One Element Behaves Like a Metal and Like a Halogen at Once
See why hydrogen fits two groups at the same time, write the equations for cold water, hot water and steam on metals, and use the activity series to decide which acid will release hydrogen.
Why can nobody agree where hydrogen belongs in the periodic table?
Every other element in the periodic table has one home. Sodium is a group 1 metal and nothing else; chlorine is a group 17 non-metal and nothing else.
Hydrogen will not settle.
It has one valence electron, exactly like sodium, and it forms a positive ion, exactly like sodium. On that evidence it is a group 1 element. But it is a gas, it travels as a diatomic molecule and it forms covalent compounds — exactly like chlorine. On that evidence it is a group 17 element.
Both cases are genuinely strong, and neither is complete. That is what "anomalous position" means, and it was named as the one limitation the Modern Periodic Law of the previous chapter did not remove.
But the interesting chemistry is not the filing problem. It is that hydrogen can be displaced from water and from acids by metals — and which metals succeed, which fail and which need steam rather than cold water turns out to follow one ordered list.
This page covers the first part of the ICSE Class 9 Chemistry chapter on hydrogen: its anomalous position, the action of cold water, hot water and steam on metals with balanced equations, the activity series applied to dilute acids, and the cases where the expected reaction does not give hydrogen at all.
Hydrogen will not settle.
It has one valence electron, exactly like sodium, and it forms a positive ion, exactly like sodium. On that evidence it is a group 1 element. But it is a gas, it travels as a diatomic molecule and it forms covalent compounds — exactly like chlorine. On that evidence it is a group 17 element.
Both cases are genuinely strong, and neither is complete. That is what "anomalous position" means, and it was named as the one limitation the Modern Periodic Law of the previous chapter did not remove.
But the interesting chemistry is not the filing problem. It is that hydrogen can be displaced from water and from acids by metals — and which metals succeed, which fail and which need steam rather than cold water turns out to follow one ordered list.
This page covers the first part of the ICSE Class 9 Chemistry chapter on hydrogen: its anomalous position, the action of cold water, hot water and steam on metals with balanced equations, the activity series applied to dilute acids, and the cases where the expected reaction does not give hydrogen at all.
How does hydrogen resemble both the alkali metals and the halogens?
Hydrogen shares the electron count and the positive ion of group 1, and the physical state, atomicity and bonding of group 17 — which is why its position is called anomalous.
Resemblances to the alkali metals of group 1.
- One valence electron. Hydrogen is ; lithium is and sodium is . All three have a single electron in the outermost shell
- It forms a positive ion. Hydrogen loses its electron to give , as sodium gives
- Its compounds have matching formulae. Its oxide is where sodium's is ; its chloride is where sodium's is ; its sulphide is where sodium's is
- It is a reducing agent, as the alkali metals are
- Its valency is 1, as theirs is
Resemblances to the halogens of group 17.
- It is a non-metal and a gas. Fluorine and chlorine are gases too; every alkali metal is a solid
- It is diatomic. Hydrogen travels as , as chlorine travels as . Alkali metals are not molecular at all
- It forms covalent compounds such as and , as the halogens form . Alkali metal compounds are ionic
- It can gain an electron. Hydrogen accepts one electron to give the hydride ion , and sodium hydride is an ionic solid closely resembling sodium chloride
**Look at what actually says. In that compound sodium has given an electron to** hydrogen, so hydrogen is the negative ion — behaving exactly as the chlorine in does. In one compound hydrogen acts as a metal-like positive ion and in another as a halogen-like negative ion, and no other element in the table does both. The evidence is not merely balanced; it is genuinely contradictory.
Where hydrogen differs from both families.
- Its outermost shell needs only two electrons to be complete — the duplet — while both the alkali metals and the halogens work towards an octet
- Its commonest isotope has no neutron at all, so its nucleus is a single proton
- It has no shell beneath the outermost one, so it has no inner electrons shielding its nucleus
Conventionally it is placed at the top of group 1, above lithium, because the single valence electron is the most fundamental of the resemblances. But the placement is a convention rather than a conclusion — and a question asking why hydrogen's position is anomalous wants both sets of resemblances, not a verdict.
Resemblances to the alkali metals of group 1.
