Frank Brothers Solutions for ICSE Class 9 Chemistry Chapter 9 Study Of The First Element Hydrogen

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Question 1:
Answer:
1. (a) Symbol of hydrogen is ‘H’ and its molecular formula is H₂.
2. (b) 2{ H} means 2 atoms of hydrogen while H₂ means one molecule of hydrogen.
3. (c) Lavoisier gave “Hydrogen” its name because it means water product.
Hydrogen is the first and simplest element in the periodic table, consisting of just one proton and one electron. Because it is highly reactive, it usually exists in pairs as a diatomic molecule rather than staying as single atoms.
Teacher's Tip: Remember that a symbol (H) is like a single person, while a molecular formula (H₂) is like a pair of twins holding hands.
Exam Tip: Whenever asked for the formula of hydrogen gas, always write H₂ with a small 2 to score full marks.

Question 2:
Answer: Hydrogen is found in free state as well as in combined state.
1. In free state, it is found in traces in the earth’s crust and atmosphere. On the sun and bright stars, hydrogen is very abundant. It is suggested that the source of sun’s energy is fusion of hydrogen to helium.
2. In combined state it is found in the compounds like water, acids, organic materials and minerals. Natural gas coming out from oil wells, coal mines and volcanoes invariably contains this gas.
While hydrogen is rare in Earth's air, it is the most common material in the entire universe, making up stars and giant planets. On Earth, we mostly find it "stuck" to other elements, such as in the water (H₂O) we drink every day.
Teacher's Tip: Think of the Sun as a giant hydrogen balloon that provides us with light and heat through nuclear reactions.
Exam Tip: Be sure to mention both "Sun/Stars" for the free state and "Water/Acids" for the combined state.

Question 3:
Answer: Hydrogen resembles the alkali in following manner -
1. Electronic configuration - Hydrogen as well as alkali metals have one electron in their valence shell.
H (1) = 1 ; Li(3) = 2, 1 ; Na(11) = 2, 8, 1
2. Ion formation - Hydrogen loses one electron to form H^{+} ion like the alkali metals which form Li^{+}, Na^{+}, K^{+} etc.
3. Valency electrons - Like alkali metals, hydrogen exhibit valency of one in its compounds, as it has only one electron in its outermost shell.
4. Combination with non-metals - Like alkali metals, hydrogen combines with non-metals such as oxygen, chlorine and sulphur forming their oxides, chlorides and sulphides respectively.
5. Reducing action - Like alkali metals, hydrogen is also a very good reducing agent.
Hydrogen acts like a metal because it has just one "outer" electron that it can give away during a reaction. This shared characteristic is why it is often placed at the very top of the first column in the periodic table.
Teacher's Tip: Remember that "Alkali" and "Hydrogen" both start with a single electron "hook" to catch other atoms.
Exam Tip: Use the electronic configuration (2, 8, 1) of Sodium as a comparison to prove hydrogen's position.

Question 4:
Answer: Hydrogen resembles the halogens in following manner -
1. Electronic configuration - All the halogens have seven electrons in their outermost shell and need just one more electron to attain stable inert gas configuration. Similarly, hydrogen with one electron in its outermost shell requires one electron to attain a stable inert gas(Helium) configuration.
2. Valency - Hydrogen and halogen both show a valency of one.
3. Non-metals - Hydrogen is non-metallic like halogen.
4. Atomicity - Hydrogen as well as halogens are diatomic gases, e.g. H₂, Cl₂, Br₂ etc.
5. Ion formation - Halogens have a strong tendency to gain an electron to form halide ions. In similar way, hydrogen shows tendency to gain one electron to form hydride ion (H^{-}).
Unlike metals, hydrogen can also behave like a non-metal by "stealing" an electron to fill its only shell. This double personality is unique to hydrogen and makes it one of the most interesting elements to study.
Teacher's Tip: Think of hydrogen as a "chameleon" that can act like a metal or a non-metal depending on its partner.
Exam Tip: When asked about resemblance to halogens, always mention the "diatomic" nature (H₂ and Cl₂).

