Selina Concise Solutions for ICSE Class 8 Chemistry Chapter 7 Hydrogen

Points to Remember:

1. Hydrogen is the most abundant element found in the universe.
2. Hydrogen is much more common in the form of compounds. The most important compound of hydrogen is water.
3. The chief sources of hydrogen are water, acids and alkalies.
4. Hydrogen is prepared by the action of water, acids or alkalies on active metals.
5. Electrolysis of water results in the formation of hydrogen and oxygen.
6. Hydrogen is lighter than air.
7. Hydrogen bums in air with pop sound.
8. Hydrogen acts as a reducing agent.
9. Hydrogen is used to produce oxyhydrogen flame and in weather forecast balloons.
 
Hydrogen is the simplest and lightest element, consisting of just one proton and one electron. It is the primary fuel for stars like our Sun, where it undergoes nuclear fusion to produce energy.
Teacher's Tip: Remember that hydrogen is so light it can escape Earth's gravity into space, which is why it's rare in our atmosphere.
Exam Tip: Always remember that "burning" hydrogen actually means reacting it with oxygen to form water vapor.

Exercise

Question 1: Fill in the blanks:

(a) Hydrogen is lighter than air.
(b) Hydrogen is sparingly soluble in water.
(c) Hydrogen bums with a oxyhydrogen pale blue flame and pop sound is heard.
(d) A metal sodium hydrogen in the reactivity series gives hydrogen with water.
(e) Hydrogen reacts with metal oxides to form metal and water.
(f) Oxidation is the removal of hydrogen and addition of oxygen.
(g) In redox reaction oxidation and reduction occur simultaneously.
 
Filling in these blanks helps you understand the physical and chemical behavior of hydrogen compared to other substances. These terms describe how hydrogen interacts with its environment, such as its density and its role in chemical reactions.
Teacher's Tip: Think of "sparingly soluble" as meaning only a very tiny amount of the gas can dissolve in the water.
Exam Tip: Use the keyword "simultaneously" when describing redox reactions to score full marks for completeness.

Question 2: Indicate which of the following statements are true and which are false:

(a) Hydrogen molecule is monovalent.
Answer: False 

(b) The removal of hydrogen from a substance is called reduction.
Answer: True 

(c) Nitric acid can not be used to prepare hydrogen by its action on active metals ?
Answer: False

(d) The reaction between hydrogen and nitrogen to form ammonia is reversible.
Answer: True 

(e) Zinc can liberate hydrogen from water, acid and alkali solution.
Answer: True 

(f) Hydrogen is combustible as well as a supporter of combustion.
Answer: False

(g) Hydrogen gas is easily liquefiable.
Answer: False 

Question 3: Complete and balance the following equations:
(a) H₂ + ... → 2HCl
(b) H₂ + S → ...
(c) Zn + ... → ZnCl₂ + H₂
(d) CuO + ... → Cu + ...
(e) Fe + H₂O → ... + ...
(f) K + H₂O → ... + ...
Answer: 
(a) H₂ + Cl₂ → 2HCl
(b) H₂ + S → H₂ S
(c) Zn + 2HCl → ZnCl₂  + H₂ 
(d) CuO + H₂ → Cu + H₂O
(e) 3Fe + 4H₂O → Fe₃O₄ + 4H₂
(f) 2K + 2H₂O → 2KOH + H₂
 
Balancing chemical equations ensures that the Law of Conservation of Mass is followed, meaning no atoms are lost during the reaction. These equations show how hydrogen reacts with non-metals like chlorine and sulfur, as well as its production from metals.
Teacher's Tip: Always check if the number of atoms on the left side (reactants) matches the right side (products).
Exam Tip: When balancing, only change the coefficients (the big numbers in front), never the subscripts (the small numbers).

