Hydrogen | ICSE Class 8 Chemistry Notes

This note covers hydrogen, its properties, preparation from acids and water, industrial manufacture, uses, reactions with oxygen, and changes involving the gain or loss of oxygen and hydrogen.

What is hydrogen, and how does it occur in compounds?

Hydrogen is a gaseous non-metal. An element is a substance that cannot be broken down chemically into simpler substances. A non-metal is an element that generally lacks characteristic metallic properties such as lustre and good electrical conductivity; hydrogen and oxygen are examples.

What do H and H₂ mean?

An atom is the smallest particle of an element that takes part in chemical reactions. The chemical symbol H represents hydrogen. Two hydrogen atoms combine to form a hydrogen molecule, written H₂.

A molecule is a particle capable of independent existence that retains the properties of the substance. Hydrogen is diatomic, meaning that each molecule contains two atoms. The small number ₂ is a subscript: it gives the number of atoms of the preceding element.

H and H₂ therefore convey different information. H identifies the element or represents one atom; H₂ represents its two-atom molecule. Writing H₂ does not make hydrogen a compound, because both atoms belong to the same element.

How is hydrogen in water different from hydrogen gas?

A compound forms when different elements combine chemically in a fixed ratio. Water is a compound of hydrogen and oxygen. The symbol O represents oxygen; H₂O, the formula of water, represents two hydrogen atoms combined with one oxygen atom.

Hydrogen occurs combined in water, acids and many carbon compounds. Sugar, for example, contains carbon, hydrogen and oxygen. These substances contain the element hydrogen chemically combined with other elements, rather than a supply of free hydrogen gas.

Note: The properties of a compound differ from those of its constituent elements. Hydrogen burns and oxygen supports burning, but water has different properties from either gas.

The difference matters when preparing hydrogen. Obtaining hydrogen from water requires a chemical change that produces a new substance. Simply changing water into ice or water vapour does not separate its constituent elements.

Which physical properties help us recognise and collect hydrogen?

A physical property can be observed without changing a substance into another substance. Hydrogen is a colourless gas and has no smell. These observations describe its appearance and odour; they do not by themselves prove that an unknown gas is hydrogen.

What do lightness and solubility mean?

Density means mass per unit volume. Hydrogen is lighter than air: equal volumes of hydrogen and air, compared under the same conditions, have different masses, with hydrogen having the smaller mass.

Solubility describes how much of a substance dissolves in a solvent under specified conditions. A solvent is the substance that dissolves another substance. Hydrogen is only slightly soluble in water, so it can be collected over water.

Why must a physical description be separated from a chemical test?

Combustion is burning, a chemical reaction that releases heat. Hydrogen is combustible, meaning that it can burn. Burning changes hydrogen into another substance, so combustibility is a chemical property rather than a physical property.

A useful account of hydrogen therefore separates two kinds of evidence. Colour, smell and solubility describe the gas before reaction. Its behaviour when a small sample is tested with a flame identifies a chemical change. Neither category should be substituted for the other.

Lightness and low solubility also answer different questions. Being lighter than air compares hydrogen with air. Being slightly soluble in water explains why most of the gas remains available for collection over water instead of dissolving.

How is hydrogen prepared from zinc and a dilute acid?

Hydrogen can be prepared by reacting zinc, a metal represented by Zn, with dilute hydrochloric acid or dilute sulphuric acid. Dilute means that the acid solution contains a relatively small proportion of acid in water.

An acid produces hydrogen ions in aqueous solution. An ion is an electrically charged atom or group of atoms; aqueous means dissolved in water. Here the reaction between the acid and zinc releases molecular hydrogen gas.

What do the equations tell us?

A chemical equation represents a chemical reaction. The starting substances are reactants; the new substances are products. The sign + separates substances, while the arrow → means “forms” and points from reactants to products.

Hydrochloric acid has the formula HCl; Cl represents chlorine. Zinc chloride, ZnCl₂, is the salt formed with this acid. A salt is a compound formed when the hydrogen of an acid is replaced by a metal or another positive ion.

