Selina Concise Solutions for ICSE Class 9 Chemistry Chapter 9 Study Of The First Element Hydrogen

Exercise 9

Question 1.
Answer: (a) The position of hydrogen in the periodic table. Hydrogen is first element in the periodic table. It has an atomic number 1 and an atomic mass of 1.00794 amu, occupying group – IA. Its position is peculiar because it is grouped with metals although it is a non-metal properties. Hydrogen relate to Group IA as well as Group VII A.
(b) The properties of hydrogen resemble the properties of Group IA elements (Alkali metals), and some of it resembles the properties of Halogens (VIIA), so Hydrogen was put at the top of the periodic table so that the symmetry of the modern periodic table is not disturbed.
1. All elements in Group- IA have one electron in outermost shell, so they have valency one.
2. These elements in Group-IA are good reducing agents.
3. All elements of this group form oxide which are highly basic and dissolves in water to form strong alkalis.
4. They impart colour to a flame.
Hydrogen is the first element with atomic number 1 and mass 1.00794 amu. It is placed in Group IA because like alkali metals, it has 1 valence electron and is a good reducing agent. However, it also behaves like Halogens (Group 17) because it is a non-metal and can gain one electron. Because it fits in two places, it is placed at the very top to maintain the table's symmetry.
In simple words: Hydrogen is like a student who belongs to two different clubs at once. It acts like the metals in Group 1 but also like the non-metals in Group 17, so scientists put it at the very top to be fair to both.

📝 Teacher's Note: Explain the "peculiar" position of hydrogen by drawing the electronic configuration of H, Li, and F on the board. This helps students visualize why it resembles both groups.

🎯 Exam Tip: When asked about the position of hydrogen, always mention its resemblance to both Alkali Metals and Halogens to get full marks.

Question 2.
Answer: Similarity of hydrogen with alkali metals and halogens

In simple words: This table compares how hydrogen is a "twin" to two different groups. It shares an electron count with Group 1 but shares its gaseous, non-metal nature with Group 17.

📝 Teacher's Note: Use the "Ion formation" row to explain the difference between the Hydrogen ion (\( \text{H}^+ \)) and the Hydride ion (\( \text{H}^- \)). This is a crucial distinction in chemistry.

🎯 Exam Tip: Memorize the equations for ion formation (\( \text{H}^{1+} \) and \( \text{H}^{1-} \)) as they are often asked in short-answer questions to show hydrogen's dual nature.

Question 3.
Answer: (a) Hydrogen is found in minute traces in the Earth’s crust and the Earth’s atmosphere. The atmosphere around the sun and stars is found to contain 1.1 % hydrogen. (b) Henry Cavendish when prepared this gas from iron and dil. acids, he established its elementary nature and showed that when the gas burns in air, water is formed. It was on account of this property that Lavoisier in 1783 named it hydrogen (Greek word meaning water-former).
In simple words: Hydrogen is rare on Earth's ground but very common in space. It got its name because when it burns, it literally creates water!

📝 Teacher's Note: Mention that while hydrogen is rare in its free form on Earth, it is extremely common in combined forms like water (\( \text{H}_2\text{O} \)) and organic matter.

🎯 Exam Tip: Remember the names of the two scientists: Cavendish (discovered its nature) and Lavoisier (named it). This is a common objective-type question.

Question 4.
Answer: (a) Monovalent metal (Sodium):
\( 2\text{Na} + \text{H}_2 \rightarrow 2\text{NaH} \) (Sodium hydride)
(b) Divalent metal (Calcium):
\( \text{Ca} + \text{H}_2 \rightarrow \text{CaH}_2 \) (Calcium hydride)
In simple words: When hydrogen reacts with metals like sodium or calcium, it forms compounds called hydrides.

📝 Teacher's Note: Explain that "monovalent" means a valency of 1 and "divalent" means a valency of 2. Show how the valency affects the chemical formula in the resulting hydride.

🎯 Exam Tip: In these reactions, hydrogen acts as an oxidizing agent. Note that the metal loses electrons to the hydrogen.