Question 4: Give reasons for the following:
(a) Hydrogen be used as a fuel?
(b) Though hydrogen is lighter than air it cannot be collected by downward displacement of air.
(c) A pop sound produced when hydrogen is burnt?
(d) Helium replaced hydrogen in weather observation balloons?
(e) Nitric acid not used for the preparation of hydrogen gas?
Answer: 
(a) Because of its high heat of combustion, it is used as a fuel.
Coal gas, water gas and liquid hydrogen are some significant fuel.
(b) Since hydrogen is lighter than air. it is possible to collect the gas by downward displacement of air. But it is not safe to do so since a mixture of hydrogen and air can lead to an explosion.
(c) Impure hydrogen gas bums in air with a pop sound. This is because of the presence of impurities in it.
(d) If there is small leakage of hydrogen in a balloon, it forms a mixture with air that can explode. So helium has replaced hydrogen.
(e) Hydrogen cannot be prepared by the action of nitric acid on metals because it also releases nitrous oxide and nitric oxide and oxides the hydrogen to form water.
These reasoning questions explain the practical safety concerns and chemical limitations involved in handling hydrogen gas. For instance, safety is the primary reason why we use helium in balloons even though hydrogen provides more lift.
Teacher's Tip: Think of the "pop" sound as a tiny explosion caused by the rapid reaction of hydrogen and oxygen.
Exam Tip: For question (e), remember that Nitric Acid is a strong "oxidizing agent" – this is the key scientific phrase teachers look for.

Question 5: Name the following:
(a) Two metals which give hydrogen with cold water.
(b) A metal which liberates hydrogen only when steam is passed over red hot metal.
(c) The process in which oxygen is added or hydrogen is removed.
(d) A metallic oxide which can be reduced into metal by hydrogen.
Answer: 
(a) Sodium (Na) and Potassium (K) give hydrogen with cold water.
(b) Iron
(c) Oxidation
(d) Copper oxide (CuO) 
Naming specific substances and processes helps you categorize chemical reactions based on their reactivity and results. Different metals react with water at different temperatures based on their position in the reactivity series.
Teacher's Tip: Highly reactive metals like Sodium and Potassium must be stored in kerosene because they react so easily.
Exam Tip: Be sure to write both the name and the chemical symbol (like Na for Sodium) to show extra knowledge.

Question 6: (a) Name the chemicals required to prepare hydrogen gas in the laboratory.
(b) Give a balanced chemical equation for the reaction.
(c) Draw a neat and well-labelled diagram for the laboratory preparation of hydrogen.
(d) How is hydrogen gas collected?
Answer: 
(a) Granulated Zinc and dil. Hydrochloric acid.
(b) Zn + 2HCl → ZnCl₂ + H₂ (g)
(c) [Diagram shows: Thistle funnel, delivery tube, gas jar, glass trough, water. Labels include: Dilute Hydrochloric Acid, Zinc Pieces, Hydrogen, Water]
(d) Hydrogen gas is collected by the down-ward displacement of water. 
Laboratory preparation uses specific apparatus like a Thistle funnel and a gas jar to safely isolate hydrogen. Collecting it over water is effective because the gas is insoluble and you can see the bubbles clearly.
Teacher's Tip: We use "granulated" zinc because it has more surface area, making the reaction happen faster than with a solid block.
Exam Tip: In your diagram, ensure the bottom of the Thistle funnel is dipped into the acid to prevent the gas from escaping through it.

Question 7: How would you show that hydrogen:
(a) is a non-supporter of combustion?
(b) is lighter than air?
Answer: 
(a) Hold a hydrogen gas filled jar with its mouth downwards. Place a lighted candle inside the jar. The candle gets extinguished but the gas bums with a pop sound. This shows that hydrogen is non-supporter of combustion.
(b) Take a delivery tube and place one of its ends in a soap solution kept in a trough and the other one in a flat bottom jar as shown in the figure. The soap bubbles containing hydrogen rise upward the air. The rising soap bubbles prove that hydrogen is lighter than air. 
Experimental proofs demonstrate the physical properties of hydrogen through visible changes, like a candle going out or bubbles floating up. These experiments are fundamental to understanding how gases behave differently from liquids and solids.
Teacher's Tip: Hydrogen is the lightest gas known, which is why the soap bubbles float up much faster than air bubbles.
Exam Tip: When describing the candle experiment, clearly state that the candle goes out while the gas at the mouth of the jar burns.

Question 8: Hydrogen is a good reducing agent: What do you understand by the above statement? Explain with the help of copper oxide as an example.
Answer: 
Hydrogen acts as a good reducing agent means, when hydrogen gas is passed over hot metallic oxides of copper, lead, iron, etc. it removes oxygen from them and thus reduces them to their corresponding metal.
Let us consider the following example, in each of which metallic oxide react with hydrogen. Metallic oxide act as oxidising agents and hydrogen acts as a reducing agents.
Removal of O₂ [reduction]:
CuO + H₂ → Cu + H₂O
Addition of O₂ [oxidation]: 
A reducing agent is a substance that helps remove oxygen from another compound during a chemical reaction. In this example, hydrogen "steals" the oxygen from the copper, leaving behind pure metallic copper.
Teacher's Tip: You can see this happen visually as black copper oxide turns into reddish-brown copper metal.
Exam Tip: Always draw brackets over the equation to show exactly which substance is being reduced and which is being oxidized.