- One valence electron. Hydrogen is ; lithium is and sodium is . All three have a single electron in the outermost shell
- It forms a positive ion. Hydrogen loses its electron to give , as sodium gives
- Its compounds have matching formulae. Its oxide is where sodium's is ; its chloride is where sodium's is ; its sulphide is where sodium's is
- It is a reducing agent, as the alkali metals are
- Its valency is 1, as theirs is
Resemblances to the halogens of group 17.
- It is a non-metal and a gas. Fluorine and chlorine are gases too; every alkali metal is a solid
- It is diatomic. Hydrogen travels as , as chlorine travels as . Alkali metals are not molecular at all
- It forms covalent compounds such as and , as the halogens form . Alkali metal compounds are ionic
- It can gain an electron. Hydrogen accepts one electron to give the hydride ion , and sodium hydride is an ionic solid closely resembling sodium chloride
**Look at what actually says. In that compound sodium has given an electron to** hydrogen, so hydrogen is the negative ion — behaving exactly as the chlorine in does. In one compound hydrogen acts as a metal-like positive ion and in another as a halogen-like negative ion, and no other element in the table does both. The evidence is not merely balanced; it is genuinely contradictory.
Where hydrogen differs from both families.
- Its outermost shell needs only two electrons to be complete — the duplet — while both the alkali metals and the halogens work towards an octet
- Its commonest isotope has no neutron at all, so its nucleus is a single proton
- It has no shell beneath the outermost one, so it has no inner electrons shielding its nucleus
Conventionally it is placed at the top of group 1, above lithium, because the single valence electron is the most fundamental of the resemblances. But the placement is a convention rather than a conclusion — and a question asking why hydrogen's position is anomalous wants both sets of resemblances, not a verdict.
Which metals react with cold water, which need steam, and why?
The more reactive the metal, the milder the form of water that will release hydrogen from it. The most reactive metals manage with cold water; less reactive ones need hot water or steam; the least reactive do not react with water at all.
Action of cold water.
Sodium reacts vigorously. The heat of the reaction melts the metal into a ball that darts about the surface, and the hydrogen released may catch fire:
Potassium reacts even more vigorously, and the hydrogen catches fire at once:
Calcium reacts steadily rather than violently, with bubbles collecting on the metal:
Action of hot water. Magnesium does not react with cold water, but hot water gives a slow steady stream of hydrogen:
Action of steam. Less reactive metals need water as steam, and at those temperatures the product is the oxide rather than the hydroxide:
Red-hot iron reacts with steam, and this one is reversible — pass the products back over the hot metal and the reaction runs the other way:
White-hot carbon also reduces steam, giving a mixture of carbon monoxide and hydrogen known as water gas:
Notice the change of product, because it is examinable. Cold and hot water give the hydroxide — , , . Steam gives the oxide — , , . The reason is that a hydroxide is not stable at the temperature of steam and breaks down to the oxide and water. **Writing as the product of zinc and steam is a standard error, and checking whether you wrote hydroxide or oxide against the temperature is the quickest way to catch it.
The iron reaction being reversible has a practical consequence. Because it can run backwards, the hydrogen must be swept away as it forms for the forward reaction to continue. An equation with a double arrow is telling you that the conditions matter as much as the reactants, which is why this one is always written with steam passing continuously over the metal.
Copper, silver and gold do not react with water in any form** — not cold, not hot, not as steam. Copper vessels can be boiled dry without attacking the metal, and that unreactivity is the whole reason copper and brass have been used for cooking and water storage in Indian kitchens for so long.
Action of cold water.
Sodium reacts vigorously. The heat of the reaction melts the metal into a ball that darts about the surface, and the hydrogen released may catch fire:
Potassium reacts even more vigorously, and the hydrogen catches fire at once:
Calcium reacts steadily rather than violently, with bubbles collecting on the metal:
Action of hot water. Magnesium does not react with cold water, but hot water gives a slow steady stream of hydrogen:
Action of steam. Less reactive metals need water as steam, and at those temperatures the product is the oxide rather than the hydroxide:
Red-hot iron reacts with steam, and this one is reversible — pass the products back over the hot metal and the reaction runs the other way:
White-hot carbon also reduces steam, giving a mixture of carbon monoxide and hydrogen known as water gas:
Notice the change of product, because it is examinable. Cold and hot water give the hydroxide — , , . Steam gives the oxide — , , . The reason is that a hydroxide is not stable at the temperature of steam and breaks down to the oxide and water. **Writing as the product of zinc and steam is a standard error, and checking whether you wrote hydroxide or oxide against the temperature is the quickest way to catch it.