Zn + 2HCl → ZnCl₂ + H₂

The number 2 before HCl is a coefficient: it multiplies the whole formula. Sulphuric acid is H₂SO₄, where S represents sulphur. With this acid, the salt is zinc sulphate, ZnSO₄.

Zn + H₂SO₄ → ZnSO₄ + H₂

What is observed in the demonstration?

1. Place zinc granules, small pieces of zinc, in a test tube or conical flask.
2. Add the selected dilute acid carefully, with the demonstration supervised.
3. Observe bubbles forming as hydrogen gas is released from the reaction mixture.
4. Lead the gas through a delivery tube, a tube that carries gas away from the reaction vessel, for collection or testing.

A clamped test tube contains zinc granules and dilute sulphuric acid. A delivery tube leads into soap solution. Gas-filled soap bubbles are shown, with a candle brought near one bubble and a pop sound indicated.

See Fig. 2.1 in your NCERT textbook

The metal replaces hydrogen from the acid. Such replacement is a displacement reaction. Gas production is evidence that a reaction is occurring, but the gas test is needed to identify hydrogen rather than merely report that bubbles appeared.

Why are zinc and particular dilute acids chosen?

Choosing a metal depends on how readily it reacts. Reactivity is the tendency of a substance to undergo chemical change. Metals differ in reactivity, so replacing zinc with another metal can change the rate of gas production or prevent the intended reaction.

How does zinc compare with other metals?

With dilute hydrochloric acid, the observed reactivity decreases from magnesium to aluminium, zinc and iron. Magnesium gives the fastest formation of bubbles among these samples. Copper produces no bubbles and does not react with dilute hydrochloric acid.

Zinc is useful because it reacts with the chosen dilute acid at a manageable rate. It provides a practical choice between metals that react more vigorously and a metal such as copper that does not release hydrogen from dilute hydrochloric acid.

Why is nitric acid excluded?

Nitric acid is a strong oxidising agent, a substance that causes another substance to gain oxygen or lose hydrogen. Such a change is called oxidation. It oxidises the hydrogen produced to water and is itself changed into nitrogen oxides, compounds of nitrogen and oxygen.

This is why nitric acid is not chosen for the usual zinc-and-acid preparation. The familiar summary “metal plus dilute acid forms salt and hydrogen” needs the correct metal and acid; it is not a rule to apply without checking the substances.

Note: Magnesium and manganese react with very dilute nitric acid to evolve hydrogen. This exception does not make nitric acid suitable for the standard preparation using zinc.

Keep very dilute in the exception. Replacing it with “dilute” or stating that every metal behaves this way changes the claim. Similarly, identifying zinc as a convenient choice does not mean that zinc is the only metal capable of producing hydrogen.

How is hydrogen collected and tested?

Hydrogen can be collected by downward displacement of water. This means that gas entering an inverted, water-filled container occupies space previously occupied by water. The water moves out while hydrogen accumulates inside the container.

Why does collection over water work?

The reason is hydrogen’s slight solubility in water. Most of the arriving gas remains gaseous instead of dissolving. Describing the method as displacement of water identifies what is pushed out of the collecting vessel.

Do not use “lighter than air” as the explanation for collection over water. Lightness compares densities with air; collection over water depends on the gas’s behaviour in water. These are separate properties even though both belong in a description of hydrogen.

What is the pop test?

In the pop test, a small collected sample of hydrogen is brought near a flame. It burns with a characteristic pop sound. The sound provides a way to distinguish hydrogen from oxygen, which makes a burning candle flame glow brighter.

The soap-bubble demonstration uses the same idea. Hydrogen passing through soap solution fills bubbles. A burning candle is brought near a gas-filled bubble, and the hydrogen burns with a pop. This connects the gas produced at the zinc to its identifying behaviour.

Note: Flame testing must be performed carefully under teacher supervision and at a safe distance from the preparation apparatus. Test a small separate sample; hydrogen mixed with air can ignite explosively.