Question 5.
Answer: (a) Calcium: is not used in lab preparation of hydrogen because: 1. The reaction and very violent and exothermic hence dangerous. 2. The heat liberated ignites the hydrogen. 3. Calcium is expensive. (b) Iron: Iron reacts slowly at ordinary temperatures, hence requires heating. The hydrogen produced also contain impurities like sulphur dioxide and hydrogen sulphide. Hence, it is not used in lab preparation of hydrogen. (c) Aluminium: It is not used in the lab preparation of hydrogen because oxides of this metal keep sticking to the surface of the metal. Thus the steam does not come in contact with metal and hence reaction stops. (d) Sodium: It is riot used in the lab preparation of hydrogen because the reaction is violent. The sodium melts into a globule and darts about freely on the surface of water hence the collection of hydrogen is difficult.
In simple words: We don't use these metals in the lab because some are too "angry" and explode, some are too slow, and some form a "skin" that stops the reaction from finishing.

📝 Teacher's Note: This is a great place to discuss lab safety. Use the example of sodium "darting" on water to explain why we use more controlled metals like Zinc for experiments.

🎯 Exam Tip: When asked why Sodium or Calcium isn't used, the keywords are "violent," "exothermic," and "dangerous."

Question 6.
Answer: Depending upon the nature of reaction taking place between metals and substances like air, water and acids, metals are arranged in a vertical series in order of their activity. Such a series is called activity series of metals. The metals places near the top of the series are the most reactive, while those placed near the bottom are the least reactive. When dilute hydrochloric acid or dilute sulphuric acid react with the metals above hydrogen in the activity series, they produce hydrogen. But the metals below hydrogen in the activity series do not.
In simple words: Think of the activity series as a "power ranking" for metals. Only the "strong" metals at the top can kick hydrogen out of an acid "team."

📝 Teacher's Note: Draw the activity series on the board and highlight Hydrogen as the "boundary line." This helps students predict which reactions will actually happen.

🎯 Exam Tip: Remember that Copper, Silver, and Gold are below Hydrogen and will never produce hydrogen gas when reacted with dilute acids.

Question 7.
Answer: (a) Reactants: Nitrogen and hydrogen (Haber process). (b) Reactants: Chlorine and hydrogen. (c) Reactants: Sulphur and hydrogen. (d) Reactants: Oxygen and hydrogen.
(a) Nitrogen: \( \text{N}_2 + 3\text{H}_2 \rightleftharpoons{\text{Fe, Mo}} 2\text{NH}_3 \). Conditions: \( 450^\circ\text{C} - 500^\circ\text{C} \), 200-900 atm pressure, Iron catalyst.
(b) Chlorine: \( \text{H}_2 + \text{Cl}_2 \xrightarrow{\text{Sunlight}} 2\text{HCl} \). Slow in diffused light, explosive in direct light.
(c) Sulphur: \( \text{H}_2 + \text{S} \rightarrow \text{H}_2\text{S} \). Occurs when hydrogen is passed through molten sulphur.
(d) Oxygen: \( 2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O} \). Hydrogen burns with a "pop" sound and a pale blue flame to form water.
In simple words: Hydrogen reacts with different elements to make Ammonia (for fertilizer), Acid, smelly Hydrogen Sulphide gas, or just plain Water.

📝 Teacher's Note: The "pop sound" is the standard test for hydrogen gas. Explain that it happens because hydrogen combines explosively with the oxygen in the air.

🎯 Exam Tip: For the Haber process, always state the catalyst (Iron) and the promoter (Molybdenum) to get full marks. For the Oxygen reaction, always mention the "pop sound" as an observation.