Question 9: (a) Name a process by which hydrogen gas is manufactured.
(b) Give equations for the reactions.
(c) How is hydrogen separated from carbon dioxide and carbon monoxide?
Answer: 
(a) Commercially, hydrogen is prepared by Bosch process.
(b) (i) Steam is passed over hot coke at 1000°C in a furnace called converters. As a result water gas is produced which is a mixture of carbon monoxide and hydrogen gases.
C + H₂O → 1000°C (CO + H₂) - Heat
This reaction is endothermic in nature.
(ii) Water gas is mixed with excess of steam and passed over a catalyst ferric oxide (Fe₂O₃) and a promotor chromium trioxide (Cr₂O₃).
(CO + H₂) + H₂O →[450°C]Fe₂O₃/Cr₂O₃ CO₂ + 2H₂+ Heat
This reaction is exothermic in nature.
(c) (i) The products are hydrogen, carbon dioxide and some unreacted carbon monoxide. Hydrogen is separated from carbon dioxide by passing the mixture through water under pressure, in which carbon dioxide gets dissolved leaving behind hydrogen. Carbon dioxide can also be separated by passing it through caustic potash (KOH) solution.
2KOH + CO₂ → K₂CO₃ + H₂O
(ii) To separate carbon monoxide the gaseous mixture is passed through ammoniacal cuprous chloride in which carbon monoxide dissolves leaving behind hydrogen. Thus hydrogen gas is obtained. 
The Bosch process is the primary industrial method for creating large quantities of hydrogen needed for making things like fertilizers. It involves complex steps to purify the hydrogen by removing other gases like carbon monoxide and dioxide.
Teacher's Tip: Notice how the first step takes in heat (endothermic) while the second step releases heat (exothermic).
Exam Tip: Remember to name the catalyst (Fe2O3) and the promoter (Cr2O3) as they are crucial for the Bosch process marks.

Question 10: Match the statements in Column A with those in Column B.
 Column A
1. A metal which reacts with cold water to form hydrogen.
2. A gas which is inflammable and a non-supporter of combustion.
3. A process in which vanaspati ghee is prepared from vegetable oils.
4. The removal of hydrogen or addition of oxygen.
5. The addition of hydrogen or removal of oxygen. 
Column B.
1. Reduction.
2. Hydrogenation
3. Oxidation
4. Sodium
5. Hydrogen.
Answer: 
1. A metal which reacts with cold water to form hydrogen. - Sodium
2. A gas which is inflammable and a non-supporter of combustion. - Hydrogen
3. A process in which vanaspati ghee is prepared from vegetable oils. - Hydrogenation
4. The removal of hydrogen or addition of oxygen. - Oxidation
5. The addition of hydrogen or removal of oxygen. - Reduction 
Matching exercises help reinforce the relationship between scientific terms and their definitions or examples. These five pairs cover the most essential concepts learned in the chapter on Hydrogen.
Teacher's Tip: Think of "Hydrogenation" as literally "adding hydrogen" to oils to make them solid.
Exam Tip: Oxidation and Reduction are opposites; if you memorize one perfectly, the other is just the reverse.

Question 11: State four uses of hydrogen:
Answer: 
1. Hydrogen with oxygen produce oxy-hydrogen flame which is used for cutting and welding.
2. Hydrogen gas is used as a fuel.
3. Hydrogen is used for hydrogenation of vegetable oil.
4. Hydrogen gas is used extensively in the manufacture of ammonia gas, which is used to produce fertilizers. 
Hydrogen's unique properties make it useful in industries ranging from food production to metalworking and rocket science. Its high energy content makes it an ideal candidate for future clean energy solutions.
Teacher's Tip: Ammonia production via the Haber process is one of the most important industrial uses for hydrogen in the world.
Exam Tip: When listing uses, always explain *why* it's used; for example, mention it's used for welding because the flame is extremely hot.