The apparatus initially contains air, so the first gas leaving it is not a pure hydrogen sample. Allow the initial gas to escape before collecting a sample for testing. A delivery tube should not be treated as a burner for the reaction flask.

A complete observation records both stages: bubbles are evolved during the reaction, and a separate gas sample burns with a pop. Saying only “bubbles form” establishes gas evolution but does not identify the gas.

How does electrolysis produce hydrogen from water?

Electrolysis is chemical decomposition brought about by passing an electric current through a suitable conducting liquid or solution. Decomposition means breaking a compound into simpler substances. Water can be decomposed into hydrogen and oxygen by this method.

What are the electrode and electrolyte?

An electrode is a conductor, a material that carries electric current, through which current enters or leaves the liquid. An electrolyte is a substance that conducts electricity when dissolved or molten (melted) and undergoes chemical change during electrolysis. The acidified water supplies the conducting solution in this demonstration.

Acidified water means water containing a little acid, here a few drops of dilute sulphuric acid. Ions carry current through an acidic solution. The added acid improves conductivity, the ability to carry electric current.

The cathode is the electrode connected to the negative terminal of the battery in this apparatus; hydrogen collects there. The anode is connected to the positive terminal; oxygen collects there. Oxygen gas has the molecular formula O₂.

What does the demonstration show?

1. Place carbon electrodes in the water and connect them to a battery through a switch.
2. Add a few drops of dilute sulphuric acid to obtain the conducting solution.
3. Invert water-filled test tubes over the electrodes so the gases can be collected separately.
4. Pass current and observe bubbles displacing water in both test tubes.
5. Compare the gas volumes and identify each gas by its separate supervised test.

2H₂O → 2H₂ + O₂, with electric current supplied, represents the overall decomposition. Hydrogen is collected in twice the volume of oxygen when the gases are compared under the same conditions. This is a volume ratio of two to one.

A plastic mug holds water and two inverted test tubes over graphite rods, made from a form of carbon. Rubber stoppers hold the rods at the base. Wires connect the anode and cathode to a battery and switch. Oxygen and hydrogen are labelled in separate tubes.

See Fig. 1.6 in your NCERT textbook

This is a chemical change because the products are different substances from water. Boiling water, by contrast, produces water vapour. The gas formed by electrolysis is identified by its chemical behaviour, not simply by noticing bubbles in a liquid.

How is hydrogen manufactured by Bosch’s process?

Bosch’s process is an industrial method of manufacturing hydrogen using steam and carbon, followed by a reaction that increases the hydrogen yield. Steam is water in the gaseous state. Yield here means the amount of hydrogen obtained.

How is water gas formed?

Steam is passed over strongly heated coke, a carbon-rich solid. The symbol C represents carbon. The reaction produces carbon monoxide, CO, and hydrogen. Carbon monoxide contains one carbon atom and one oxygen atom per molecule.

C + H₂O → CO + H₂, with strong heating.

The mixture of carbon monoxide and hydrogen is called water gas. It is a mixture because the gases are present together without being combined into a single new compound. The name does not mean that water gas is simply water vapour.

How is more hydrogen obtained?

Water gas is mixed with more steam and passed over a heated catalyst. A catalyst changes the rate of a reaction without being used up overall. Carbon monoxide reacts with steam to form carbon dioxide and additional hydrogen.

Carbon dioxide, written CO₂, contains one carbon atom and two oxygen atoms per molecule. The second reaction is called the water-gas shift reaction.

CO + H₂O → CO₂ + H₂, with heating and a catalyst.

The first reaction already supplies hydrogen. The second increases the supply by converting carbon monoxide with additional steam. Carbon dioxide is then removed, and remaining impurities, unwanted substances mixed with the hydrogen, must also be removed.

Follow the order carefully: make water gas, increase its hydrogen content, then purify the gas. Treating the first mixture as pure hydrogen misses the purpose of the later stages. Similarly, saying that the catalyst is a source of hydrogen confuses its role with that of steam.