The iron reaction being reversible has a practical consequence. Because it can run backwards, the hydrogen must be swept away as it forms for the forward reaction to continue. An equation with a double arrow is telling you that the conditions matter as much as the reactants, which is why this one is always written with steam passing continuously over the metal.
Copper, silver and gold do not react with water in any form** — not cold, not hot, not as steam. Copper vessels can be boiled dry without attacking the metal, and that unreactivity is the whole reason copper and brass have been used for cooking and water storage in Indian kitchens for so long.
How does the activity series tell you which acid will give hydrogen?
A metal displaces hydrogen from a dilute acid only if it lies above hydrogen in the activity series.
The series arranges metals in decreasing order of reactivity, with hydrogen inserted in its place:
- , , , , , , , , ****, , , , ,
Everything above hydrogen is more reactive than hydrogen and can push it out of an acid. Everything below cannot.
Worked predictions with dilute sulphuric acid.
Zinc is above hydrogen, so it reacts:
Magnesium is above hydrogen, so it reacts, and faster than zinc because it is higher up:
Iron is above hydrogen, so it reacts, and more slowly than zinc:
Aluminium is above hydrogen, and needs three acid molecules for two atoms of metal:
The same predictions with dilute hydrochloric acid, which gives the chloride instead of the sulphate:
And the metals that fail. Copper, mercury, silver and gold all lie below hydrogen, so dilute sulphuric acid and dilute hydrochloric acid leave them untouched. Drop a copper wire into dilute hydrochloric acid and nothing whatever happens — no bubbles, no change of colour, no warmth.
Now the boundary cases, which is where the series has to be read carefully.
Potassium, sodium and calcium are above hydrogen and react far too violently with acids to be used for preparing the gas — the reaction is dangerous and cannot be controlled.
Lead is above hydrogen and should work, but in practice it stops almost at once. The lead sulphate formed with sulphuric acid and the lead chloride formed with hydrochloric acid are both insoluble, so they coat the metal in a layer the acid cannot get through. The reaction is not prevented by the activity series but by the solubility of its own product — a reason of a completely different kind, and one that a question asking why lead is unsuitable is looking for.
So the series answers only half the question. It tells you whether a reaction is possible; it says nothing about whether the reaction is usable. A metal is suitable for preparing hydrogen only if it is above hydrogen, reacts at a controllable rate, and gives a soluble salt. That triple test is what the next chapter's choice of granulated zinc actually rests on.
The series arranges metals in decreasing order of reactivity, with hydrogen inserted in its place:
- , , , , , , , , ****, , , , ,
Everything above hydrogen is more reactive than hydrogen and can push it out of an acid. Everything below cannot.
Worked predictions with dilute sulphuric acid.
Zinc is above hydrogen, so it reacts:
Magnesium is above hydrogen, so it reacts, and faster than zinc because it is higher up:
Iron is above hydrogen, so it reacts, and more slowly than zinc:
Aluminium is above hydrogen, and needs three acid molecules for two atoms of metal:
The same predictions with dilute hydrochloric acid, which gives the chloride instead of the sulphate:
And the metals that fail. Copper, mercury, silver and gold all lie below hydrogen, so dilute sulphuric acid and dilute hydrochloric acid leave them untouched. Drop a copper wire into dilute hydrochloric acid and nothing whatever happens — no bubbles, no change of colour, no warmth.
Now the boundary cases, which is where the series has to be read carefully.
Potassium, sodium and calcium are above hydrogen and react far too violently with acids to be used for preparing the gas — the reaction is dangerous and cannot be controlled.
Lead is above hydrogen and should work, but in practice it stops almost at once. The lead sulphate formed with sulphuric acid and the lead chloride formed with hydrochloric acid are both insoluble, so they coat the metal in a layer the acid cannot get through. The reaction is not prevented by the activity series but by the solubility of its own product — a reason of a completely different kind, and one that a question asking why lead is unsuitable is looking for.