Question 8.
Answer: (a) Among the given metals Zinc is most suitable. (i) Copper: In case of copper, It is placed below hydrogen in the activity series. So it does not displace hydrogen from acid. (ii) In case of Mg; it is a very expensive metal. (iii) In case of sodium, it reacts with explosively and violently. (b) Among the given acids we prefer dilute sulphuric acid. We reject concentrated sulphuric, dilute nitric and concentrated nitric acid because these are powerful oxidising agents and oxygen formed due to its decomposition oxidises the hydrogen to water. (c) Modification: Collect the gas by downward displacement of water when all the air from the apparatus has been expelled. Drying Agent used is Calcium Chloride.
In simple words: Zinc is the "Goldilocks" metal—not too fast, not too slow, and not too expensive. We use dilute acid to make sure we get gas instead of just making more water.

📝 Teacher's Note: Explain that "downward displacement of water" is used because hydrogen doesn't dissolve in water and it's lighter than water, so it pushes the water down as it fills the jar.

🎯 Exam Tip: If asked why Nitric acid isn't used, the answer is always that it's a "powerful oxidizing agent" that oxidizes hydrogen to water.

Question 9.
Answer:  (a) Iron reacts reversibly with steam. Hence the hydrogen formed is removed as it is released to prevent reduction of triferric tetraoxide. \( \text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2 \) (Steam)

📝 Teacher's Note: This is a classic example of Le Chatelier's principle in action. Removing a product (hydrogen) forces the reaction to keep moving forward.

🎯 Exam Tip: Memorize the balanced equation \( 3\text{Fe} + 4\text{H}_2\text{O} \rightleftharpoons \text{Fe}_3\text{O}_4 + 4\text{H}_2 \) and use the reversible arrows \( \rightleftharpoons \).

Question 10. (a) The metal is magnesium (b) \( \text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2 \)
Answer: Magnesium reacts with boiling water or steam to produce Magnesium oxide and Hydrogen gas.
In simple words: Magnesium is "lazier" than sodium; it needs hot water to start giving off hydrogen gas.

📝 Teacher's Note: Contrast this with Sodium, which reacts with *cold* water. This demonstrates the difference in reactivity levels in the activity series.

🎯 Exam Tip: For Magnesium, always specify that the water must be "boiling" or in the form of "steam" for the reaction to occur efficiently.

Question 11.
Answer: (a) Substance A is \( \text{CuO} \) and substance B is \( \text{Cu} \).
(b) Test for water (i) It is neutral to litmus (ii) It changes anhydrous copper sulphate into blue salt.
(c) When substance A i.e, \( \text{CuO} \) reacts with hydrogen, it removes oxygen and we get free metal i.e. \( \text{Cu} \).
(d) \( \text{CuO} + \text{H}_2 \rightarrow \text{Cu} + \text{H}_2\text{O} \)
(e) No, there is no reaction between substance B and dilute hydrochloric acid because copper does not displace hydrogen from acids.
(f) \( \text{Cu} + \text{HCl} \rightarrow \text{No reaction} \)

In simple words: Hydrogen acts like a thief that steals oxygen from Copper Oxide, leaving behind pure copper. Copper is too "weak" to kick hydrogen out of an acid later.

📝 Teacher's Note: This is a Red-Ox (Reduction-Oxidation) reaction. Hydrogen is oxidized (gains O) and Copper Oxide is reduced (loses O). This is a core concept for Class 8-9.

🎯 Exam Tip: The color change from black (CuO) to reddish-brown (Cu) is a very common observation question in exams.

Question 12.
Answer: Magnesium lies above Hydrogen in reactivity series and can displace hydrogen from acid whereas, Mercury and silver lie below hydrogen in reactivity series and cannot displace hydrogen from acid and hence nothing happens.
In simple words: Only metals at the top of the "power list" (Activity Series) can make hydrogen gas from an acid. The ones at the bottom can't do anything.

📝 Teacher's Note: Use the "displacement" analogy—like a stronger player taking a spot on a sports team. This makes the Activity Series much easier to understand.

🎯 Exam Tip: If an exam gives you a metal and an acid, check the activity series first! If the metal is below H (like Cu, Ag, Hg, Au), write "No Reaction."

Question 13.
Answer: Soap bubbles containing hydrogen rapidly rise up in air as hydrogen is lighter than air.
In simple words: Hydrogen is like the "helium" of the lab; it's so light that bubbles filled with it fly away just like party balloons.