Question 12: Define: (a) catalytic hydrogenation (b) oxidation (c) reduction (d) redox reaction
Answer: 
(a) Catalytic hydrogenation: catalytic hydrogenation is a process by which hydrogen gas is passed through vegetable oils in the presence of catalyst like Ni, Pt or Pd to convert them into solid vanaspati ghee.
(b) Oxidation: A reaction in which a substance combine with oxygen or in which hydrogen is removed is called oxidation reaction. Example: H₂S + Cl₂ → 2HCl + S
(c) Reduction: Those reactions in which hydrogen combines with a substance or oxygen is removed from a substance, are known as reduction reactions. Example : 2HgO →heat 2Hg + O₂
(d) Redox reaction: Redox reactions are those in which reduction and oxidation both takes place simultaneously i.e. one substance is reduced while the other gets oxidised. 
Definitions provide the precise language needed to describe chemical changes accurately and professionally. Knowing these definitions allows you to analyze any chemical equation and identify what is happening to each atom.
Teacher's Tip: Use the acronym "OIL RIG" - Oxidation Is Loss (of electrons/hydrogen), Reduction Is Gain.
Exam Tip: For definitions, always include a chemical equation as an example to demonstrate your understanding.

Question 13: Multiple Choice Questions

(a) Equal volumes of hydrogen and chlorine are exposed to diffused sunlight to prepare
1. hydrogen chloride
2. water
3. sodium hydroxide
4. hydrochloric acid
Answer: 1. hydrogen chloride

(b) The metal which reacts with cold water to produce hydrogen is
1. magnesium
2. aluminium
3. calcium
4. iron
Answer: 3. calcium

(c) In metal activity series the more reactive metals are at
1. top
2. bottom
3. middle
4. none
Answer: 1. top

(d) Hydrogen is responsible for producing
1. heat and light
2. hydrogenated oil
3. fertilizers
4. all of the above
Answer: 4. all of the above

(e) Hydrogen is
1. combustible
2. non-combustible
3. supporter of combustion
4. neither supporter nor combustible
Answer: 1. combustible

(f) Water gas is a mixture of
1. carbon monoxide and oxygen
2. carbon monoxide and hydrogen
3. hydrogen and oxygen
4. hydrogen and nitrogen.
Answer: 2. carbon monoxide and hydrogen

ADDITIONAL QUESTIONS

MULTIPLE CHOICE QUESTIONS
Tick the most appropriate answer.
 
Question 1: The name hydrogen was given by
1. Cavendish
2. Lavoisier
3. Haber
4. none of these
Answer: 2. Lavoisier
 
Question 2: Which is the lightest of all elements?
1. hydrogen
2. helium
3. lithium
4. none of these
Answer: 1. hydrogen
 
Question 3: Hydrogen burns in oxygen to form
1. hydrogen sulphide
2. nitrates
3. water
4. ammonia
Answer: 3. water
 
Question 4: The process of adding oxygen to a substance is called
1. oxidation.
2. reduction.
3. displacement.
4. hydrogenation.
Answer: 1. oxidation.
 
These additional multiple-choice questions reinforce the basic historical and physical facts about the element hydrogen. Understanding who discovered it and its fundamental role in forming water is the foundation of chemistry.
Teacher's Tip: While Cavendish discovered the gas, Lavoisier was the one who named it "Hydro-gen" (meaning water-maker).
Exam Tip: Pay attention to names of scientists; historical context is often used for introductory marks.

FILL IN THE BLANKS

1. Hydrogen means ‘maker of water’ in Greek.
2. In nature, hydrogen occurs as a diatomic molecule represented as H₂.
3. Sodium liberates hydrogen when treated with cold water.
4. Granulated zinc is preferred over pure zinc in the laboratory preparation of hydrogen.
5. The compounds of carbon and hydrogen are called hydrocarbons.
6. Hydrogen is present abundantly in the atmosphere.
7. In electrolysis of water, dilute sulphuric acid is added to increase conductivity current of water.
8. Ammonia is used to make fertilizers.
 
Filling in these blanks helps build your scientific vocabulary, specifically regarding terminology like "diatomic" and "hydrocarbons." These words are specific to chemistry and describe the structure and categories of matter.
Teacher's Tip: Pure water doesn't conduct electricity well, which is why we add acid to help the electrolysis process along.
Exam Tip: If a question asks for a molecule representation, always write the subscript like H₂, not just H.

TRUE \ FALSE
Write true or false for each statement. Rewrite the false statements correctly.

1. Lead reacts briskly with dilute hydrochloric acid to form hydrogen.
Answer: False. Lead reacts very slowly with dilute hydrocloric acid to form hydrogen.