So the series answers only half the question. It tells you whether a reaction is possible; it says nothing about whether the reaction is usable. A metal is suitable for preparing hydrogen only if it is above hydrogen, reacts at a controllable rate, and gives a soluble salt. That triple test is what the next chapter's choice of granulated zinc actually rests on.
Why does nitric acid refuse to give hydrogen, and what happens with alkali?
Dilute nitric acid is not used to prepare hydrogen because it is a powerful oxidising agent: it oxidises the hydrogen to water as fast as it is formed, so oxides of nitrogen come off instead of the gas you wanted.
The displacement does happen. Zinc above hydrogen does push hydrogen out of nitric acid — but the hydrogen never escapes as hydrogen. The acid immediately supplies oxygen to it, giving water, and the nitrogen is left as nitric oxide with dilute acid or nitrogen dioxide with concentrated acid.
So the gas jar fills with an oxide of nitrogen, and the hydrogen you were trying to collect has become water in the flask.
One exception is worth knowing. Very dilute nitric acid does give hydrogen with magnesium and with manganese. Those two are reactive enough, and the acid weak enough as an oxidiser, for the hydrogen to survive.
Concentrated sulphuric acid fails for the same reason. It too is an oxidising agent, and it gives sulphur dioxide rather than hydrogen. So sulphuric acid is suitable only when dilute, and specifying the concentration in your answer is part of getting it right.
Copper fails for a different reason entirely, and the distinction matters. Copper lies below hydrogen in the activity series, so it cannot displace hydrogen from any acid — the problem is the metal, not the acid. Copper does react with nitric acid, because nitric acid attacks it as an oxidising agent rather than by displacement, but the products are a copper salt, water and an oxide of nitrogen. No hydrogen at any stage.
Keep those two failures apart. Nitric acid fails because of what the acid does to hydrogen; copper fails because of where the metal sits in the series. A question asking why copper and nitric acid are both unsuitable is testing whether you know they fail for unrelated reasons — and answering both with "it is below hydrogen" loses half the marks.
Now the alkalis. A few metals release hydrogen not from acids but from hot concentrated solutions of caustic alkali. These are the amphoteric metals — aluminium, zinc and lead — whose oxides react with both acids and alkalis.
With sodium hydroxide:
The products are sodium aluminate, sodium zincate and sodium plumbite.
With potassium hydroxide the reactions are the same with potassium in place of sodium, giving potassium aluminate, potassium zincate and potassium plumbite:
Only the amphoteric metals do this. Sodium, magnesium, iron and copper do not give hydrogen with alkali. So "which metals react with caustic alkali to give hydrogen" has exactly three answers at this level — aluminium, zinc and lead — and the reason is amphoteric character, not position in the activity series.
There is a small domestic consequence. An aluminium vessel should not be used to store a strongly alkaline cleaning solution, because the alkali attacks the metal and releases hydrogen. The reaction on the page is the reason for the warning on the vessel.
The displacement does happen. Zinc above hydrogen does push hydrogen out of nitric acid — but the hydrogen never escapes as hydrogen. The acid immediately supplies oxygen to it, giving water, and the nitrogen is left as nitric oxide with dilute acid or nitrogen dioxide with concentrated acid.
So the gas jar fills with an oxide of nitrogen, and the hydrogen you were trying to collect has become water in the flask.
One exception is worth knowing. Very dilute nitric acid does give hydrogen with magnesium and with manganese. Those two are reactive enough, and the acid weak enough as an oxidiser, for the hydrogen to survive.
Concentrated sulphuric acid fails for the same reason. It too is an oxidising agent, and it gives sulphur dioxide rather than hydrogen. So sulphuric acid is suitable only when dilute, and specifying the concentration in your answer is part of getting it right.
Copper fails for a different reason entirely, and the distinction matters. Copper lies below hydrogen in the activity series, so it cannot displace hydrogen from any acid — the problem is the metal, not the acid. Copper does react with nitric acid, because nitric acid attacks it as an oxidising agent rather than by displacement, but the products are a copper salt, water and an oxide of nitrogen. No hydrogen at any stage.
