Hydrogen Burns Itself but Puts Out a Burning Splint
Learn the physical properties of hydrogen, how it burns and combines with metals and non-metals, how it strips oxygen out of heated metal oxides, and the test and uses that follow from all of it.
How can a gas burn and still extinguish a flame put into it?
Because burning and supporting burning are two different jobs, and hydrogen does only the first.
Hold a lighted splint at the mouth of a jar of hydrogen and the gas burns, with a pale blue flame and a sharp pop. Push the same splint down into the jar and it goes out — because there is no oxygen in there to keep it alight.
So hydrogen is combustible but is not a supporter of combustion. That one distinction is both the standard laboratory test for the gas and a favourite exam trap. This page covers the second part of the ICSE Class 8 Chemistry chapter on hydrogen: its properties, its reducing action, and its uses.
Hold a lighted splint at the mouth of a jar of hydrogen and the gas burns, with a pale blue flame and a sharp pop. Push the same splint down into the jar and it goes out — because there is no oxygen in there to keep it alight.
So hydrogen is combustible but is not a supporter of combustion. That one distinction is both the standard laboratory test for the gas and a favourite exam trap. This page covers the second part of the ICSE Class 8 Chemistry chapter on hydrogen: its properties, its reducing action, and its uses.
What are the physical properties of hydrogen?
- Colour, odour and taste — colourless, odourless and tasteless. So none of the senses can detect a hydrogen leak, which is why the gas is hazardous to handle.
- Density — the lightest of all substances. Its vapour density is
which is the smallest possible value. Taking the average molecular mass of air as , hydrogen is
times lighter than air — which is why it was once used to fill balloons and airships.
- Solubility — almost insoluble in water. This is exactly why it can be collected by the downward displacement of water, as the previous part described.
- Effect on litmus — neutral. It changes neither blue nor red litmus, because it is neither acidic nor basic.
- Liquefaction — it can be liquefied only at very low temperature under high pressure, and liquid hydrogen is used as a rocket fuel.
- Diffusion — it diffuses faster than any other gas, being the lightest. A vessel of hydrogen loses its contents through a porous plug more quickly than a vessel of any other gas would.
Why these properties hang together. Almost all of them follow from one fact: the hydrogen molecule is the lightest there is. Low density, fastest diffusion, and escape from the atmosphere are all the same property seen three ways.
The safety consequence of being colourless and odourless. Cooking gas has a smelly compound deliberately added so a leak can be detected. Hydrogen offers no such warning of its own, and it also forms an explosive mixture with air over a wide range of proportions — which is why it is handled in closed systems and why a jar of it is tested at the mouth and never from above.
- Density — the lightest of all substances. Its vapour density is
which is the smallest possible value. Taking the average molecular mass of air as , hydrogen is
times lighter than air — which is why it was once used to fill balloons and airships.
- Solubility — almost insoluble in water. This is exactly why it can be collected by the downward displacement of water, as the previous part described.
- Effect on litmus — neutral. It changes neither blue nor red litmus, because it is neither acidic nor basic.
- Liquefaction — it can be liquefied only at very low temperature under high pressure, and liquid hydrogen is used as a rocket fuel.
- Diffusion — it diffuses faster than any other gas, being the lightest. A vessel of hydrogen loses its contents through a porous plug more quickly than a vessel of any other gas would.
Why these properties hang together. Almost all of them follow from one fact: the hydrogen molecule is the lightest there is. Low density, fastest diffusion, and escape from the atmosphere are all the same property seen three ways.
The safety consequence of being colourless and odourless. Cooking gas has a smelly compound deliberately added so a leak can be detected. Hydrogen offers no such warning of its own, and it also forms an explosive mixture with air over a wide range of proportions — which is why it is handled in closed systems and why a jar of it is tested at the mouth and never from above.
How does hydrogen react with oxygen, metals and non-metals?
With oxygen — combustion. Hydrogen burns in air or oxygen with a pale blue flame, forming water and giving out a great deal of heat:
The only product is water, which is why hydrogen is called a clean fuel. A mixture of hydrogen and oxygen in the ratio explodes violently on ignition and is known as detonating gas.
With non-metals it forms covalent compounds, and here it behaves as a non-metal:
with nitrogen, under high pressure with heat and an iron catalyst, giving ammonia.
with chlorine, in diffused sunlight, giving hydrogen chloride. In direct sunlight the same reaction is explosive.
with boiling sulphur, giving the foul-smelling hydrogen sulphide.