Question 5:
Answer: By the action of dilute sulphuric acid or hydrochloric acid with zinc we can prepare hydrogen.
Zn + H₂SO₄ → ZnSO₄ + H₂
Zinc Sulphuric acid Zinc sulphate Hydrogen
Zn + 2HCl → ZnCl₂ + H₂
Zinc Hydrochloric acid Zinc chloride Hydrogen
Zinc liberate hydrogen from hot concentrated solution of alkali.
Zn + 2NaOH → Na₂ZnO₂ + H₂
Zinc Sodium hydroxide Sodium zincate Hydrogen
Zn + 2KOH → K₂ZnO₂ + H₂
Zinc Potassium hydroxide Potassium zincate Hydrogen
In these reactions, the zinc metal "kicks out" the hydrogen from the acid or the strong base. This process is a common way to collect hydrogen gas for experiments in a school laboratory.
Teacher's Tip: Zinc is like a bully that pushes the smaller hydrogen out of its chemical house!
Exam Tip: Remember to name the salt formed, like "Sodium zincate," to get full marks for the equation.

Question 6:
Answer: (i)Calcium-
Ca + 2H₂O → Ca(OH)₂ + H₂
This reaction is vigorous but less vigorous as compared to reaction of sodium and potassium. So, not used for preparation of hydrogen. Also, Calcium is expensive.
(ii) Iron-
3Fe + 4H₂O → Fe₃O₄+ 4H₂
This reaction is a reversible reaction. Hence, the hydrogen formed should be continuously drawn out. Otherwise, it will reduce the iron oxide (ferrous-ferric oxide) back to iron. And, also the hydrogen thus produced contains impurities like hydrogen sulphide and sulphur dioxide.
(iii) Aluminium-It forms a protective coating of Al₂O₃ due to its great affinity for oxygen. So, it does not give hydrogen with acid after the reaction has produced for some time.
(iv) Sodium-
2Na + 2H₂O → 2NaOH + H₂
This reaction is violent. Sodium melts into silvery globules and darts about freely on the surface of water, making collection of hydrogen gas is difficult.
Not all metals are suitable for making hydrogen in a lab because some are too dangerous or too slow. For example, sodium reacts so fast it can cause an explosion, while aluminium creates a "skin" that stops the reaction halfway.
Teacher's Tip: Use the "Goldilocks Rule" - Sodium is too hot, Iron is too slow, but Zinc is just right!
Exam Tip: If asked why Sodium isn't used, always mention that the reaction is "violent" and "hard to collect."

Question 7:
Answer: (i) Reactants used - Zinc and sulphuric acid/Hydrochloric acid
(ii) Reaction -
Zn + H₂SO₄ → ZnSO₄ + H₂
Zinc Sulphuric acid Zinc sulphate Hydrogen
Zn + 2HCl → ZnCl₂ + H₂
Zinc Hydrochloric acid Zinc chloride Hydrogen
(iii) Procedure - Some pieces of granulated zinc are placed in the Woulfe’s bottle and the apparatus is made air tight. Now, dilute sulphuric acid or dilute hydrochloric acid is poured through thistle funnel. Hydrogen gas is evolved.
(iv) Complete labelled diagram -
[Diagram labels: Dil. H₂SO₄ (or Dil. HCl), Thistle funnel, Woulfe's bottle, Granulated zinc, H₂ gas, Water]
(v) Method of collection - Hydrogen is collected by downward displacement of water.
(vi) Drying agent for gas - Hydrogen gas is dried by passing it through anhydrous calcium chloride or P₂O₅.
The lab setup uses a special bottle with two necks and a funnel that looks like a thistle flower. We collect the gas under water because hydrogen doesn't dissolve in water and it is much lighter than air.
Teacher's Tip: The gas is collected "downward" because hydrogen wants to float "upward" like a balloon.
Exam Tip: When drawing the diagram, ensure the bottom of the thistle funnel is dipped into the acid to prevent gas from escaping.

