Hydrogen and Its Compounds Archives

11. Which one of the following is the colour of hydrogen gas?

Which one of the following is the colour of hydrogen gas?

Light yellow

Orange

Black

Colourless

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This question was previously asked in

UPSC NDA-1 – 2022

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The correct answer is Colourless. Hydrogen gas (H₂) is a colourless gas.

– Hydrogen is the lightest and most abundant element in the universe.
– At standard temperature and pressure, hydrogen exists as a diatomic molecule (H₂) and is a gas.
– Pure hydrogen gas is colourless, odourless, and tasteless.
– It is also highly flammable.

Many common gases like nitrogen (N₂), oxygen (O₂), noble gases (He, Ne, Ar, etc.), and carbon dioxide (CO₂) are also colourless. Some gases do have colour, such as chlorine (greenish-yellow), bromine vapour (reddish-brown), iodine vapour (violet), and nitrogen dioxide (reddish-brown).

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12. Which one of the following reactions does not result in the evolutio

Which one of the following reactions does not result in the evolution of hydrogen gas ?

Reaction of zinc metal with dilute sulphuric acid solution

Mixing water to Plaster of Paris

Heating zinc metal with sodium hydroxide solution

Reaction of potassium metal with water

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This question was previously asked in

UPSC NDA-1 – 2021

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Mixing water with Plaster of Paris (Calcium sulfate hemihydrate, CaSO₄·½H₂O) results in a hydration reaction where water is absorbed to form gypsum (Calcium sulfate dihydrate, CaSO₄·2H₂O). This reaction is exothermic and causes the mixture to set and harden, but it does not produce any gas, including hydrogen.

The reactions in options A, C, and D all involve the liberation of hydrogen gas:
A) Active metals like zinc react with dilute acids (Zn + H₂SO₄ → ZnSO₄ + H₂).
C) Amphoteric metals like zinc react with strong bases upon heating (Zn + 2NaOH → Na₂ZnO₂ + H₂).
D) Highly reactive metals like potassium react vigorously with water (2K + 2H₂O → 2KOH + H₂).

Plaster of Paris is commonly used in construction, casting, and medical applications (like casts for broken bones) due to its ability to quickly set after mixing with water. The setting process involves the crystallization of gypsum.

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13. Which one of the following statements about dihydrogen (H₂) is not cor

Which one of the following statements about dihydrogen (H₂) is not correct ?

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H₂ is lighter than air and insoluble in water.

H₂ is inert at room temperature due to high H – H bond dissociation enthalpy.

H₂ reacts with alkali metals at high temperature to yield metal hydrides.

A mixture of NO₂ and H₂ is known as Syngas.

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This question was previously asked in

UPSC NDA-1 – 2021

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The statement “A mixture of NO₂ and H₂ is known as Syngas” is not correct. Syngas (synthesis gas) is primarily a mixture of carbon monoxide (CO) and hydrogen (H₂).

Syngas is a fuel gas mixture that can be produced from various carbon-containing materials like natural gas, coal, biomass, or plastic waste. It is used as a fuel or as an intermediate for the production of other chemicals like methanol and ammonia. Nitrogen dioxide (NO₂) is a toxic gas primarily associated with air pollution.

Dihydrogen (H₂) is indeed lighter than air and practically insoluble in water. Its high H–H bond dissociation enthalpy (435 kJ/mol) makes it quite stable and inert at room temperature. At high temperatures, it reacts with alkali metals (like Na, K, Li) to form ionic hydrides (e.g., 2Na + H₂ → 2NaH), where hydrogen acts as an anion (H⁻).

14. Temporary hardness in water is due to which one of the following of Ca

Temporary hardness in water is due to which one of the following of Calcium and Magnesium ?

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Hydrogen carbonates

Carbonates

Chlorides

Sulphates

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UPSC NDA-1 – 2017

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Temporary hardness in water is caused by the presence of dissolved bicarbonates (also known as hydrogen carbonates) of calcium and magnesium, such as Ca(HCO3)2 and Mg(HCO3)2. These can precipitate out as insoluble carbonates when the water is boiled, thereby removing the hardness.

Temporary hardness is due to soluble bicarbonates of calcium and magnesium and can be removed by boiling.

Permanent hardness is caused by dissolved chlorides and sulphates of calcium and magnesium (e.g., CaCl2, MgCl2, CaSO4, MgSO4) and cannot be removed by boiling. Water softening methods are required to remove permanent hardness.

15. The radioactive isotope of hydrogen is

The radioactive isotope of hydrogen is

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protium

deuterium

tritium

hydronium

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UPSC CDS-2 – 2020

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Tritium (³H) is the radioactive isotope of hydrogen. It contains one proton and two neutrons.

