Hydrogen Sits in a Place Where It Does Not Quite Belong
Learn where hydrogen occurs free and combined, why it resembles two different groups at once, how it is prepared in the laboratory from zinc and dilute acid, how metals behave with water, steam and acids, and how it is made industrially.
Why is hydrogen placed with the metals when it is a gas?
Because it behaves partly like a metal and partly like a non-metal, and no single position in the periodic table fits both.
Hydrogen has one electron in its only shell. That makes it look like sodium and potassium, which also have one outer electron and which form positive ions. But hydrogen is also one electron short of a full shell, which makes it look like chlorine and fluorine, which are one short too.
So it genuinely resembles two groups that sit at opposite ends of the table. It is placed provisionally at the top of the first group, and its refusal to settle is called its anomalous position. This page covers the first part of the ICSE Class 8 Chemistry chapter on hydrogen: where it is found, how it is prepared, and how metals release it.
Hydrogen has one electron in its only shell. That makes it look like sodium and potassium, which also have one outer electron and which form positive ions. But hydrogen is also one electron short of a full shell, which makes it look like chlorine and fluorine, which are one short too.
So it genuinely resembles two groups that sit at opposite ends of the table. It is placed provisionally at the top of the first group, and its refusal to settle is called its anomalous position. This page covers the first part of the ICSE Class 8 Chemistry chapter on hydrogen: where it is found, how it is prepared, and how metals release it.
Where does hydrogen occur, and why is its position anomalous?
In the free state hydrogen is rare on Earth. There are only traces in the atmosphere, a little in volcanic gases and in natural gas wells. The reason is its lightness — being the least dense of all gases, free hydrogen drifts to the upper atmosphere and escapes into space.
Beyond the Earth the picture reverses completely: hydrogen is the most abundant element in the universe, and it is the main constituent of the Sun and the stars.
In the combined state it is everywhere:
- Water — by far the largest store. Since has molecular mass of which is hydrogen, the percentage of hydrogen in water by mass is
- All acids — , ,
- All alkalis — ,
- Hydrocarbons — petroleum, natural gas, methane
- Carbohydrates, fats and proteins — so all food and all living matter
Why its position is anomalous. Hydrogen shows two sets of resemblances.
Like the alkali metals of group 1:
- One electron in the outermost shell
- Forms a positive ion, , by losing that electron
- Displaces from acids and is itself displaced by metals
- Forms an oxide and a chloride of similar formula type — beside , beside
- Acts as a reducing agent, as metals do
Like the halogens of group 17:
- One electron short of a complete shell
- Exists as a diatomic molecule, , just as does
- Is a non-metal and a gas
- Forms hydrides with active metals, such as , in which hydrogen carries a negative charge exactly as chlorine does in
The honest conclusion. Neither group is right. It is placed at the top of group 1 because the single valence electron is the more useful resemblance, but the placement is a compromise and the syllabus expects you to say so — naming resemblances on both sides is what a full answer requires.
Beyond the Earth the picture reverses completely: hydrogen is the most abundant element in the universe, and it is the main constituent of the Sun and the stars.
In the combined state it is everywhere:
- Water — by far the largest store. Since has molecular mass of which is hydrogen, the percentage of hydrogen in water by mass is
- All acids — , ,
- All alkalis — ,
- Hydrocarbons — petroleum, natural gas, methane
- Carbohydrates, fats and proteins — so all food and all living matter
Why its position is anomalous. Hydrogen shows two sets of resemblances.
Like the alkali metals of group 1:
- One electron in the outermost shell
- Forms a positive ion, , by losing that electron
- Displaces from acids and is itself displaced by metals
- Forms an oxide and a chloride of similar formula type — beside , beside
- Acts as a reducing agent, as metals do
Like the halogens of group 17:
- One electron short of a complete shell
- Exists as a diatomic molecule, , just as does
- Is a non-metal and a gas
- Forms hydrides with active metals, such as , in which hydrogen carries a negative charge exactly as chlorine does in
The honest conclusion. Neither group is right. It is placed at the top of group 1 because the single valence electron is the more useful resemblance, but the placement is a compromise and the syllabus expects you to say so — naming resemblances on both sides is what a full answer requires.