Question 8:
Answer: Hydrogen gas obtained in the laboratory by dilute H₂SO₄ and granulated zinc have following impurities -
1. Hydrogen sulphide (H₂S),
2. Sulphur dioxide (SO₂),
3. Oxides of nitrogen,
4. Phosphine (PH₃),
5. Arsine (AsH₃),
6. Carbon dioxide, Nitrogen oxides,
7. Water vapour
They are removed by passing through -
1. Lead nitrate solution - It absorbs H₂S.
2. Silver nitrate solution (AgNO₃) - It absorbs PH₃, AsH₃
3. Caustic potash (solid KOH) - It absorbs CO₂, SO₂, oxides of nitrogen
4. Anhydrous calcium chloride or P₂O₅ - It absorbs moisture.
Gas produced in a lab is rarely 100\% pure and often contains smelly or dangerous side-gases. To clean it, we bubble the gas through different chemical "baths" that catch the unwanted parts.
Teacher's Tip: Think of these solutions as "filters" that scrub the gas clean before we use it.
Exam Tip: Match the specific impurity to the correct chemical solution (e.g., Lead nitrate for H₂S) for full marks.

Question 9:
Answer: (a) When passed over heated metallic oxides, hydrogen reduces the oxides to the metals by taking away oxygen.
(i) CuO + H₂ → Cu + H₂O
(ii) ZnO + H₂ → Zn + H₂O
(b) MgO and PbO cannot be reduced to metal by hydrogen.
Hydrogen acts like a "thief" that steals oxygen away from metals to turn them into pure elements. This is very useful in factories for getting pure copper or zinc from their ores.
Teacher's Tip: Reduction is just a fancy way of saying "stealing oxygen."
Exam Tip: Remember that hydrogen can't steal from very strong metals like Magnesium (Mg).

Question 10:
Answer: Potassium hydroxide on reaction with zinc produces potassium zincate and hydrogen.
Zn + 2KOH → K₂ZnO₂ + H₂
Zinc Potassium hydroxide Potassium zincate Hydrogen
Bosch Process -
The various steps involved in the process are as follows:
(1) Super heated steam is passed over heated coke at about 1000°C in special furnaces called converters to form water gas.
C + H₂O →{1000°C} CO + H₂
White hot coke Super heated steam Water gas
This reaction is endothermic.
(2) The water gas is so obtained is then mixed with more steam and passed over heated ferric oxide and chromic oxide at a temperature of about 450°C. Ferric oxide acts as a catalyst and chromic oxide acts as promoter. The CO from water gas takes away the oxygen from steam to produce carbon dioxide and more hydrogen. In this reaction poisonous carbon monoxide is converted to non-poisonous carbon dioxide and more hydrogen is produced.
CO + H₂ + H₂O →[450°C] Fe₂O₃/Cr₂O₃ CO₂ + 2H₂(g)
Water gas Steam Carbon dioxide Hydrogen
(3) Separation of hydrogen from a mixture of CO₂ and H₂ is carried out by -
(a) Both the carbon dioxide and hydrogen obtained are then passed through water under high pressure to form carbonic acid. Hydrogen hardly dissolves in carbonic acid, so easily collected.
CO₂ + H₂O →{{under high pressure}} H₂}CO₃
Carbon dioxide Water Carbonic acid
(b) The mixture of CO₂ and H₂ can also be passed through caustic potash which reacts with CO₂. Hydrogen gas does not react and can be collected.
2KOH + CO₂ → K₂CO₃ + H₂
Caustic potash
(4) Separation of carbon monoxide: Hydrogen gas is generally contaminated with carbon monoxide. In order to remove it hydrogen is dissolved in ammonical cuprous chloride solution. This solution absorbs any carbon monoxide gas present in the mixture. Finally, H₂ gas is dried over by passing through anhydrous CaCl₂ or P₂O₃.
(iii) Account for the following facts:
(a) Lead reacts with dilute sulphuric or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride. Therefore, further reaction is prevented.
(b) Potassium and sodium are not used to react with dilute hydrochloric acid or dilute sulphuric acid in the laboratory preparation of hydrogen since they react with acid violently.
The Bosch Process is the "industrial" way to make huge amounts of hydrogen for factories using coal and water. It is a multi-step process that cleans the gas and even turns poisonous waste into safer carbon dioxide.
Teacher's Tip: Think of a "promoter" like a cheerleader that helps the "catalyst" do its job even better!
Exam Tip: Be sure to memorize the specific temperature (1000°C) for the first step of the Bosch process.