📝 Teacher's Note: This is a simple way to demonstrate density. You can contrast this with carbon dioxide bubbles, which would sink because they are heavier than air.

🎯 Exam Tip: This property is the reason why hydrogen was once used in airships (though it was later replaced by helium because hydrogen is flammable).

Question 14.
Answer: Bosch Process consists of following steps.
Step 1: Steam is passed over a hot coke (at \( 1000^\circ\text{C} \)) in a special type of a furnaces called converters. In this step carbon reacts with water to form carbon monoxide and hydrogen gas. This mixture is called water gas.
Step 2: In this step excess of steam is mixed with water gas and entire mixture is passed over heated ferric oxide and chromic oxide. Ferric oxide acts as catalyst and chromic oxide as promoter.
Step 3: In this step removal of carbon dioxide from reaction mixture takes place. The mixture of carbon dioxide and hydrogen is forced through cold water under pressure at 30 atmospheric pressure or through caustic potash solution which dissolve carbon dioxide leaving behind hydrogen gas. \( 2\text{KOH} + \text{CO}_2 \rightarrow \text{K}_2\text{CO}_3 + \text{H}_2\text{O} \)
Step 4: In this last step, mixture is passed through ammonical solution of cuprous chloride solution so as to dissolve carbon monoxide.
The Bosch process is a four-step industrial method to make hydrogen. First, steam and hot coke make "water gas." Next, more steam is added with a catalyst to make more hydrogen and \( \text{CO}_2 \). Then, the \( \text{CO}_2 \) is washed away with water or potash. Finally, any leftover Carbon Monoxide is removed using a special copper solution.
In simple words: Making industrial hydrogen is like a giant cleaning job. We start with coal and steam, make a gas mix, and then carefully wash away all the unwanted parts until only pure hydrogen is left.

📝 Teacher's Note: This is an important industrial process. Focus on "Water Gas" (\( \text{CO} + \text{H}_2 \)) as it's a key term students need to know.

🎯 Exam Tip: Know the catalysts for Step 2: Ferric oxide (catalyst) and Chromic oxide (promoter). These specific names are high-yield for exams.

Question 15.
Answer: (a) Cold water: Sodium metal wrapped in small piece of wire gauze or Sodium amalgamated with mercury is used. This prevents sodium from darting about. \( 2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2 \uparrow \)
(b) Hot water: \( \text{Zn} \) or \( \text{Mg} \) can be used. \( \text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2 \uparrow \) (boiling water)
(C) Steam: Iron reacts with the steam and the reaction is reversible. Iron reacts with steam when it is red hot as hydrogen is blown out of contact with iron by the force of current of the steam.
Different metals need different "encouragement" to make hydrogen. Sodium reacts with cold water. Magnesium needs boiling water. Iron is the least reactive here and requires red-hot heat and steam.
In simple words: Some metals are eager (Cold water), some are lazy (Hot water), and some need a "fire" under them (Steam) to produce hydrogen gas.

📝 Teacher's Note: Explain that "amalgamated with mercury" just means mixing the metal with mercury to slow down its reaction and make it safer to handle.

🎯 Exam Tip: Be careful with the products! Sodium and water make a hydroxide (\( \text{NaOH} \)), but Magnesium/Iron and steam make an oxide (\( \text{MgO} \) or \( \text{Fe}_3\text{O}_4 \)).