How is hydrogen prepared in the laboratory?
By the action of dilute sulphuric acid on granulated zinc.
The apparatus. Granulated zinc is placed in a flask or a Woulfe's bottle. Dilute sulphuric acid is added through a thistle funnel whose stem dips below the acid surface, so the gas cannot escape back up it. The hydrogen leaves by a delivery tube and is collected by the downward displacement of water — that is, over water, in an inverted gas jar.
Why collected over water. Hydrogen is almost insoluble in water, so water will not absorb it. And it is lighter than air, so it rises and fills the jar from the top.
Why dilute nitric acid is not used. Nitric acid is a strong oxidising agent. Any hydrogen it liberates is immediately oxidised to water, so instead of collecting hydrogen you get oxides of nitrogen and nothing useful. This is the single most asked question on this preparation.
Why pure zinc is not used. Pure zinc reacts with dilute acid very slowly. Commercial granulated zinc contains small amounts of other metals as impurities, and these set up tiny local cells at the surface that speed the reaction up considerably. So impure zinc is deliberately preferred — an unusual case where the impurity is the point.
Why not sodium or potassium. They react with the acid explosively, which makes them unsafe and uncontrollable for a preparation.
Why not copper. Copper sits below hydrogen in the reactivity series, so it cannot displace hydrogen from a dilute acid at all — the reactivity-series rule of the previous chapter, applied directly.
Drying the gas. If dry hydrogen is needed, the gas is passed through a tube of calcium chloride or over concentrated sulphuric acid as a drying agent, and then collected by the downward displacement of air.
One safety point that follows from the chemistry. Hydrogen forms an explosive mixture with air, so the apparatus must be flushed through before any flame is brought near, and the first jar collected is discarded because it contains the air that was in the flask.
The apparatus. Granulated zinc is placed in a flask or a Woulfe's bottle. Dilute sulphuric acid is added through a thistle funnel whose stem dips below the acid surface, so the gas cannot escape back up it. The hydrogen leaves by a delivery tube and is collected by the downward displacement of water — that is, over water, in an inverted gas jar.
Why collected over water. Hydrogen is almost insoluble in water, so water will not absorb it. And it is lighter than air, so it rises and fills the jar from the top.
Why dilute nitric acid is not used. Nitric acid is a strong oxidising agent. Any hydrogen it liberates is immediately oxidised to water, so instead of collecting hydrogen you get oxides of nitrogen and nothing useful. This is the single most asked question on this preparation.
Why pure zinc is not used. Pure zinc reacts with dilute acid very slowly. Commercial granulated zinc contains small amounts of other metals as impurities, and these set up tiny local cells at the surface that speed the reaction up considerably. So impure zinc is deliberately preferred — an unusual case where the impurity is the point.
Why not sodium or potassium. They react with the acid explosively, which makes them unsafe and uncontrollable for a preparation.
Why not copper. Copper sits below hydrogen in the reactivity series, so it cannot displace hydrogen from a dilute acid at all — the reactivity-series rule of the previous chapter, applied directly.
Drying the gas. If dry hydrogen is needed, the gas is passed through a tube of calcium chloride or over concentrated sulphuric acid as a drying agent, and then collected by the downward displacement of air.
One safety point that follows from the chemistry. Hydrogen forms an explosive mixture with air, so the apparatus must be flushed through before any flame is brought near, and the first jar collected is discarded because it contains the air that was in the flask.
How do metals react with water, steam, acids and alkalis?
The reactivity series decides everything, and the more reactive the metal, the milder the reagent needed to get hydrogen out of it.
With cold water — only the most reactive metals: potassium, sodium and calcium. They give a hydroxide and hydrogen.
Sodium reacts so vigorously that the hydrogen catches fire, which is why sodium is stored under kerosene.