Keep those two failures apart. Nitric acid fails because of what the acid does to hydrogen; copper fails because of where the metal sits in the series. A question asking why copper and nitric acid are both unsuitable is testing whether you know they fail for unrelated reasons — and answering both with "it is below hydrogen" loses half the marks.
Now the alkalis. A few metals release hydrogen not from acids but from hot concentrated solutions of caustic alkali. These are the amphoteric metals — aluminium, zinc and lead — whose oxides react with both acids and alkalis.
With sodium hydroxide:
The products are sodium aluminate, sodium zincate and sodium plumbite.
With potassium hydroxide the reactions are the same with potassium in place of sodium, giving potassium aluminate, potassium zincate and potassium plumbite:
Only the amphoteric metals do this. Sodium, magnesium, iron and copper do not give hydrogen with alkali. So "which metals react with caustic alkali to give hydrogen" has exactly three answers at this level — aluminium, zinc and lead — and the reason is amphoteric character, not position in the activity series.
There is a small domestic consequence. An aluminium vessel should not be used to store a strongly alkaline cleaning solution, because the alkali attacks the metal and releases hydrogen. The reaction on the page is the reason for the warning on the vessel.
Exam tip
Exam tip: match the water to the metal and the product to the temperature
For the anomalous position, give BOTH sets of resemblances. Group 1: one valence electron, forms , valency 1, matching formulae and , reducing agent. Group 17: gas, diatomic, covalent compounds, forms the hydride ion as in .
Cold water works with sodium, potassium and calcium; hot water with magnesium; steam with aluminium, zinc, red-hot iron and white-hot carbon.
Cold and hot water give the HYDROXIDE; steam gives the OXIDE. Check this before you write the product — a hydroxide is not stable at steam temperature.
The iron and steam reaction is reversible — write it with a double arrow.
Carbon and steam give water gas, .
Learn the activity series in order with hydrogen in its place, and state the rule: a metal displaces hydrogen from a dilute acid only if it is above hydrogen.
Copper, mercury, silver and gold are below hydrogen and give no reaction with dilute acids.
Lead is above hydrogen but unusable, because the insoluble or coats the metal. Say insoluble, not unreactive.
Dilute nitric acid is an OXIDISING AGENT — it oxidises the hydrogen to water and gives oxides of nitrogen. Concentrated sulphuric acid does the same and gives .
Copper fails because of the metal; nitric acid fails because of the acid. Two different reasons — keep them apart.
And for alkali, remember exactly three metals — aluminium, zinc and lead, the amphoteric ones — and name the product as an aluminate, zincate or plumbite.
Cold water works with sodium, potassium and calcium; hot water with magnesium; steam with aluminium, zinc, red-hot iron and white-hot carbon.
Cold and hot water give the HYDROXIDE; steam gives the OXIDE. Check this before you write the product — a hydroxide is not stable at steam temperature.
The iron and steam reaction is reversible — write it with a double arrow.
Carbon and steam give water gas, .
Learn the activity series in order with hydrogen in its place, and state the rule: a metal displaces hydrogen from a dilute acid only if it is above hydrogen.
Copper, mercury, silver and gold are below hydrogen and give no reaction with dilute acids.
Lead is above hydrogen but unusable, because the insoluble or coats the metal. Say insoluble, not unreactive.
Dilute nitric acid is an OXIDISING AGENT — it oxidises the hydrogen to water and gives oxides of nitrogen. Concentrated sulphuric acid does the same and gives .
Copper fails because of the metal; nitric acid fails because of the acid. Two different reasons — keep them apart.
And for alkali, remember exactly three metals — aluminium, zinc and lead, the amphoteric ones — and name the product as an aluminate, zincate or plumbite.
Did you know
Why the same metal needs cold water, hot water or steam
There is something slightly strange about the list of reactions in this chapter. Water is water. Why should a metal that ignores cold water react happily with hot water, and a metal that ignores hot water react with steam?
The reactant has not changed at all. Only its temperature has.
What changes is how much energy each collision carries. A reaction between a metal and water needs bonds broken before new ones can form, and there is an energy price for that. A very reactive metal like sodium pays it easily, so cold water is enough. Magnesium cannot pay it at room temperature and can at the temperature of hot water. Zinc and iron cannot pay it even then, and need the much greater energy that steam brings.