The industrial process and the laboratory zinc reaction have the same desired product but different starting materials. In the laboratory method, hydrogen is released from an acid; in this industrial sequence, steam supplies hydrogen.

How do hydrogen’s reactions explain its uses?

A substance’s uses are connected to its properties. Hydrogen can burn, remove oxygen from certain compounds and combine with other substances. These different reactions explain why hydrogen is useful both as a fuel and as a chemical starting material.

Why can hydrogen serve as a fuel?

A fuel is a substance used to release useful energy. Hydrogen burns in oxygen to form water. Its combustion releases heat, so it can be used as a fuel. The equation must show oxygen as a reactant as well as hydrogen.

2H₂ + O₂ → 2H₂O, when ignited.

The two gases combine to form a compound with different properties. Hydrogen is consumed during burning; oxygen participates in the reaction. The presence of hydrogen atoms in the resulting water does not mean that water behaves like hydrogen gas.

How is hydrogen used to change vegetable oils?

A chemical bond is the force holding atoms together. Single, double and triple bonds involve one, two and three shared pairs of electrons respectively. Electrons are negatively charged particles in atoms.

Hydrogenation is the addition of hydrogen to an unsaturated compound. An unsaturated carbon compound contains a carbon-carbon double or triple bond; a saturated carbon compound has single bonds between its carbon atoms.

Unsaturated compounds add hydrogen in the presence of catalysts such as nickel or palladium to give saturated compounds. Vegetable oils generally have long unsaturated carbon chains. Their hydrogenation commonly uses nickel as the catalyst.

These uses should be distinguished by their purpose. Fuel use releases energy, hydrogenation changes an unsaturated compound, and obtaining copper involves removing oxygen from its oxide. A single list of uses becomes more meaningful when each use is linked to the relevant reaction.

What are oxidation and reduction, and how does hydrogen take part?

Oxidation is gain of oxygen or loss of hydrogen. Reduction is loss of oxygen or gain of hydrogen. These definitions describe changes in particular substances during a reaction, so identify the substance before deciding which change has occurred.

What happens when copper is heated in air?

The symbol Cu represents copper. On heating, copper gains oxygen and forms a black coating of copper(II) oxide, written CuO. The Roman numeral II indicates a valency of two for copper here; valency means combining capacity.

2Cu + O₂ → 2CuO, with heating.

A china dish containing copper powder rests on wire gauze over a tripod stand. A burner heats it from below. The labels identify the dish with copper powder, wire gauze, tripod stand and burner.

See Fig. 1.10 in your NCERT textbook

What happens when hydrogen passes over heated copper oxide?

Hydrogen removes oxygen from heated copper(II) oxide. The black material turns brown as copper is obtained, while water forms from hydrogen and the removed oxygen.

CuO + H₂ → Cu + H₂O, with heating.

A reaction involving both oxidation and reduction is a redox reaction. Here the same oxygen transfer explains both changes: the oxide loses oxygen while hydrogen gains it. Do not describe the entire reaction as if only one substance changes.

A reducing agent causes another substance to lose oxygen or gain hydrogen. Hydrogen is the reducing agent in this reaction because it removes oxygen from copper oxide. While doing so, hydrogen is itself oxidised.

Copper oxide is the oxidising agent because it supplies oxygen to hydrogen. It is itself reduced. The name of an agent describes what it does to the other substance, not the change that the agent itself undergoes.

Note: “Hydrogen is a reducing agent” and “hydrogen is oxidised” are both correct for the heated copper oxide reaction. The statements describe two different aspects of the same oxygen transfer.

How can hydrogen equations be read and checked accurately?

A balanced equation has the same number of atoms of each element on both sides. Balancing records the rearrangement of atoms during a reaction. It does not change the identity of the starting substances or products.

Why must formulae remain unchanged?

A subscript is part of a substance’s formula, whereas a coefficient multiplies the complete formula. To balance water formation, change the number of molecules represented; do not alter the formula H₂O to make the atom count convenient.