With active metals it forms hydrides, and here it behaves as a halogen would:
Why the hydrides matter so much. In the hydrogen has gained an electron and carries a negative charge, exactly as chlorine does in . In every other compound in this chapter hydrogen is positive. So hydrogen genuinely switches sides depending on its partner — and this is the strongest single piece of evidence for the anomalous position described in the previous part.
And notice which reactions need what. Hydrogen is not a reactive gas at room temperature; almost every reaction here needs heat, light, pressure or a catalyst to proceed. This is a direct application of the conditions for a chemical reaction, and it is why a jar of hydrogen can stand in air indefinitely without anything happening until a flame arrives.
The only product is water, which is why hydrogen is called a clean fuel. A mixture of hydrogen and oxygen in the ratio explodes violently on ignition and is known as detonating gas.
With non-metals it forms covalent compounds, and here it behaves as a non-metal:
with nitrogen, under high pressure with heat and an iron catalyst, giving ammonia.
with chlorine, in diffused sunlight, giving hydrogen chloride. In direct sunlight the same reaction is explosive.
with boiling sulphur, giving the foul-smelling hydrogen sulphide.
With active metals it forms hydrides, and here it behaves as a halogen would:
Why the hydrides matter so much. In the hydrogen has gained an electron and carries a negative charge, exactly as chlorine does in . In every other compound in this chapter hydrogen is positive. So hydrogen genuinely switches sides depending on its partner — and this is the strongest single piece of evidence for the anomalous position described in the previous part.
And notice which reactions need what. Hydrogen is not a reactive gas at room temperature; almost every reaction here needs heat, light, pressure or a catalyst to proceed. This is a direct application of the conditions for a chemical reaction, and it is why a jar of hydrogen can stand in air indefinitely without anything happening until a flame arrives.
How does hydrogen strip the oxygen out of a metal oxide?
By combining with that oxygen itself to form water. Hydrogen is a strong reducing agent — it removes oxygen from a compound.
With copper(II) oxide. Dry hydrogen passed over black copper oxide heated in a tube:
The black solid turns reddish-brown as copper metal is left behind, and droplets of water condense on the cooler parts of the tube. Both changes together are the evidence.
With lead(II) oxide:
The yellow oxide becomes grey lead.
With iron oxide:
A worked calculation. How much copper is obtained from of copper oxide? Take — copper's atomic mass is , and rounding it to here keeps the arithmetic clean — with and . The equation reads
So of copper oxide gives of copper. For :
Checking the mass balance: in, and out. Conservation of mass holds, which confirms the arithmetic.
Which oxides are not reduced. Hydrogen cannot reduce the oxides of metals that are more reactive than itself — , , , and resist it. Those metals hold their oxygen more tightly than hydrogen can pull, so heating them in hydrogen achieves nothing.
This is the reactivity series again, in its other role. Hydrogen reduces the oxides of metals below it — copper, lead, iron — and fails with those above it. Knowing where hydrogen sits in the series therefore predicts every one of these reactions, and predicts the failures just as reliably.
With copper(II) oxide. Dry hydrogen passed over black copper oxide heated in a tube:
The black solid turns reddish-brown as copper metal is left behind, and droplets of water condense on the cooler parts of the tube. Both changes together are the evidence.
With lead(II) oxide:
The yellow oxide becomes grey lead.
With iron oxide:
A worked calculation. How much copper is obtained from of copper oxide? Take — copper's atomic mass is , and rounding it to here keeps the arithmetic clean — with and . The equation reads
So of copper oxide gives of copper. For :
Checking the mass balance: in, and out. Conservation of mass holds, which confirms the arithmetic.
Which oxides are not reduced. Hydrogen cannot reduce the oxides of metals that are more reactive than itself — , , , and resist it. Those metals hold their oxygen more tightly than hydrogen can pull, so heating them in hydrogen achieves nothing.
This is the reactivity series again, in its other role. Hydrogen reduces the oxides of metals below it — copper, lead, iron — and fails with those above it. Knowing where hydrogen sits in the series therefore predicts every one of these reactions, and predicts the failures just as reliably.
How do you test for hydrogen, and what is it used for?
The test. Bring a burning splint to the mouth of the gas jar. Hydrogen burns with a pale blue flame and a characteristic pop sound, and the splint goes out if pushed inside. Water droplets form on the cooler walls.