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Question 11:
Answer: (a) Sodium > Calcium > Magnesium > Iron
(i) Sodium
2Na + 2H₂O → 2NaOH + H₂
(ii) Calcium
Ca + 2H₂O → Ca(OH)₂ + H₂
(iii) Magnesium
Mg + H₂O → MgO + H₂
(iv) Iron
3Fe + 4H₂O \rightleftharpoons Fe₃O₄ + 4H₂
(b) Copper is not used to prepare hydrogen by the action of dilute HCl or dilute H₂SO₄ on the metal since it is less reactive than hydrogen so cannot displace hydrogen from acid. Only the metals that are more reactive than the hydrogen itself can displace hydrogen from acids.
This order shows which metals are the most "energetic" when they touch water. Metals at the top react instantly with cold water, while metals like iron need to be heated until they are red-hot to react with steam.
Teacher's Tip: Remember the Reactivity Series: Metals below Hydrogen are "peaceful" and won't fight to push it out of an acid.
Exam Tip: Always use the "greater than" symbol (>) when asked to arrange elements in order of reactivity.

Question 12:
Answer: (a) (i) When magnesium is heated in air, it reacts with oxygen. During this oxidation reaction, magnesium oxide is produced.
2Mg + O₂ → 2MgO (s)
When copper is heated, copper oxide is formed.
Cu + O₂ → CuO
When iron is heated in presence of air, it will also form iron oxide.
4Fe + 3O₂ → Fe₂O₃
(ii) Magnesium and iron will form chlorides whereas copper does not show any reaction
Mg + 2HCl → MgCl₂ + H₂
Cu + HCl → {No reaction}
Fe + 2HCl → FeCl₂+ H₂
(iii) Magnesium being at the higher position than zinc in the reactivity series will displace Zinc from zinc sulphate.
Mg + ZnSO₄ → MgSO₄ + Zn
Copper and iron being at lower position than zinc will not displace zinc from zinc sulphate.
Cu + ZnSO₄ → {No reaction}
Fe + ZnSO₄ → {No reaction}
(b) Magnesium > Iron > Copper
Chemical reactions are like a game of musical chairs; only the stronger, more reactive metals can take a seat in a compound. This is why Magnesium can push Zinc out, but Copper is too weak to do the same.
Teacher's Tip: Magnesium is the "king" of this group because it reacts with everything on the list!
Exam Tip: If there is no reaction, write "No reaction" clearly to show you know the metal is not reactive enough.

Question 13:
Answer:
1. (a) Nickel, hydrogen
2. (b) above, dilute mineral
3. (c) covalent, electronegative
4. (d) CuO, hydrogen, water
5. (e) CO, H₂
6. (f) alkali
7. (g) nascent hydrogen
8. (h) water
These key terms and formulas summarize the most important facts about how hydrogen is made and used. For example, "nascent hydrogen" refers to hydrogen that has just been born and is extra powerful in reactions.
Teacher's Tip: Think of "nascent" like a "newborn baby" - it has a lot of energy!
Exam Tip: When filling in blanks, ensure the chemical symbols like H₂ are written with the correct subscripts.

Question 14:
Answer:
1. (a) False
2. (b) False
3. (c) True
4. (d) True
5. (e) True
6. (f) True
7. (g) True
8. (h) False
True and false questions help you spot common mistakes, like thinking hydrogen is heavier than air. In reality, hydrogen is the lightest thing in the world, which is why it floats up away from us.
Teacher's Tip: Don't be tricked - hydrogen is definitely NOT a supporter of combustion; it burns itself!
Exam Tip: Read every word carefully; a single word like "heavier" can make a whole scientific statement false.