In 2H₂ + O₂ → 2H₂O, the left side represents four hydrogen atoms and two oxygen atoms. The right side also represents four hydrogen atoms and two oxygen atoms. The equation is balanced without changing either gas’s molecular formula.

How can the zinc reaction be checked?

For Zn + H₂SO₄ → ZnSO₄ + H₂, count every element separately. Zinc and sulphur each occur once on both sides. The two hydrogen atoms of the acid appear in hydrogen gas, while four oxygen atoms remain in the sulphate compound.

1. Identify each formula and name the substance it represents before counting atoms.
2. Count the atoms of each element on the reactant side, including any coefficients.
3. Repeat the count on the product side and compare the totals element by element.
4. Check the stated conditions, such as dilute acid, heating or electric current, as well as the balance.

Balancing alone does not prove that a reaction occurs under any conditions. Zinc needs the appropriate acid in the preparation described here; copper oxide needs heating for the hydrogen reaction; water needs an electric current for electrolysis.

An equation and an observation therefore complement each other. The equation records substances and atom balance. Observations such as a pop sound, gas bubbles or a black-to-brown colour change supply evidence about what occurred in the demonstration.

Glossary

• Hydrogen — A gaseous non-metal whose molecules contain two hydrogen atoms and whose combustion with oxygen forms water.
• Diatomic molecule — A molecule containing two atoms, as in the hydrogen molecule represented by the formula H₂.
• Compound — A substance formed when different elements combine chemically in a fixed ratio, producing properties different from its constituents.
• Dilute acid — An acid solution containing a relatively small proportion of acid compared with the water present.
• Displacement reaction — A reaction in which one element replaces another from a compound, as zinc releases hydrogen from a suitable acid.
• Electrolysis — Chemical decomposition caused by passing electric current through a suitable conducting liquid or solution.
• Electrode — A conductor through which electric current enters or leaves the liquid in an electrolytic apparatus.
• Electrolyte — A substance that conducts electricity when dissolved or molten and undergoes chemical change during electrolysis.
• Water gas — A mixture of carbon monoxide and hydrogen, formed when steam reacts with strongly heated carbon.
• Catalyst — A substance that changes the rate of a chemical reaction without being used up overall.
• Hydrogenation — Addition of hydrogen to an unsaturated compound, commonly used with a nickel catalyst for vegetable oils.
• Oxidation — Gain of oxygen or loss of hydrogen by a substance during a chemical reaction.
• Reduction — Loss of oxygen or gain of hydrogen by a substance during a chemical reaction.
• Reducing agent — A substance that causes another substance to lose oxygen or gain hydrogen while itself undergoing oxidation.
• Balanced equation — A chemical equation containing equal numbers of atoms of each element on its two sides.

Common errors and misconceptions

• Misconception: H and H₂ represent the same particle. Correct: H represents a hydrogen atom; H₂ represents a molecule containing two hydrogen atoms.
• Misconception: Hydrogen is collected over water because it is lighter than air. Correct: Collection over water depends on hydrogen being only slightly soluble in water.
• Misconception: Any metal and any dilute acid produce hydrogen. Correct: The metal and acid matter; copper does not react with dilute hydrochloric acid, and nitric acid is unsuitable for zinc preparation.
• Misconception: Nitric acid cannot release hydrogen with any metal under any conditions. Correct: Magnesium and manganese release hydrogen with very dilute nitric acid.
• Misconception: Electrolysis produces water vapour. Correct: It chemically decomposes water into hydrogen and oxygen, which are collected and identified separately.
• Misconception: Water gas is pure hydrogen. Correct: It is a mixture of carbon monoxide and hydrogen; further reaction and purification are needed in manufacture.
• Misconception: A reducing agent is itself reduced. Correct: Hydrogen reduces heated copper oxide while itself being oxidised to water.
• Misconception: Changing a subscript is an acceptable way to balance an equation. Correct: Keep formulae unchanged and adjust coefficients so the atom counts agree.