2. Hydrogen does not combine with nitrogen under ordinary conditions.
Answer: True.

3. Copper reacts with hot water to form copper oxide and hydrogen.
Answer: False. Copper does not react with water.

4. Hydrogen is a good oxidizing agent.
Answer: False. Hydrogen is a reducing agent.
 
Correcting false statements is a higher-level skill that proves you truly understand the concepts rather than just memorizing facts. For example, knowing that copper is non-reactive with water explains why we can use copper pipes for plumbing.
Teacher's Tip: Metals like copper, gold, and silver are at the very bottom of the reactivity series, meaning they are very stable.
Exam Tip: When rewriting a false statement, only change the essential part (like changing "oxidizing" to "reducing") to keep it clear.

MATCH THE COLUMNS

Column A
1. 2H₂ + O₂ → 2H₂O
2. H₂ + Cl₂ → 2HCl
3. ZnO + H₂ → Zn + H₂O
4. Cu + H₂SO₄ → CuSO₄ + H₂
5. N₂ + 3H₂ → 2NH₃ + heat
Column B
a. This reaction is not possible.
b. This reaction is exothermic to the extent of explosion.
c. This reaction is known as Haber process.
d. This reaction illustrates the reducing property of hydrogen.
e. This reaction takes place in diffused sunlight.
f. This reaction is a method of preparation of hydrogen.
Answer: 
1. 2H₂ + O₂ → 2H₂O - b. This reaction is exothermic to the extent of explosion.
2. H₂ + Cl₂ → 2HCl - e. This reaction takes place in diffused sunlight.
3. ZnO + H₂ → Zn + H₂O - d. This reaction illustrates the reducing property of hydrogen.
4. Cu + H₂SO₄ → CuSO₄ + H₂ - a. This reaction is not possible.
5. N₂ + 3H₂ → 2NH₃ + heat - c. This reaction is known as Haber process.
 
Matching chemical equations to their specific characteristics helps you associate abstract symbols with real-world physical reactions. For instance, knowing which reactions need sunlight or which are explosive is critical for safety and lab work.
Teacher's Tip: "Diffused sunlight" means the light is scattered; direct sunlight might make the reaction with chlorine happen too violently.
Exam Tip: Question 4 is a trick! Copper cannot displace hydrogen from acids because it is less reactive than hydrogen.

WRITE SHORT ANSWERS

Question 1: Why is granulated zinc preferred in the laboratory preparation of hydrogen?
Answer: Granulated zinc contains traces of copper as an impurity which acts as a catalyst. It also prevents the deposition of the gas on zinc. 
The impurities in granulated zinc are actually helpful because they speed up the chemical reaction without being used up themselves. This makes the production of hydrogen much more efficient for a classroom experiment.
Teacher's Tip: A catalyst is like a "helper" that speeds up a reaction without joining the final product.
Exam Tip: Mention both the "copper impurity" and the "catalytic action" to get full marks.

Question 2: Give a test to identify hydrogen gas.
Answer: Hydrogen is a highly inflammable gas which bums with a ‘pop’ sound and blue flame and forms water. 
The "pop test" is the standard way to confirm you have collected hydrogen because no other common gas behaves this way. The blue flame and formation of water droplets on the jar confirm the chemical change.
Teacher's Tip: Be careful! Always perform this test with a small amount of gas to avoid a large explosion.
Exam Tip: Always include the "pop sound" and the "pale blue flame" in your description of the test.

Question 3: Hydrogen gas is collected by the downward displacement of water. Give reason.
Answer: Hydrogen gas is collected by the downward displacement of water because hydrogen gas is practically insoluble in water. 
Because hydrogen doesn't dissolve, it simply pushes the water out of the jar as it fills the space. This allows you to collect a pure sample of the gas without it mixing with air.
Teacher's Tip: This method also lets you see exactly how much gas you have produced by looking at the water level.
Exam Tip: Clearly state that it is "insoluble" or "sparingly soluble" in water to explain this collection method.

Question 4: Write two physical properties of hydrogen.
Answer: Physical properties of hydrogen gas
1. It is a colourless, odourless and tasteless gas.
2. It is highly inflammable and bums with a blue flame. 
Physical properties are characteristics you can observe without changing the chemical identity of the substance. Being colorless and odorless makes hydrogen difficult to detect without scientific tests.
Teacher's Tip: Note that "inflammable" means the same thing as "flammable"—it burns very easily!
Exam Tip: If asked for more properties, you could also mention it is the lightest known gas.