So the three forms of water are not three different reagents. They are one reagent offered at three prices, and the activity series is the list of who can afford which.
There is a second, quieter reason, and it explains the change of product. Sodium reacting with cold water gives sodium hydroxide. Zinc reacting with steam gives zinc oxide, not zinc hydroxide. That is not because zinc prefers a different product — it is because a hydroxide simply cannot survive at the temperature of steam. It breaks down into the oxide and water as soon as it forms.
So the equation for steam already has a second reaction hidden inside it. The hydroxide forms and is destroyed, and what you collect is whatever is left standing at that temperature.
This is a general habit of chemistry worth carrying forward. When the same reactants give different products under different conditions, the usual explanation is not that the reactants changed their minds but that one of the possible products is unstable under the new conditions. You will meet the same reasoning again in the decomposition of bicarbonates on heating, and again in the nitrates of the practical chapter — the same starting material, a different temperature, and a different survivor.
The reactant has not changed at all. Only its temperature has.
What changes is how much energy each collision carries. A reaction between a metal and water needs bonds broken before new ones can form, and there is an energy price for that. A very reactive metal like sodium pays it easily, so cold water is enough. Magnesium cannot pay it at room temperature and can at the temperature of hot water. Zinc and iron cannot pay it even then, and need the much greater energy that steam brings.
So the three forms of water are not three different reagents. They are one reagent offered at three prices, and the activity series is the list of who can afford which.
There is a second, quieter reason, and it explains the change of product. Sodium reacting with cold water gives sodium hydroxide. Zinc reacting with steam gives zinc oxide, not zinc hydroxide. That is not because zinc prefers a different product — it is because a hydroxide simply cannot survive at the temperature of steam. It breaks down into the oxide and water as soon as it forms.
So the equation for steam already has a second reaction hidden inside it. The hydroxide forms and is destroyed, and what you collect is whatever is left standing at that temperature.
This is a general habit of chemistry worth carrying forward. When the same reactants give different products under different conditions, the usual explanation is not that the reactants changed their minds but that one of the possible products is unstable under the new conditions. You will meet the same reasoning again in the decomposition of bicarbonates on heating, and again in the nitrates of the practical chapter — the same starting material, a different temperature, and a different survivor.
Exam relevance
How does the activity series feed into JEE Main and NEET?
Because the ordering of metals by reactivity is rewritten in Class 11 and 12 as a quantitative scale, and then used in nearly every question about a metal.
This is the foundation for Class 11 Chemistry Redox Reactions and Class 12 Electrochemistry and General Principles and Processes of Isolation of Elements, examined in both JEE Main and NEET. The activity series becomes the electrochemical series, with each metal given a standard electrode potential instead of a place in a list. The rule "a metal above hydrogen displaces it from acid" becomes "a metal with a negative standard electrode potential displaces hydrogen" — the same statement with a number attached, and the hydrogen electrode is the zero the whole scale is measured against.
Displacement then becomes a calculation. Class 12 asks whether a given reaction is feasible by computing the cell potential from two electrode potentials, and whether it is spontaneous from the sign of the Gibbs energy. Feasibility questions of this kind are a recurring JEE Main type, and the qualitative prediction made here is what they are checking numerically.
The reactions of metals with water and acid reappear in the s-block chapter. Class 11 The s-Block Elements covers the reaction of the alkali and alkaline earth metals with water, the increasing reactivity down each group, and the basic character of the hydroxides — the same reactions written here, now explained by ionisation enthalpy and hydration energy.
The nitric acid behaviour becomes a major topic. Class 12 The p-Block Elements treats nitric acid as an oxidising agent in detail, including which oxide of nitrogen is produced with which metal at which concentration, and the passivity of some metals towards it. The reason given here — the hydrogen is oxidised to water — is exactly the reasoning that chapter formalises, and questions naming the gas evolved when a specific metal meets nitric acid of a specific concentration are common.
Amphoteric behaviour is examined directly. Class 11 and 12 ask which oxides and hydroxides are amphoteric, and aluminium and zinc are the standard examples. The aluminate and zincate products written here are the products those questions expect.