Exam-style questions with model answers

Q1. A hydrogen molecule contains two hydrogen atoms. Using H as the symbol for hydrogen, write its molecular formula and explain the term diatomic. [2 marks]

1. The molecular formula is H₂, with the subscript showing the two hydrogen atoms in one molecule.
2. Diatomic means consisting of two atoms; therefore a hydrogen molecule is diatomic.

Q2. Hydrogen is only slightly soluble in water. In an inverted water-filled container, incoming hydrogen pushes water out. Name this collection method and explain why it works. [2 marks]

1. The method is collection by downward displacement of water, because incoming hydrogen occupies the space from which water leaves.
2. It works because hydrogen is only slightly soluble in water, so most of the gas remains available for collection.

Q3. In a supervised demonstration, zinc, Zn, reacts with dilute sulphuric acid, H₂SO₄, to form zinc sulphate, ZnSO₄, and hydrogen, H₂. The gas produces bubbles and burns with a pop near a flame. Give the balanced equation, the reaction observation and the identifying test. [3 marks]

1. The balanced equation is Zn + H₂SO₄ → ZnSO₄ + H₂. The numbers of zinc, hydrogen, sulphur and oxygen atoms agree on both sides.
2. Bubbles form as the gas is released from the reacting mixture of zinc and dilute sulphuric acid. This shows gas evolution during the chemical change.
3. A small separate sample burns with a pop near a flame, identifying hydrogen. This flame test should be performed carefully under teacher supervision.

Q4. Nitric acid oxidises hydrogen to water. Copper does not react with dilute hydrochloric acid, while zinc does. Magnesium and manganese release hydrogen with very dilute nitric acid. Explain the choice of zinc and dilute hydrochloric acid, and state the nitric acid exception. [4 marks]

1. Zinc is suitable because it reacts with dilute hydrochloric acid and releases hydrogen, providing the gas required in the laboratory preparation.
2. Copper is unsuitable for this preparation because it does not react with dilute hydrochloric acid and therefore cannot provide hydrogen by that route.
3. Nitric acid is not selected for zinc preparation because it oxidises the hydrogen produced to water, interfering with collection of hydrogen gas.
4. The stated exception is magnesium or manganese with very dilute nitric acid. Retaining “very dilute” is necessary when describing these exceptional reactions.

Q5. In electrolysis, electric current passes through water containing a little dilute sulphuric acid, whose ions carry current. Hydrogen collects at the negative electrode (cathode), oxygen at the positive electrode (anode), in a two-to-one volume ratio under the same conditions. H₂O, H₂ and O₂ represent water, hydrogen and oxygen. Explain the chemical change, acid’s role, gas locations, volume relationship and balanced equation. [5 marks]

1. Electrolysis causes decomposition: the electric current brings about a chemical change in which water breaks down into the different substances hydrogen and oxygen.
2. The little dilute sulphuric acid makes the water conduct current more readily. Its ions carry current through the solution used in the apparatus.
3. Hydrogen collects at the negative electrode, called the cathode in this apparatus, while oxygen collects at the positive electrode, called the anode.
4. The hydrogen volume is twice the oxygen volume when the gases are compared under the same conditions, giving the stated two-to-one ratio.
5. The balanced equation is 2H₂O → 2H₂ + O₂, with electric current supplied. It accounts for four hydrogen atoms and two oxygen atoms on each side.

Q6. Heated copper(II) oxide reacts as CuO + H₂ → Cu + H₂O. CuO is black copper oxide, H₂ is hydrogen, Cu is brown copper, and H₂O is water. Using oxygen gain or loss, state the colour change, identify reduction and oxidation, name the reducing agent and explain why the reaction is redox. [5 marks]

1. The black copper oxide turns brown as copper is formed. This observed colour change accompanies the chemical conversion of the oxide into the metal.
2. Copper oxide loses oxygen and becomes copper. Loss of oxygen is reduction, so copper oxide is the substance reduced in this reaction.
3. Hydrogen gains oxygen and forms water. Gain of oxygen is oxidation, so hydrogen is the substance oxidised while the copper oxide is reduced.
4. Hydrogen is the reducing agent because it removes oxygen from copper oxide. Its name describes the change it causes in the other substance.
5. The reaction is redox because reduction and oxidation occur together: oxygen is transferred from copper oxide to hydrogen during the same chemical reaction.

