How is Hydrogen stored physically ?

Question

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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.

Why the other options are wrong — (b) 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) 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) 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.

Concept: 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.

State PCS 2025 Q66 — Science & Technology PYQ with Answer

State PCS 2025 Q66 — Science & Technology PYQ with Answer