With steam — magnesium, aluminium, zinc and iron. They give an oxide and hydrogen, not a hydroxide.
The iron reaction is reversible, which matters industrially: removing the hydrogen as it forms keeps it going forward.
With dilute acids — every metal above hydrogen in the series. They give a salt and hydrogen.
Metals below hydrogen — copper, silver, gold, mercury — give no reaction with dilute acids.
With alkalis — only the amphoteric metals: zinc, aluminium and lead. They give a special salt and hydrogen.
The products are sodium zincate and sodium aluminate. Most metals do nothing at all with alkalis, so this reaction is a test for an amphoteric metal.
The pattern to carry away. Reading down the series, the reagent has to get harsher: cold water for potassium, sodium and calcium; steam for magnesium through iron; dilute acid for everything above hydrogen; and nothing works for copper and below. If you know where a metal sits, you can predict its behaviour with all four reagents without memorising a table.
And the product changes too, which is the detail most often missed: cold water gives a hydroxide, while steam gives an oxide.
With cold water — only the most reactive metals: potassium, sodium and calcium. They give a hydroxide and hydrogen.
Sodium reacts so vigorously that the hydrogen catches fire, which is why sodium is stored under kerosene.
With steam — magnesium, aluminium, zinc and iron. They give an oxide and hydrogen, not a hydroxide.
The iron reaction is reversible, which matters industrially: removing the hydrogen as it forms keeps it going forward.
With dilute acids — every metal above hydrogen in the series. They give a salt and hydrogen.
Metals below hydrogen — copper, silver, gold, mercury — give no reaction with dilute acids.
With alkalis — only the amphoteric metals: zinc, aluminium and lead. They give a special salt and hydrogen.
The products are sodium zincate and sodium aluminate. Most metals do nothing at all with alkalis, so this reaction is a test for an amphoteric metal.
The pattern to carry away. Reading down the series, the reagent has to get harsher: cold water for potassium, sodium and calcium; steam for magnesium through iron; dilute acid for everything above hydrogen; and nothing works for copper and below. If you know where a metal sits, you can predict its behaviour with all four reagents without memorising a table.
And the product changes too, which is the detail most often missed: cold water gives a hydroxide, while steam gives an oxide.
How is hydrogen manufactured on an industrial scale?
Two methods, and they are chosen for opposite reasons — cost and purity.
The Bosch process, which is the cheap large-scale route, works in two stages.
Stage 1 — making water gas. Steam is passed over red-hot coke at about :
The mixture of carbon monoxide and hydrogen produced is called water gas. It is only about half hydrogen, so it is not yet useful.
Stage 2 — converting the carbon monoxide. More steam is mixed in and the gases are passed over a catalyst of ferric oxide with chromium oxide as a promoter, at about :
The carbon monoxide has become carbon dioxide, and a second molecule of hydrogen has been won from the steam.
Stage 3 — removing the carbon dioxide. The mixture is passed into water under pressure, in which carbon dioxide dissolves readily while hydrogen does not. What remains is hydrogen, with any last traces of carbon dioxide removed by passing through a solution of an alkali such as potassium hydroxide.
Why the second stage exists at all. Without it, half the product is carbon monoxide — a poisonous gas that would have to be discarded. The stage converts the waste into more of the product while using nothing but extra steam, which is what makes the process economic.
Electrolysis of water, which is the pure but expensive route.
Water made conducting with a little acid or alkali is electrolysed. Hydrogen collects at the cathode and oxygen at the anode, in a **volume ratio of — a ratio you can read directly off the balanced equation.
Comparing the two. Electrolysis gives very pure** hydrogen and the oxygen is a useful by-product, but it consumes a great deal of electricity and is therefore costly. The Bosch process is far cheaper per unit of gas but gives a less pure product needing purification. So the choice depends on what the hydrogen is for — electrolysis where purity matters, the Bosch process where quantity does.
The Bosch process, which is the cheap large-scale route, works in two stages.