**Hydrogen's anomalous position carries into Hydrogen as a chapter of its own.** Class 11 covers its position, its isotopes, its hydrides — ionic, covalent and metallic, with as the ionic example used here — heavy water and hydrogen peroxide. **The against contrast on this page is what makes the three classes of hydride make sense.
For NEET, this material is examined as reasoning and recall: predict whether a given metal reacts with a given acid, name the gas evolved, identify the oxidising agent, or match a metal to its behaviour with alkali. Biology needs the underlying idea too** — the electron-transfer sequences of Respiration in Plants and Photosynthesis are ordered by the same kind of scale.
What the questions look like. For board work, expect explain the anomalous position of hydrogen, write equations for the action of cold water, hot water and steam on named metals, state the activity series and predict a reaction, explain why dilute nitric acid is not used, explain why lead and copper are unsuitable, and write the equations for aluminium and zinc with caustic alkali. Equations need the correct product for the stated temperature. For JEE Main and NEET, expect feasibility from electrode potentials, the nitrogen oxide produced under given conditions, and amphoteric identification.
How board and competitive emphasis differ. A board paper rewards the balanced equation with the right product and the named reason. A competitive paper assumes the chemistry and asks whether a cell reaction will go, and in which direction.
The single trap that costs the most marks. Writing hydrogen as a product of a metal and nitric acid. The displacement happens and the hydrogen does not survive — it is oxidised to water, and an oxide of nitrogen comes off instead. The defence is to ask what kind of acid you are dealing with before you write any product: a dilute non-oxidising acid gives hydrogen, and an oxidising acid gives water plus a reduced form of the acid's own non-metal.
This is the foundation for Class 11 Chemistry Redox Reactions and Class 12 Electrochemistry and General Principles and Processes of Isolation of Elements, examined in both JEE Main and NEET. The activity series becomes the electrochemical series, with each metal given a standard electrode potential instead of a place in a list. The rule "a metal above hydrogen displaces it from acid" becomes "a metal with a negative standard electrode potential displaces hydrogen" — the same statement with a number attached, and the hydrogen electrode is the zero the whole scale is measured against.
Displacement then becomes a calculation. Class 12 asks whether a given reaction is feasible by computing the cell potential from two electrode potentials, and whether it is spontaneous from the sign of the Gibbs energy. Feasibility questions of this kind are a recurring JEE Main type, and the qualitative prediction made here is what they are checking numerically.
The reactions of metals with water and acid reappear in the s-block chapter. Class 11 The s-Block Elements covers the reaction of the alkali and alkaline earth metals with water, the increasing reactivity down each group, and the basic character of the hydroxides — the same reactions written here, now explained by ionisation enthalpy and hydration energy.
The nitric acid behaviour becomes a major topic. Class 12 The p-Block Elements treats nitric acid as an oxidising agent in detail, including which oxide of nitrogen is produced with which metal at which concentration, and the passivity of some metals towards it. The reason given here — the hydrogen is oxidised to water — is exactly the reasoning that chapter formalises, and questions naming the gas evolved when a specific metal meets nitric acid of a specific concentration are common.
Amphoteric behaviour is examined directly. Class 11 and 12 ask which oxides and hydroxides are amphoteric, and aluminium and zinc are the standard examples. The aluminate and zincate products written here are the products those questions expect.
**Hydrogen's anomalous position carries into Hydrogen as a chapter of its own.** Class 11 covers its position, its isotopes, its hydrides — ionic, covalent and metallic, with as the ionic example used here — heavy water and hydrogen peroxide. **The against contrast on this page is what makes the three classes of hydride make sense.
For NEET, this material is examined as reasoning and recall: predict whether a given metal reacts with a given acid, name the gas evolved, identify the oxidising agent, or match a metal to its behaviour with alkali. Biology needs the underlying idea too** — the electron-transfer sequences of Respiration in Plants and Photosynthesis are ordered by the same kind of scale.
What the questions look like. For board work, expect explain the anomalous position of hydrogen, write equations for the action of cold water, hot water and steam on named metals, state the activity series and predict a reaction, explain why dilute nitric acid is not used, explain why lead and copper are unsuitable, and write the equations for aluminium and zinc with caustic alkali. Equations need the correct product for the stated temperature. For JEE Main and NEET, expect feasibility from electrode potentials, the nitrogen oxide produced under given conditions, and amphoteric identification.