Hydrogen has three naturally occurring isotopes: Protium (¹H, 1 proton, 0 neutrons), Deuterium (²H or D, 1 proton, 1 neutron), and Tritium (³H or T, 1 proton, 2 neutrons). Protium and Deuterium are stable isotopes.

Tritium is a beta emitter with a relatively short half-life of approximately 12.3 years. Hydronium (H₃O⁺) is a polyatomic cation formed when a proton (H⁺) is added to a water molecule (H₂O); it is not an isotope of hydrogen.

16. Reaction between which of the following two reactants will produce hyd

Reaction between which of the following two reactants will produce hydrogen gas?

Magnesium and hydrochloric acid

Copper and dilute nitric acid

Calcium carbonate and hydrochloric acid

Zinc and nitric acid

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UPSC CDS-2 – 2018

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The reaction between Magnesium and hydrochloric acid will produce hydrogen gas.

Reactive metals, generally those above hydrogen in the reactivity series (like Magnesium, Zinc, etc.), react with dilute acids to displace hydrogen and produce hydrogen gas and a salt.

The reaction in option A is: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
Option B (Copper and dilute nitric acid): Nitric acid is an oxidizing acid, and while it reacts with copper, it typically produces nitrogen oxides rather than hydrogen gas.
Option C (Calcium carbonate and hydrochloric acid): This produces carbon dioxide gas (CO₂), water, and calcium chloride (CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂).
Option D (Zinc and nitric acid): Similar to copper, zinc reacts with nitric acid to produce zinc nitrate and nitrogen oxides (or ammonium nitrate with very dilute acid), not primarily hydrogen gas.

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17. Which one of the following statements is correct?

Which one of the following statements is correct?

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The oxidation number for hydrogen is always zero.

The oxidation number for hydrogen is always +1.

The oxidation number for hydrogen is always -1.

Hydrogen can have more than one oxidation number.

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UPSC CDS-2 – 2016

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Hydrogen can exist in different chemical environments where it exhibits more than one oxidation number.

The oxidation number for hydrogen is typically +1 when bonded to nonmetals (e.g., H₂O, HCl, NH₃). However, when bonded to more active metals (forming metal hydrides like NaH, CaH₂), hydrogen is more electronegative than the metal and has an oxidation number of -1. In its elemental form (H₂), the oxidation number is 0.

The oxidation number reflects the hypothetical charge an atom would have in a molecule if all bonds were ionic. The electronegativity difference between hydrogen and the atom it is bonded to determines whether hydrogen takes a positive or negative oxidation state.

18. Which of the following substances cause temporary hardness in water?

Which of the following substances cause temporary hardness in water?

1. Mg(HCO₃)₂
2. Ca(HCO₃)₂
3. CaCl₂
4. MgSO₄

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3 and 4

2 and 3

1 and 4

1 and 2

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This question was previously asked in

UPSC CDS-1 – 2018

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Temporary hardness in water is caused by the presence of dissolved bicarbonates of calcium (Ca(HCO3)2) and magnesium (Mg(HCO3)2). These compounds decompose when water is boiled, forming insoluble calcium carbonate and magnesium carbonate, which precipitate out, thereby removing the hardness.
From the given list:
1. Mg(HCO3)2 – Magnesium bicarbonate (causes temporary hardness)
2. Ca(HCO3)2 – Calcium bicarbonate (causes temporary hardness)
3. CaCl2 – Calcium chloride (causes permanent hardness)
4. MgSO4 – Magnesium sulfate (causes permanent hardness)
Therefore, substances 1 and 2 cause temporary hardness.

Temporary hardness is primarily caused by dissolved calcium and magnesium bicarbonates.

Permanent hardness is caused by dissolved sulfates and chlorides of calcium and magnesium. It cannot be removed by simple boiling but requires other methods like ion exchange or chemical precipitation using agents like washing soda.

19. Which one of the following statements is not correct?

Which one of the following statements is not correct?

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Hydrogen is an element

Hydrogen is the lightest element

Hydrogen has no isotopes

Hydrogen and oxygen form an explosive mixture

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This question was previously asked in

UPSC CDS-1 – 2016

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The statement that is not correct is “Hydrogen has no isotopes”.

Hydrogen is indeed an element and the lightest element. A mixture of hydrogen and oxygen is highly explosive. However, hydrogen has three naturally occurring isotopes: Protium (¹H), Deuterium (²H or D), and Tritium (³H or T). They differ in the number of neutrons (0, 1, and 2 respectively) while all having one proton.

Protium is the most common isotope of hydrogen, making up over 99.98% of natural hydrogen. Deuterium is also stable, while Tritium is radioactive. Deuterium is often referred to as heavy hydrogen, and water containing it (D₂O) is called heavy water.