Question 16.
Answer: Metals \( \text{Mg, Al, Zn, Fe} \) do not react with cold water. It reacts with boiling water liberating hydrogen gas but the reaction is very slow. \( \text{Mg, Al, Zn, Fe} \) react with the hot steam in the heated state and form the corresponding oxide and hydrogen gas. Iron reacts with the steam and the reaction is reversible.
Magnesium: Reaction of boiling water with steam is slow but \( \text{Mg} \) liberates Hydrogen rapidly with steam. \( \text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2 \uparrow \) (boiling water). Aluminium: Gets coated with \( \text{Al}_2\text{O}_3 \) on rubbing with sand paper its oxide coating is removed and then it reacts with steam to produce hydrogen. \( 2\text{Al} + \text{H}_2\text{O} \rightarrow \text{Al}_2\text{O}_3 + 3\text{H}_2 \uparrow \) (steam).
Zinc: Zinc reacts with steam and produce zinc oxide and \( \text{H}_2 \). \( \text{Zn} + \text{H}_2\text{O} \rightarrow \text{ZnO} + \text{H}_2 \uparrow \) (steam).
Medium-reactive metals like Magnesium, Aluminium, and Zinc won't work with cold water. They require steam to produce hydrogen and their respective metal oxides. Aluminium specifically needs to be sanded first to remove its protective "skin" (oxide layer) before it can react.
In simple words: These metals are like "slow burners." They won't do much with cold water, but once you turn them into steam and heat them up, they start producing plenty of hydrogen.

📝 Teacher's Note: The "oxide layer" on Aluminium is a vital concept. It's why aluminium foil doesn't dissolve when you cook with it, but it also means it's tricky to use in these specific reactions.

🎯 Exam Tip: Always remember that reactions with steam produce the metal oxide, while reactions with cold water produce the metal hydroxide.

Question 17.
Answer: (a) Zinc and iron lie above hydrogen in reactivity series and can displace hydrogen from acid. \( \text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2 \uparrow \) (dilute). \( \text{Fe} + \text{H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2 \uparrow \) (dilute). (b) \( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \uparrow \) (dilute). \( \text{Fe} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2 \uparrow \) (dilute). Copper lies below hydrogen. Thus, it cannot displace hydrogen from acids.
Zinc and Iron are more reactive than hydrogen, so they can displace it from dilute sulphuric and hydrochloric acids. Copper is less reactive, so it won't react with these acids at all.
In simple words: Zinc and Iron are "stronger" than hydrogen and can steal its place in an acid. Copper is "weaker" and can't push hydrogen out.

📝 Teacher's Note: Differentiate between "dilute" and "concentrated" acids. For lab preparation of hydrogen, we almost always use dilute acids to ensure the reaction is safe and efficient.

🎯 Exam Tip: Always write "(dilute)" next to the acid in these equations to show you know the correct lab conditions.

Question 18.
Answer: Two alkalis which can displace hydrogen are \( \text{NaOH} \) and \( \text{KOH} \). Aluminium: \( 2\text{Al} + 6\text{NaOH} \rightarrow 2\text{Na}_3\text{AlO}_3 + 3\text{H}_2 \uparrow \) (Sodium aluminate). \( 2\text{Al} + 2\text{KOH} + 2\text{H}_2\text{O} \rightarrow 2\text{KAlO}_2 + 3\text{H}_2 \uparrow \) (Potassium meta aluminate). Zinc: \( \text{Zn} + 2\text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2 \uparrow \) (Sodium zincate). \( \text{Zn} + 2\text{KOH} \rightarrow \text{K}_2\text{ZnO}_2 + \text{H}_2 \uparrow \) (Potassium zincate). Aluminium and Zinc have unique nature; They react with acids and can even react with hot concentrated alkalis to form hydrogen and a soluble salt. Salts (oxides and hydroxide) of these metals are Amphoteric.
Zinc and Aluminium are "amphoteric" metals, meaning they can react with both acids and strong bases (alkalis like Sodium Hydroxide) to create hydrogen gas. These reactions form complex salts like Sodium Zincate and Sodium Aluminate.
In simple words: Most metals only react with acids, but Zinc and Aluminium are "multi-talented"—they can react with acids AND with strong soapy liquids called alkalis to make hydrogen.

📝 Teacher's Note: Introduce the word Amphoteric. It's a vital vocabulary word that means a substance can behave as both an acid and a base. Zinc and Aluminium are the primary examples in school chemistry.

🎯 Exam Tip: Memorize the formula for Sodium Zincate (\( \text{Na}_2\text{ZnO}_2 \)) and Sodium Aluminate (\( \text{NaAlO}_2 \)). These are very common "complete the reaction" questions.