That combination — burns with a pop, extinguishes a splint pushed in, produces water — identifies hydrogen and distinguishes it from oxygen, which does the opposite in every respect: oxygen does not burn but makes a glowing splint burst into flame.
Its uses, each traceable to a property from this page:
- As a fuel. Liquid hydrogen is a rocket fuel, chosen for its very high heat output per unit mass. Hydrogen fuel cells produce electricity with water as the only product, which is why it is regarded as a clean fuel.
- Hydrogenation of oils. Hydrogen is passed through warm vegetable oil with a nickel catalyst, converting the liquid oil into a semi-solid fat — this is how vanaspati is made.
- Manufacture of ammonia. , the route to nitrogenous fertilisers, which is the largest industrial use of hydrogen.
- Manufacture of hydrogen chloride and of methanol.
- The oxy-hydrogen flame, produced by burning hydrogen in oxygen, which reaches a high enough temperature for welding and cutting metals.
- Extraction of metals, using its reducing action on oxides of tungsten and molybdenum.
- Filling balloons — for which it is lightest and cheapest, but it is inflammable, so helium is preferred despite being heavier and costlier.
The pattern worth noticing. Every use rests on one of three properties: its lightness (balloons), the heat released when it burns (rocket fuel, welding, fuel cells), or its reducing power (hydrogenation, metal extraction). Nothing in the list is arbitrary.
And the one property that limits it. Being inflammable and explosive with air, and giving no smell as a warning, hydrogen is difficult to store and transport safely. That single drawback is why it has not simply replaced other fuels, and why the balloon use was given up in favour of helium.
That combination — burns with a pop, extinguishes a splint pushed in, produces water — identifies hydrogen and distinguishes it from oxygen, which does the opposite in every respect: oxygen does not burn but makes a glowing splint burst into flame.
Its uses, each traceable to a property from this page:
- As a fuel. Liquid hydrogen is a rocket fuel, chosen for its very high heat output per unit mass. Hydrogen fuel cells produce electricity with water as the only product, which is why it is regarded as a clean fuel.
- Hydrogenation of oils. Hydrogen is passed through warm vegetable oil with a nickel catalyst, converting the liquid oil into a semi-solid fat — this is how vanaspati is made.
- Manufacture of ammonia. , the route to nitrogenous fertilisers, which is the largest industrial use of hydrogen.
- Manufacture of hydrogen chloride and of methanol.
- The oxy-hydrogen flame, produced by burning hydrogen in oxygen, which reaches a high enough temperature for welding and cutting metals.
- Extraction of metals, using its reducing action on oxides of tungsten and molybdenum.
- Filling balloons — for which it is lightest and cheapest, but it is inflammable, so helium is preferred despite being heavier and costlier.
The pattern worth noticing. Every use rests on one of three properties: its lightness (balloons), the heat released when it burns (rocket fuel, welding, fuel cells), or its reducing power (hydrogenation, metal extraction). Nothing in the list is arbitrary.
And the one property that limits it. Being inflammable and explosive with air, and giving no smell as a warning, hydrogen is difficult to store and transport safely. That single drawback is why it has not simply replaced other fuels, and why the balloon use was given up in favour of helium.
Exam tip
Exam tip: burns is not the same as supports burning
State the test in both halves: hydrogen burns with a pale blue flame and a pop at the mouth of the jar, and it does not support combustion, so a splint pushed inside is extinguished. Writing only the first half is the commonest lost mark in this chapter.
Give colour changes in every reduction answer: black to reddish-brown, yellow to grey. Add that water droplets appear, since both observations together are the evidence.
Name hydrogen as a reducing agent and define the action as removal of oxygen.
Remember which oxides resist it — , , , , — because those metals are above hydrogen in the reactivity series.
For a mass calculation, write the equation with the masses underneath (), then use proportion, then check the totals balance.
Quote **vapour density ** and *about times lighter than air*, with the working .
Say hydrogen is neutral to litmus — not that it has no effect because it is a gas.
And when listing uses, pair each with the property responsible: lightness, heat of combustion, or reducing power. For balloons, add that helium is preferred because hydrogen is inflammable.
Give colour changes in every reduction answer: black to reddish-brown, yellow to grey. Add that water droplets appear, since both observations together are the evidence.
Name hydrogen as a reducing agent and define the action as removal of oxygen.
Remember which oxides resist it — , , , , — because those metals are above hydrogen in the reactivity series.
For a mass calculation, write the equation with the masses underneath (), then use proportion, then check the totals balance.
