BPSC 70th CCE Re-exam Prelims General Studies · Question 66 of 142

Correct — A, By compressing hydrogen gas. The word doing the work in this stem is 'physically', because hydrogen storage is formally divided into exactly two families and the division turns on whether the hydrogen molecule survives unchanged. The United States Department of Energy sets it out in these words: 'Hydrogen can be stored physically as either a gas or a liquid. Storage of hydrogen as a gas typically requires high-pressure tanks (350–700 bar) … Storage of hydrogen as a liquid requires cryogenic temperatures because the boiling point of hydrogen at one atmosphere pressure is −252.8 °C. Hydrogen can also be stored on the surfaces of solids (by adsorption) or within solids (by absorption).' The first two of those are the physical methods; the third family — hydrogen bound into or onto a material — is the material-based or chemical route. Compressing the gas into a high-pressure tank changes nothing about the molecule; it only pushes more of them into the same volume, which is precisely what 'physical' means here. Why compression is needed at all is worth knowing, because it is the whole engineering problem of a hydrogen economy: hydrogen has the highest energy per unit mass of any fuel, but at ordinary temperature and pressure it is so thin that its energy per unit volume is very low. A kilogram of it carries a lot of energy and occupies an absurd amount of space, so the fuel must be squeezed to 350 or 700 bar, or chilled to 20 kelvin as a liquid, before a vehicle can carry a useful amount.

• (b)In form of hydrides — This is real storage, and it is the standard example of the other family. In a metal hydride, a complex hydride or a chemical hydrogen carrier, the hydrogen is chemically bound into a solid compound and has to be released by heating or by a chemical reaction. The Department of Energy's own classification puts hydrogen held within solids by absorption on the material-based side of the line, opposite the physical side. It is the correct answer to the question 'how is hydrogen stored chemically', which is not what was asked.
• (c)In form of water — Water is where hydrogen comes from, not where it is kept. H₂O is a stable compound; recovering hydrogen from it means electrolysis, which costs more energy than the hydrogen will later give back. Calling water a storage medium confuses a feedstock with a store — and in any case a compound is chemical, not physical, so this option fails the stem's adverb twice over.
• (d)None of the above — The escape needs all three named statements to fail, and the first states the textbook physical method exactly. The only reading that could support it is pedantic — that liquefaction at −252.8 °C is also a physical method and the option names only compression — but naming one true physical method is not the same as claiming it is the only one, and the stem asks how, not how exclusively.

Hydrogen is attractive as a fuel for two reasons and difficult for one. It has the highest energy per kilogram of any fuel — roughly three times that of petrol — and burning it yields only water vapour, with no carbon dioxide, sulphur oxides or particulates. The difficulty is density: as a gas at room temperature and atmospheric pressure it carries very little energy per litre, so storage dominates the engineering. The three physical answers are compression to 350 or 700 bar in a reinforced tank, liquefaction at cryogenic temperature below −252.8 °C, and cryo-compression combining both. The material answers store the hydrogen inside or on a substance — metal hydrides, complex hydrides, chemical carriers such as ammonia or liquid organic hydrogen carriers, and high-surface-area sorbents that hold hydrogen by adsorption. India's policy interest is recent and specific: the Union Cabinet approved the National Green Hydrogen Mission on 4 January 2023 with an initial outlay of Rs 19,744 crore, targeting at least 5 million metric tonnes of green hydrogen a year by 2030 alongside about 125 GW of new renewable capacity, and the abatement of nearly 50 million tonnes of greenhouse gas emissions annually.

Answer this by classifying rather than recalling. Ask of each option: does the hydrogen molecule remain a hydrogen molecule? Compress a gas and it does — you have simply crowded the molecules. Bind it into a hydride and it does not — it is now part of a compound and must be chemically released. Put it in water and it never was hydrogen gas in the first place. That single test sorts all three named options without any memory of the Department of Energy's terminology, and it generalises: in physics and chemistry, 'physical' change means composition unaltered, 'chemical' change means a new substance. Two extras are worth carrying. The pressures are examinable numbers — 350 bar and 700 bar for compressed gas — as is the liquefaction temperature, −252.8 °C at one atmosphere. And the reason storage matters at all is the contrast between hydrogen's energy per mass, which is the highest of any fuel, and its energy per volume, which is very low.

• US Department of Energy: hydrogen can be stored physically as a gas, in high-pressure tanks at 350 to 700 bar, or as a liquid at cryogenic temperature; its boiling point at one atmosphere is −252.8 °C.
• Material-based storage is the other family — hydrogen held on the surfaces of solids by adsorption or within solids by absorption, as in metal hydrides, complex hydrides and chemical hydrogen carriers.
• Hydrogen has the highest energy per unit mass of any fuel, but its low density at ambient temperature gives it a very low energy per unit volume — the reason storage is the central engineering problem.
• India's National Green Hydrogen Mission was approved by the Union Cabinet on 4 January 2023 with an initial outlay of Rs 19,744 crore, of which Rs 17,490 crore is for the SIGHT programme.
• The Mission's 2030 targets: at least 5 million metric tonnes of green hydrogen a year, about 125 GW of associated renewable capacity, over Rs 8 lakh crore of investment and abatement of nearly 50 million tonnes of greenhouse gas emissions a year.

• Reading past the adverb. 'Physically' is the entire question — hydrides are a correct answer to the chemical-storage version of it.
• Treating water as a hydrogen store. It is a feedstock that must be electrolysed, and the electrolysis consumes more energy than the hydrogen returns.
• Assuming compression is the only physical method. Liquefaction at −252.8 °C and cryo-compression are physical too; the option names one true method, which is enough.

BPSC asks new-energy technology as a one-line classification and hides the discrimination in a single adverb, so the item rewards careful reading over technical depth. UPSC prefers the surrounding properties — which fuel has the highest fuel value, which fuel causes the least pollution, what a fuel cell emits — and now asks green hydrogen through the Mission's targets rather than through storage engineering.

• practice — not a real PYQ

  Storing hydrogen as a liquid requires cooling it below approximately

  • (a)−78 °C
  • (b)−183 °C
  • (c)−253 °C
  • (d)−40 °C

  Answer(c) −253 °C — the boiling point of hydrogen at one atmosphere is −252.8 °C, which is why liquid hydrogen storage needs cryogenic equipment. About −183 °C is the boiling point of oxygen and −78 °C is where solid carbon dioxide sublimes.

• practice — not a real PYQ

  The National Green Hydrogen Mission, approved by the Union Cabinet in January 2023, targets an annual green hydrogen production capacity by 2030 of at least

  • (a)1 million metric tonnes
  • (b)5 million metric tonnes
  • (c)20 million metric tonnes
  • (d)50 million metric tonnes

  Answer(b) 5 million metric tonnes — with about 125 GW of associated renewable energy capacity, against an initial Mission outlay of Rs 19,744 crore.