jar.
- Bosch process, stage 1: over white-hot coke, giving water gas.
- Stage 2, the shift: with about twice the volume of steam, over ferric oxide with a chromium oxide promoter.
- Stage 3: carbon dioxide dissolved out in water under pressure, or in caustic potash solution.
- Stage 4: residual carbon monoxide absorbed in ammoniacal cuprous chloride.
- Why the shift stage exists: carbon monoxide is insoluble and cannot be washed out of hydrogen, while carbon dioxide dissolves easily — so the impurity is converted into a separable one, and extra hydrogen is the bonus.
Sketch the apparatus from memory and label six parts, then write one reason beside each of the four choices — metal, acid, granule size and collection method.