Quote **vapour density ** and *about times lighter than air*, with the working .
Say hydrogen is neutral to litmus — not that it has no effect because it is a gas.
And when listing uses, pair each with the property responsible: lightness, heat of combustion, or reducing power. For balloons, add that helium is preferred because hydrogen is inflammable.
Did you know
Why does the same element form both a positive and a negative ion?
In hydrochloric acid, hydrogen exists as — it has given its electron away. In sodium hydride, the same element exists as — it has taken an electron in.
No other element in Class 8 chemistry does this so plainly. Sodium is always positive; chlorine is always negative. Hydrogen chooses depending on the company it keeps.
The reason is that it sits exactly on the fence. With one electron in one shell, hydrogen is both one electron above an empty shell and one electron below a full one. Losing that electron and gaining another are equally plausible moves, and which happens depends entirely on whether the partner holds electrons more or less tightly than hydrogen does.
Meet chlorine, which holds electrons tightly, and hydrogen surrenders its own. Meet sodium, which holds its electron loosely, and hydrogen takes it.
This is what the anomalous position really means. Hydrogen is not awkwardly placed because chemists could not decide. It is awkwardly placed because it genuinely behaves as two different kinds of element, and a table with one box per element cannot say so.
No other element in Class 8 chemistry does this so plainly. Sodium is always positive; chlorine is always negative. Hydrogen chooses depending on the company it keeps.
The reason is that it sits exactly on the fence. With one electron in one shell, hydrogen is both one electron above an empty shell and one electron below a full one. Losing that electron and gaining another are equally plausible moves, and which happens depends entirely on whether the partner holds electrons more or less tightly than hydrogen does.
Meet chlorine, which holds electrons tightly, and hydrogen surrenders its own. Meet sodium, which holds its electron loosely, and hydrogen takes it.
This is what the anomalous position really means. Hydrogen is not awkwardly placed because chemists could not decide. It is awkwardly placed because it genuinely behaves as two different kinds of element, and a table with one box per element cannot say so.
Key takeaways
Properties and uses of hydrogen: quick revision
- Physical: colourless, odourless, tasteless; the lightest substance, vapour density , about times lighter than air; almost insoluble in water; neutral to litmus; liquefied only at very low temperature; diffuses fastest of all gases.
- Combustion: , with a pale blue flame and water as the only product. A mixture with oxygen is detonating gas.
- With non-metals: (pressure, heat, iron catalyst); (diffused sunlight); .
- With active metals it forms hydrides where hydrogen is negative: , — the strongest evidence for its anomalous position.
- Reducing action — it removes oxygen: (black to reddish-brown), (yellow to grey), .
- Worked from : of gives of copper, needing of hydrogen and forming of water — and confirms it.
- It cannot reduce , , , or , because those metals are above hydrogen in the reactivity series.
- Test: burns at the mouth of the jar with a pale blue flame and a pop, and extinguishes a splint pushed inside — combustible but not a supporter of combustion.
- Uses: rocket fuel and fuel cells (water the only product), hydrogenation of oils to vanaspati with a nickel catalyst, manufacture of ammonia for fertilisers, hydrogen chloride and methanol, the oxy-hydrogen flame for welding, metal extraction, and balloons — for which helium is preferred because hydrogen is inflammable.
Work through a few oxide-reduction mass calculations and write the colour change for each — those two together are what this chapter is marked on.
- Combustion: , with a pale blue flame and water as the only product. A mixture with oxygen is detonating gas.
- With non-metals: (pressure, heat, iron catalyst); (diffused sunlight); .
- With active metals it forms hydrides where hydrogen is negative: , — the strongest evidence for its anomalous position.
- Reducing action — it removes oxygen: (black to reddish-brown), (yellow to grey), .
- Worked from : of gives of copper, needing of hydrogen and forming of water — and confirms it.
- It cannot reduce , , , or , because those metals are above hydrogen in the reactivity series.
- Test: burns at the mouth of the jar with a pale blue flame and a pop, and extinguishes a splint pushed inside — combustible but not a supporter of combustion.
- Uses: rocket fuel and fuel cells (water the only product), hydrogenation of oils to vanaspati with a nickel catalyst, manufacture of ammonia for fertilisers, hydrogen chloride and methanol, the oxy-hydrogen flame for welding, metal extraction, and balloons — for which helium is preferred because hydrogen is inflammable.
Work through a few oxide-reduction mass calculations and write the colour change for each — those two together are what this chapter is marked on.