Question 19.
Answer:
(a) \( 2\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_2 \);
(b) \( \text{Ca} + 2\text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{H}_2 \);
(c) \( \text{Mg} + \text{H}_2\text{O} \rightarrow \text{MgO} + \text{H}_2 \);
(d) \( \text{Zn} + \text{H}_2\text{O} \rightarrow \text{ZnO} + \text{H}_2 \);
(e) \( 3\text{Fe} + 4\text{H}_2\text{O} \rightarrow \text{Fe}_3\text{O}_4 + 4\text{H}_2 \);
(f) \( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \);
(g) \( 2\text{Al} + 3\text{H}_2\text{SO}_4 \rightarrow \text{Al}_2\text{(SO}_4)_3 + 3\text{H}_2 \);
(h) \( \text{Fe} + 2\text{HCl} \rightarrow \text{FeCl}_2 + \text{H}_2 \);
(i) \( \text{Zn} + 2\text{NaOH} \rightarrow \text{Na}_2\text{ZnO}_2 + \text{H}_2 \);
(j) \( 2\text{Al} + 2\text{KOH} + 2\text{H}_2\text{O} \rightarrow 2\text{KAlO}_2 + 3\text{H}_2 \)
This is a list of balanced equations showing how various metals (Na, Ca, Mg, Zn, Fe, Al) react with water, steam, acids, or alkalis to produce hydrogen gas.
In simple words: This is a summary of all the different "recipes" used to create hydrogen gas in a laboratory or factory.

📝 Teacher's Note: Ask students to categorize these equations into groups: Metal + Water, Metal + Acid, and Metal + Alkali. This helps them see the patterns instead of just memorizing single lines.

🎯 Exam Tip: When balancing these, always start with the metal atoms, then balance the non-metals, and save Hydrogen and Oxygen for last. It makes the process much simpler.

Question 20.
Answer:  (a) Lead reacts with dilute sulphuric acid and hydrochloric acid and forms an insoluble coating of lead sulphate and lead chloride respectively. Hence, further reaction is prevented.
(b) When potassium and sodium react with dilute sulphuric acid, \( \text{H}_2\text{SO}_4 \) or dilute \( \text{HCl} \), the reaction is highly explosive and practically not feasible.
(a) Lead isn't used because it forms a waterproof "shell" of salt that stops the acid from reaching the rest of the metal. (b) Sodium and Potassium are not used because the reaction with acid is so powerful it would explode.
In simple words: Lead "chokes" its own reaction with a protective skin, and Sodium/Potassium are so "angry" they would just blow up the lab equipment!

📝 Teacher's Note: The lead coating is a "passive" layer. Use the analogy of a person wearing a raincoat—the water (acid) can't get through to wet the person (metal) underneath.

🎯 Exam Tip: For lead, the specific terms are "insoluble coating" or "impervious layer." Using these terms will help you score full marks.

Question 21.
Answer: (a) Sodium hydroxide + zinc \( \rightarrow \) hydrogen + Sodium zincate (b) Calcium + water \( \rightarrow \) calcium hydroxide + Hydrogen
(a) Reacting Zinc with Sodium Hydroxide makes Sodium Zincate and Hydrogen gas. (b) Reacting Calcium with water creates Calcium Hydroxide and Hydrogen gas.
In simple words: These are word equations for common ways to make hydrogen in a lab.

📝 Teacher's Note: Word equations are the first step before writing balanced chemical equations. Ensure students can translate these names into formulas like \( \text{NaOH} \) and \( \text{Zn} \).

🎯 Exam Tip: Pay attention to the "ate" ending in Sodium Zincate. This indicates a complex salt where the metal is part of the negative radical.