How board and competitive emphasis differ. A board paper rewards the balanced equation with the right product and the named reason. A competitive paper assumes the chemistry and asks whether a cell reaction will go, and in which direction.
The single trap that costs the most marks. Writing hydrogen as a product of a metal and nitric acid. The displacement happens and the hydrogen does not survive — it is oxidised to water, and an oxide of nitrogen comes off instead. The defence is to ask what kind of acid you are dealing with before you write any product: a dilute non-oxidising acid gives hydrogen, and an oxidising acid gives water plus a reduced form of the acid's own non-metal.
Key takeaways
Hydrogen's position, water on metals and the activity series: quick revision
- Hydrogen's position is anomalous because it resembles two groups at once.
- Like group 1: one valence electron, forms , valency 1, formulae matching sodium's ( and , and ), and it is a reducing agent.
- Like group 17: a non-metal gas, diatomic as , forms covalent compounds, and can gain an electron to give the hydride ion — as in , which resembles .
- It differs from both: its shell needs only two electrons, and its commonest isotope has no neutron.
- Cold water: ; ; .
- Hot water: — magnesium does not react with cold water.
- Steam: ; ; (reversible); (water gas).
- Cold and hot water give the hydroxide; steam gives the oxide, because a hydroxide is unstable at that temperature.
- Copper, silver and gold do not react with water in any form.
- Activity series: , , , , , , , , H, , , , , .
- A metal displaces hydrogen from a dilute acid only if it is above hydrogen: ; ; .
- Potassium, sodium and calcium are too violent to use; lead fails because insoluble or coats the metal.
- A usable metal must be above hydrogen, react controllably, and give a soluble salt.
- Dilute nitric acid gives no hydrogen — it is an oxidising agent and oxidises the hydrogen to water, releasing oxides of nitrogen. Very dilute nitric acid does give hydrogen with magnesium and manganese.
- Concentrated sulphuric acid is also oxidising and gives .
- Copper gives no hydrogen with any acid because it is below hydrogen — a different reason from nitric acid's.
- Amphoteric metals with hot concentrated alkali give hydrogen: ; ; — aluminate, zincate, plumbite, with potassium salts from .
- Only aluminium, zinc and lead react this way; sodium, magnesium, iron and copper do not.
Pick any metal from the series and any of the three forms of water, and write the equation from scratch — if you get the hydroxide-or-oxide choice right every time, this chapter is secure.
- Like group 1: one valence electron, forms , valency 1, formulae matching sodium's ( and , and ), and it is a reducing agent.
- Like group 17: a non-metal gas, diatomic as , forms covalent compounds, and can gain an electron to give the hydride ion — as in , which resembles .
- It differs from both: its shell needs only two electrons, and its commonest isotope has no neutron.
- Cold water: ; ; .
- Hot water: — magnesium does not react with cold water.
- Steam: ; ; (reversible); (water gas).
- Cold and hot water give the hydroxide; steam gives the oxide, because a hydroxide is unstable at that temperature.
- Copper, silver and gold do not react with water in any form.
- Activity series: , , , , , , , , H, , , , , .
- A metal displaces hydrogen from a dilute acid only if it is above hydrogen: ; ; .
- Potassium, sodium and calcium are too violent to use; lead fails because insoluble or coats the metal.
- A usable metal must be above hydrogen, react controllably, and give a soluble salt.
- Dilute nitric acid gives no hydrogen — it is an oxidising agent and oxidises the hydrogen to water, releasing oxides of nitrogen. Very dilute nitric acid does give hydrogen with magnesium and manganese.
- Concentrated sulphuric acid is also oxidising and gives .
- Copper gives no hydrogen with any acid because it is below hydrogen — a different reason from nitric acid's.
- Amphoteric metals with hot concentrated alkali give hydrogen: ; ; — aluminate, zincate, plumbite, with potassium salts from .
- Only aluminium, zinc and lead react this way; sodium, magnesium, iron and copper do not.
Pick any metal from the series and any of the three forms of water, and write the equation from scratch — if you get the hydroxide-or-oxide choice right every time, this chapter is secure.