Q7. In Bosch’s process, steam reacts with hot carbon to produce carbon monoxide and hydrogen. Carbon monoxide then reacts with more steam, with heating and a catalyst, to form carbon dioxide and more hydrogen. Unwanted gases are removed afterwards. Explain these three stages. [3 marks]

1. First, steam reacts with strongly heated carbon to produce water gas, the mixture of carbon monoxide and hydrogen. The first product mixture is therefore not pure hydrogen.
2. Next, carbon monoxide reacts with additional steam under the stated catalytic conditions. Carbon dioxide and more hydrogen form, increasing the amount of hydrogen available.
3. Finally, carbon dioxide and remaining impurities are removed from the gas mixture. Purification is necessary because making additional hydrogen does not itself separate all the gases.

Q8. Hydrogen is used as a fuel and in hydrogenation of vegetable oils. It burns in oxygen to form water, releasing heat; unsaturated compounds add hydrogen with a catalyst to become saturated. Explain the reaction and purpose of each use. [4 marks]

1. For fuel use, hydrogen reacts with oxygen to form water. Hydrogen and oxygen are the starting substances, while water is the product of combustion.
2. The purpose of burning hydrogen as a fuel is to obtain useful energy, released as heat during this chemical reaction.
3. During hydrogenation, hydrogen is added to an unsaturated compound in the presence of a catalyst, converting it into a saturated compound.
4. The purpose is to harden liquid vegetable oils into solid or semi-solid fats, such as vanaspati, by adding hydrogen to their unsaturated carbon chains.

Key takeaways

• Hydrogen is a gaseous non-metal whose diatomic molecules contain two atoms and are represented by H₂.
• Zinc reacts with dilute hydrochloric acid or dilute sulphuric acid to produce a salt and hydrogen gas.
• Hydrogen’s slight solubility in water explains collection over water; its low density describes a different property.
• A small sample of hydrogen burns with a pop, while oxygen makes a burning candle flame glow brighter.
• Electrolysis decomposes water into hydrogen and oxygen, with twice as much hydrogen by volume under the same conditions.
• Bosch’s process forms water gas, increases its hydrogen content through reaction with steam, and then removes impurities.
• Hydrogen acts as a reducing agent when it removes oxygen from heated copper oxide and is itself oxidised.
• Balance chemical equations by adjusting coefficients, keeping formulae unchanged and retaining the conditions needed for each reaction.

Test yourself

Why is H₂ an element rather than a compound?

Both atoms in its molecule belong to hydrogen, the same element; a compound contains different elements chemically combined.

Which salt forms when zinc reacts with dilute sulphuric acid?

Zinc sulphate forms, while hydrogen is released as the gaseous product of the reaction.

Which property explains hydrogen collection over water?

Hydrogen is only slightly soluble in water, so most of the incoming gas remains available for collection.

Where does hydrogen collect during electrolysis of acidified water?

Hydrogen collects at the cathode, the electrode connected to the negative battery terminal in this apparatus.

What is water gas?

Water gas is a mixture of carbon monoxide and hydrogen, not pure hydrogen or water vapour.

Why does nitric acid usually fail in the zinc preparation?

Nitric acid acts as an oxidising agent and converts the hydrogen produced into water.

What happens to hydrogen when it reduces heated copper oxide?

Hydrogen gains oxygen and forms water, so it is itself oxidised while reducing the copper oxide.

Why should subscripts remain unchanged when balancing?

Subscripts belong to chemical formulae; altering them changes the substances represented rather than balancing their amounts.