Stage 1 — making water gas. Steam is passed over red-hot coke at about :
The mixture of carbon monoxide and hydrogen produced is called water gas. It is only about half hydrogen, so it is not yet useful.
Stage 2 — converting the carbon monoxide. More steam is mixed in and the gases are passed over a catalyst of ferric oxide with chromium oxide as a promoter, at about :
The carbon monoxide has become carbon dioxide, and a second molecule of hydrogen has been won from the steam.
Stage 3 — removing the carbon dioxide. The mixture is passed into water under pressure, in which carbon dioxide dissolves readily while hydrogen does not. What remains is hydrogen, with any last traces of carbon dioxide removed by passing through a solution of an alkali such as potassium hydroxide.
Why the second stage exists at all. Without it, half the product is carbon monoxide — a poisonous gas that would have to be discarded. The stage converts the waste into more of the product while using nothing but extra steam, which is what makes the process economic.
Electrolysis of water, which is the pure but expensive route.
Water made conducting with a little acid or alkali is electrolysed. Hydrogen collects at the cathode and oxygen at the anode, in a **volume ratio of — a ratio you can read directly off the balanced equation.
Comparing the two. Electrolysis gives very pure** hydrogen and the oxygen is a useful by-product, but it consumes a great deal of electricity and is therefore costly. The Bosch process is far cheaper per unit of gas but gives a less pure product needing purification. So the choice depends on what the hydrogen is for — electrolysis where purity matters, the Bosch process where quantity does.
Exam tip
Exam tip: give the reason nitric acid is excluded
Two questions come up almost every time on the laboratory preparation, so have both ready. Nitric acid is avoided because it is an oxidising agent and oxidises the hydrogen to water. Pure zinc is avoided because it reacts too slowly, and the impurities in granulated zinc speed the reaction up.
Name the collection method precisely: downward displacement of water, because hydrogen is almost insoluble in water and lighter than air.
Distinguish the products carefully: cold water gives a hydroxide, steam gives an oxide. Writing for a cold-water reaction, or for steam, loses the mark.
Use the reactivity series to justify every prediction, and write no reaction for copper, silver and gold with dilute acids.
Remember only zinc, aluminium and lead react with alkalis, giving zincate and aluminate salts — this is the test for an amphoteric metal.
For the anomalous position, give resemblances to both the alkali metals and the halogens. Naming only one side is half the answer.
For the Bosch process, give both stages with their catalysts and state how the carbon dioxide is removed — dissolving in water under pressure.
And quote the ** volume ratio for electrolysis, with hydrogen at the cathode**.
Name the collection method precisely: downward displacement of water, because hydrogen is almost insoluble in water and lighter than air.
Distinguish the products carefully: cold water gives a hydroxide, steam gives an oxide. Writing for a cold-water reaction, or for steam, loses the mark.
Use the reactivity series to justify every prediction, and write no reaction for copper, silver and gold with dilute acids.
Remember only zinc, aluminium and lead react with alkalis, giving zincate and aluminate salts — this is the test for an amphoteric metal.
For the anomalous position, give resemblances to both the alkali metals and the halogens. Naming only one side is half the answer.
For the Bosch process, give both stages with their catalysts and state how the carbon dioxide is removed — dissolving in water under pressure.
And quote the ** volume ratio for electrolysis, with hydrogen at the cathode**.
Did you know
Why is hydrogen abundant in the universe but scarce in our air?
Hydrogen makes up most of the matter in the stars, and almost none of the air you are breathing. The same property explains both.
Hydrogen is the lightest substance there is. In a gas, lighter molecules move faster at a given temperature, and a molecule moving fast enough simply leaves — it climbs out of the Earth's gravitational reach and does not come back. Over a long enough stretch of time, essentially all the free hydrogen the Earth ever had has drifted away.
A star is a different case entirely. It is vastly more massive, so its gravity holds hydrogen easily, and there was nothing to pull the hydrogen away from the material the star formed from in the first place.