Question 15:
Answer: (a) Although hydrogen is lighter than air yet it cannot be collected by the downward displacement of air as it forms an explosive mixture with air. So, it is collected by downward displacement of water not air.
(b) Hydrogen is highly inflammable and forms explosive mixture with air so apparatus for the laboratory preparation of hydrogen should be air tight and away from the flame.
(c) Nitric acid is not used in preparation of hydrogen because it is strong oxidizing agent and oxidises H₂ to H₂O.
Hot concentrated H₂SO₄ is not used in preparation of hydrogen as a part of it is reduced to SO₂.
(d) As sodium reacts violently with water.
2Na + 2H₂O → 2NaOH + H₂
This reaction is very violent. Sodium melts into silvery globules and darts about freely on the surface of water, making collection of hydrogen gas is difficult so large piece of sodium should not be placed in water.
(e) Na, K { and } Ca cannot be used to prepare hydrogen from acids because these displace hydrogen from dilute acids with explosive violence.
(f) In the laboratory preparation of hydrogen zinc granules are preferred over pure zinc metal because copper is present as impurity in granulated zinc, which catalyse the reaction and increase its rate.
(g) This process requires heat (endothermic). So the steam should not be passed over hot coke as it may cool down the coke. So after some time steam is alternatively replaced by air stream.
2nd-
This reaction is - C({white hot}) + H₂O({steam}) → CO + H₂
if steam is passed for a long time, it may react with carbon mono oxide (CO) and make carbon dioxide which is not a required product.
(h) Since hydrogen is a non-supporter of combustion so it does not allow substances to burn in it and hence when a burning splinter is introduced into a jar of hydrogen, the splinter gets extinguished.
(i) In nature, most dietary fats and oils exist in a structural form which is called the "cis" form. When these natural cis form fats are processed by bubbling hydrogen gas through them at high temperatures, they become partially hydrogenated which changes their structure to the "trans" form. The natural cis fat has a bend and the processed trans fat is a straight molecule. This difference in cis and trans shapes is of major significance. When eaten, fats and oils are incorporated into cell membranes altering the composition of these delicate structures. When they interact with normal fat metabolism, they disturb function in a most deleterious manner. Hence, these substances meet the definition of a poison. Trans fats interfere with important, normal functions by inhibiting enzymes which are necessary for the body's normal metabolism of fats and they keep doing it for a long time.
(j) Since hydrogen is highly inflammable in nature so inspite of being the lightest element it is not used in weather balloons.
(k) Lead reacts with dilute sulphuric acid or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride which prevents further reaction.
Safety is the most important part of studying hydrogen. Because it can explode or catch fire easily, we must follow strict rules, like making sure there are no leaks and avoiding very reactive metals.
Teacher's Tip: Think of "Trans Fats" as "Broken Fats" that don't fit into your body's puzzle correctly.
Exam Tip: For part (f), remember the keyword "catalyse" to explain why we use impure zinc granules.

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Question 16:
Answer: (a) When hydrogen is passed over heated copper oxide reddish brown copper and water is obtained.
CuO + H₂ → Cu + H₂O
Copper oxide Reddish brown
black copper
(b) When hydrogen is passed over heated sodium, sodium hydride is obtained.
2Na + H₂ → 2NaH
Sodium Sodium hydride
These reactions show hydrogen's two different "jobs." It can act as a cleaner that takes away oxygen (reduction) or it can bond directly with a metal to form a new solid called a hydride.
Teacher's Tip: Notice the color change! Black powder turns into shiny reddish copper.
Exam Tip: Be sure to write the physical colors (black, reddish-brown) under the chemical names in the equation.

Question 17:
Answer: H₂ + Cl₂ → 2HCl
In diffused sunlight, the reaction takes place smoothly and forms HCl gas. While, in direct sunlight, the reaction occurs with an explosion.
Light acts as a trigger for this reaction. While a little bit of light helps it happen safely, too much energy from the sun makes the atoms snap together so fast it creates a bang!
Teacher's Tip: This is like a "solar-powered" chemical reaction!
Exam Tip: Specify "diffused sunlight" as the condition to show the reaction is being done safely.