Question 5: What is hydrogenation?
Answer: Hydrogenation is a process in which vegetable oils are converted into fats on treating with hydrogen. 
This process is used in the food industry to change liquid oils into solid fats, which stay fresh longer and are easier to transport. It is an example of how organic chemistry is used in our daily lives and the food we eat.
Teacher's Tip: This is how "margarine" or "vanaspati ghee" is made from liquid vegetable oil.
Exam Tip: Mention that a catalyst like Nickel (Ni) is usually required for this process to happen.

ANSWER IN DETAIL

Question 1: Discuss the occurrence of hydrogen in the universe and on the earth.
Answer: Hydrogen is the most abundant element on Earth. It is present in large amounts in stars and sun. In our atmosphere and Earth’s crust, it is found in very small amount. Hydrogen compounds are found abundantally e.g. acids, bases and hydrocarbons. Nearly three fourth of earth’s surface is covered with water which is formed from hydrogen itself. 
Hydrogen is the primary matter of the universe, making up the vast majority of all atoms in existence. On Earth, however, it is rarely found alone as a gas because it is so reactive and light that it mostly exists locked inside other molecules.
Teacher's Tip: While it's rare as a gas in our air, remember it's everywhere in the form of water (H₂O).
Exam Tip: Differentiate between "free state" (stars/atmosphere) and "combined state" (water/acids) for a more structured answer.

Question 2: How do different metals displace hydrogen from water? Explain with the help of equations.
Answer: Almost all metals react with water but with different intensities.
1. Sodium and potassium react violently with cold water to an extent that hydrogen gas so produced catches fire. 2K + 2H₂O → 2KOH + H₂ ↑ and 2Na + 2H₂O → 2NaOH + H₂ ↑
2. Calcium reacts less violently with water. Ca + 2H₂O → Ca(OH)₂ + H₂ ↑
3. Magnesium reacts with hot water to form magnesium oxide and hydrogen. Mg + H₂O → MgO + H₂ ↑
4. Metals like aluminium zinc and iron reacts only with steam. 3Fe + 4H₂O → Fe₃O₄ + 4H₂ ↑
5. Metals like gold, silver and copper do not react with water. 
The reactivity of a metal determines if it needs cold water, hot water, or steam to trigger a chemical reaction. This progression shows that more energy (heat) is needed to force less reactive metals to give up their hydrogen.
Teacher's Tip: Notice that for cold water, you get a "hydroxide," but for steam, you get an "oxide."
Exam Tip: Include the upward arrow ↑ next to H₂ to show that a gas is being released.

Question 3: Describe two chemical properties of hydrogen with equations.
Answer: Chemical properties of hydrogen gas:
1. Reaction with oxygen: Hydrogen bums with a pale blue flame in air or oxygen forming water. This reaction is highly explosive. 2H₂ + O₂ → 2H₂O
2. Reaction with chlorine: Hydrogen reacts with chlorine in diffused sunlight to form hydrogen chloride. H₂ + Cl₂ → 2HCl 
Chemical properties describe how a substance changes into a completely new substance through a reaction. These two examples show hydrogen combining with non-metals to form stable compounds like water and acid.
Teacher's Tip: The reaction with oxygen is what provides the power for rocket engines!
Exam Tip: Make sure to state the "condition" for the chlorine reaction, which is "diffused sunlight."

Question 4: Why is hydrogen called a reducing agent? Give chemical equation to support your answer.
Answer: Hydrogen has the capacity to remove oxygen from a number of metal oxide. Therefore hydrogen is a good reducing agent. ZnO + H₂ → Zn + H₂O 
By removing oxygen, hydrogen reduces the metal oxide back into its pure metallic state. This property is very useful in metallurgy for refining metals from their natural ores.
Teacher's Tip: In this reaction, hydrogen is oxidized (gains oxygen) while the zinc is reduced (loses oxygen).
Exam Tip: Define "reduction" in your answer to make the explanation more complete.

Question 5: Discuss the use of hydrogen as a fuel.
Answer: Liquid hydrogen is used as a fuel in rockets and guided missiles. Hydrogen can also be used as a fuel in automobiles. Researches are being done to discover use of hydrogen as an alternative source of energy in future since this would cut down pollution also. 
Hydrogen is considered a "green fuel" because when it burns, it only produces water vapor instead of harmful carbon dioxide. While it is currently expensive and difficult to store, it holds great promise for a zero-emission future.
Teacher's Tip: Hydrogen fuel cells are already being used in some buses and cars today!
Exam Tip: Highlight that the main benefit of hydrogen fuel is that it is "environmentally friendly" or "non-polluting."