So the Earth kept its hydrogen only where it was locked into heavier compounds — chiefly water, but also every acid, every hydrocarbon and every living thing. The hydrogen is still here in enormous quantity; it just cannot be found as a gas, which is precisely why the whole of this chapter is about extracting it from something else.
Hydrogen is the lightest substance there is. In a gas, lighter molecules move faster at a given temperature, and a molecule moving fast enough simply leaves — it climbs out of the Earth's gravitational reach and does not come back. Over a long enough stretch of time, essentially all the free hydrogen the Earth ever had has drifted away.
A star is a different case entirely. It is vastly more massive, so its gravity holds hydrogen easily, and there was nothing to pull the hydrogen away from the material the star formed from in the first place.
So the Earth kept its hydrogen only where it was locked into heavier compounds — chiefly water, but also every acid, every hydrocarbon and every living thing. The hydrogen is still here in enormous quantity; it just cannot be found as a gas, which is precisely why the whole of this chapter is about extracting it from something else.
Key takeaways
Occurrence and preparation of hydrogen: quick revision
- Free hydrogen is rare on Earth because it is the lightest gas and escapes, but it is the most abundant element in the universe and the main constituent of the Sun.
- Combined: water (which is hydrogen by mass), all acids and alkalis, hydrocarbons, and all food and living matter.
- Anomalous position — like alkali metals: one valence electron, forms , similar oxide and chloride formulae, acts as a reducing agent. Like halogens: one electron short, diatomic , a non-metal gas, forms hydrides such as where it is negative. Placed in group 1 as a compromise.
- Laboratory preparation: , using granulated (impure) zinc and dilute sulphuric acid, collected by downward displacement of water.
- No nitric acid — it is an oxidising agent and oxidises hydrogen to water. No pure zinc — too slow; impurities accelerate it. No sodium — explosive. No copper — below hydrogen in the series.
- Cold water with K, Na, Ca gives a hydroxide hydrogen: .
- Steam with Mg, Al, Zn, Fe gives an oxide hydrogen: (reversible).
- Dilute acids with every metal above hydrogen give a salt hydrogen: . Copper, silver and gold give no reaction.
- Alkalis with the amphoteric metals Zn, Al, Pb: and .
- Bosch process: over red-hot coke, then over ferric oxide with chromium oxide, then the is dissolved out in water under pressure.
- Electrolysis: , hydrogen at the cathode, in a **** volume ratio. Very pure but expensive; Bosch is cheaper but needs purification.
Practise predicting the reagent and the product for each metal in the series — getting hydroxide-versus-oxide right is where most of the marks in this chapter sit.
- Combined: water (which is hydrogen by mass), all acids and alkalis, hydrocarbons, and all food and living matter.
- Anomalous position — like alkali metals: one valence electron, forms , similar oxide and chloride formulae, acts as a reducing agent. Like halogens: one electron short, diatomic , a non-metal gas, forms hydrides such as where it is negative. Placed in group 1 as a compromise.
- Laboratory preparation: , using granulated (impure) zinc and dilute sulphuric acid, collected by downward displacement of water.
- No nitric acid — it is an oxidising agent and oxidises hydrogen to water. No pure zinc — too slow; impurities accelerate it. No sodium — explosive. No copper — below hydrogen in the series.
- Cold water with K, Na, Ca gives a hydroxide hydrogen: .
- Steam with Mg, Al, Zn, Fe gives an oxide hydrogen: (reversible).
- Dilute acids with every metal above hydrogen give a salt hydrogen: . Copper, silver and gold give no reaction.
- Alkalis with the amphoteric metals Zn, Al, Pb: and .
- Bosch process: over red-hot coke, then over ferric oxide with chromium oxide, then the is dissolved out in water under pressure.
- Electrolysis: , hydrogen at the cathode, in a **** volume ratio. Very pure but expensive; Bosch is cheaper but needs purification.
Practise predicting the reagent and the product for each metal in the series — getting hydroxide-versus-oxide right is where most of the marks in this chapter sit.