Question 18:
Answer: (a) Occlusion - Adsorption of gases at the surface of finely divided metal is called occlusion. Hydrogen gas is readily adsorbed or occluded by the metals like palladium, platinum or nickel at ordinary temperature.
(b) Hydrogenation - Unsaturated compounds combine with hydrogen in presence of catalyst like palladium, platinum or nickel to form saturated compounds. These reactions are known as hydrogenation reactions.
C₂H₂ + 2H₂ →{Ni/\Delta} C₂H₆
Acetylene
C₂H₄ + H₂ →{Ni/\Delta} C₂H₆
Ethylene
(c) Nascent hydrogen - It means newly born hydrogen. Hydrogen which is just formed at the time of its generation is called nascent hydrogen. It is more powerful reducing agent than molecular hydrogen.
Hydrogen has a special relationship with metals like Nickel; it can "stick" to their surface or use them as a stage to join with other molecules. This is how we turn liquid vegetable oils into solid margarine through the process of hydrogenation.
Teacher's Tip: Occlusion is like a "metal sponge" soaking up gas instead of water!
Exam Tip: Mention the specific catalysts (Ni, Pt, Pd) whenever you define hydrogenation.

Question 19:
Answer: Some pieces of zinc granules are placed in Woulfe’s bottle and the apparatus is made air tight. Now dilute sulphuric acid is poured through the thistle funnel.
Observation: The reaction takes place at room temperature even without heating. A brisk effervescence with the evolution of gas is seen inside the bottle. If burning candle is brought near the bubbles of hydrogen gas, they explode. This proves that bubbles were full of hydrogen gas and they move up in the air because they are lighter than air.
Brisk effervescence is just a scientific way of saying "lots of fast bubbles." The fact that the bubbles float upward proves that hydrogen is much lighter than the oxygen and nitrogen that make up our normal air.
Teacher's Tip: "Effervescence" is the same thing you see when you open a fresh bottle of soda!
Exam Tip: Use the "pop sound" or "explosion" with a candle as the standard test for identifying hydrogen.

Question 20:
Answer: 1. (a) Meteorological balloons - The low density and high lifting power of hydrogen made it useful in meteorological balloons used for studying air currents and weather conditions. However, due to its highly inflammable nature it has been replaced by helium which has a lifting only slightly less than that of hydrogen.
2. (b) In metallurgy - Hydrogen acts as a very good reducing agent. It is used to obtain metals by reducing their oxides.
3. (c) In fuel - Hydrogen has very high heat of combustions, therefore it is used as fuel in the form of coal gas, water gas and liquid hydrogen(for rocket propulsion).
4. (d) In making fertilizers - A large quantity of hydrogen is used in the manufacture of ammonia by Haber process. Ammonia is used in manufacture of HCl and methyl alcohol.
Hydrogen is like a super-tool for humans; it can lift balloons, clean metals, power giant rockets, and even help grow food! Because it packs so much energy, it is considered one of the cleanest and most powerful fuels for the future.
Teacher's Tip: Hydrogen is the "Rocket Fuel" that takes astronauts to the space station!
Exam Tip: Mention "Haber Process" specifically when talking about fertilizer production to impress the examiner.

Question 21:
Answer: (a) As an oxidizing agent - With metals hydrogen acts as an oxidizing agent. When dry hydrogen is passed over heated reactive metals (K, Na { and } Ca) the corresponding metal hydrides, are formed.
2Na + H₂ → 2NaH
(b) As a reducing agent - When passed over heated metallic oxides, hydrogen reduces the oxides to the metals by taking away oxygen.
CuO + H₂ → Cu + H₂O
These two roles show that hydrogen can either "take" or "give" depending on its environment. When it works with metals, it forms salt-like solids, but when it works with oxides, it creates water and pure metal.
Teacher's Tip: Think of Hydrogen as a "Flexible Friend" that changes its behavior to match its partner!
Exam Tip: Provide both equations (2NaH and Cu + H₂O) to show both sides of hydrogen's character.