Question 22.
Answer: 1. Hydrogen gas is collected by the downward displacement of water. This is because – It is virtually insoluble in water. It forms an explosive mixture with air and therefore, cannot be collected by downward displacement of air, even though it is lighter than the air.
2. A candle when brought near the mouth of the jar containing hydrogen gas starts burning but the candle is extinguished when pushed inside the jar because hydrogen is a combustible gas but a non-supporter of combustion i.e. it burns itself but does not allow substances to burn in it.
3. An oxy-hydrogen flame is used for welding and cutting metals because, when a mixture, of hydrogen and oxygen is burnt, temperatures as high as \( 2500^\circ\text{C} \) is produced.
4. Apparatus for the laboratory preparation of hydrogen should be air tight and away from a naked flame because, mixture of hydrogen and air explodes violently when brought near a flame.
Hydrogen is collected over water because it doesn't dissolve and it's too dangerous to collect in air. Hydrogen is "combustible" (it burns) but not a "supporter of combustion" (things won't burn *inside* it). Because it burns so hot with oxygen, it's used for welding metal. Lab equipment must be airtight to prevent explosive leaks.
In simple words: Hydrogen is very "shy" with water but very "explosive" with air. It's a great fuel for melting metal, but you have to be very careful not to let any escape or it might go "bang!"

📝 Teacher's Note: The distinction between "combustible" and "supporter of combustion" is vital. Hydrogen burns, but a candle goes out inside a jar of it because there's no oxygen there to help the candle burn.

🎯 Exam Tip: For collection, you must mention that hydrogen is "lighter than air" AND "insoluble in water." Both points are needed for a full answer.

Question 23.
Answer: (a) (iii) non-combustible
(b) (iii) a base
(c) (iii) \( \text{CaO} \)
(d) (iv) It is strong oxidising agent.
(e) (iii) \( \text{PbO} \) is oxidised to \( \text{Pb} \) (f) (ii) \( \text{Zn} \) (g) (ii) Copper
In simple words: These are the correct choices for multiple-choice questions about the traits and reactions of hydrogen and related metals.

📝 Teacher's Note: Review each MCQ option. For (e), passing hydrogen over a metal oxide is always a "Reduction" of that oxide because oxygen is removed. Correcting textbooks is a great way to engage critical thinking.

🎯 Exam Tip: For (g), remember the "Activity Series" rule again: Copper is below Hydrogen and cannot displace it. This is a favorite "trick" question.

Question 24.
Answer:  
(a) \( \text{CuO, H}_2\text{O} \)
(b) sparingly
(c) amalgam
(d) iron, magnesium, aluminium
(e) above, dilute hydrochloric, dilute sulphuric.
In simple words: These are the correct fill-in-the-blank words for sentences describing hydrogen experiments and properties.

📝 Teacher's Note: "Sparingly" means very little. Even though we say hydrogen is insoluble in water, a tiny, tiny amount does actually dissolve, so "sparingly soluble" is the most scientific term.

🎯 Exam Tip: When filling in (e), always make sure to include the word "dilute." Concentrated acids behave differently and won't produce pure hydrogen gas in these setups.

Question 25.
Answer:  (a) Hydrogen is used as a fuel in the form of coal gas, water gas and liquid hydrogen. (b) All metals above hydrogen in reactivity series react with acids to give hydrogen. (c) Metals like palladium or platinum or nickel absorb hydrogen at room temperature. (d) The reaction of hydrogen with oxygen is explosive with pop sound. (e) Concentrated sulphuric acid reacts with zinc to liberate sulphur dioxide.
Hydrogen is a great fuel (especially in liquid form for rockets). Metals higher than hydrogen in the activity series make it from acids. Certain expensive metals can actually "soak up" hydrogen gas. When hydrogen meets oxygen and a spark, it pops! Finally, using the "wrong" acid (concentrated) with zinc gives off smelly sulphur dioxide instead of hydrogen.
In simple words: Hydrogen is a powerful rocket fuel. It pops when it burns with oxygen and can be soaked up by some special metals like sponges soak up water.

📝 Teacher's Note: The absorption of hydrogen by metals like Palladium is called Occlusion. This is how hydrogen might be stored in future "hydrogen-powered" cars.

🎯 Exam Tip: For sub-part (e), this is why we must use dilute acid for lab prep. Concentrated acid changes the whole chemical result! Always write "dilute" in your lab reports.