NDA Chemistry · Hydrogen and Water

Properties of Hydrogen

Hydrogen is the lightest element — a colourless, odourless, diatomic gas that is almost insoluble in water and unreactive at room temperature because the H–H bond is very strong.

All Hydrogen and Water notes NDA Chemistry strategy

Why this matters

Four PYQs, mostly EASY-to-MODERATE recall: the colour of the gas, a 'which statement about dihydrogen is NOT correct' trap, and one HARD question on how a large volume of hydrogen is stored. The traps live in the named facts — syngas is CO plus hydrogen, not nitrogen-anything, and large volumes of hydrogen are held by interstitial transition-metal hydrides. Learn the physical-property line and the three hydride types and the marks are free.

Concept 1 of 2

Physical properties of dihydrogen

Intuition

Hydrogen is element number 1 — one proton, one electron, the simplest and lightest atom. In its natural state it exists as a diatomic molecule H₂ (dihydrogen). Because it is so light and the two atoms are held by a very strong bond, the gas is colourless, lighter than air, almost insoluble in water, and slow to react unless heated.

Definition

The physical-property facts the bank tests:

State and look — a colourless, odourless, tasteless gas at room temperature.
Density — the lightest of all gases; lighter than air, so it rises.
Solubility — almost insoluble in water.
Molecular form — exists as diatomic H₂ (dihydrogen), not single atoms.
Reactivity — inert (unreactive) at room temperature because the H–H bond dissociation enthalpy is very high (about 436 kJ/mol); it needs heat, light or a catalyst to react.

Property	Value / fact
Colour	Colourless (also odourless, tasteless)Q Hydrogen gas is colourless — a coloured-flame or coloured-gas option is always wrong.
Density vs air	Lighter than air (lightest of all gases)
Solubility in water	Almost insoluble
Molecular form	Diatomic, H₂ (dihydrogen)
Reactivity at room temperature	Inert — strong H–H bond, about 436 kJ/mol

Lightest, colourless, insoluble, diatomic, and unreactive until heated.

Practice this conceptself-check · 5 quick reps

From the bank · past-year question

Example 1Hydrogen and WaterEASY

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

[Q87 · Apr · 2022]

Hydrogen is colourless, not pale-blue

Hydrogen gas itself is colourless. It burns with a pale-blue flame, but the gas you collect is colourless — do not confuse the flame colour with the gas colour.

Concept 2 of 2

Types of hydrides and hydrogen storage

Intuition

When hydrogen combines with another element it forms a hydride, and the kind of hydride depends on the partner. Reactive metals give salt-like (ionic) hydrides; non-metals give covalent (molecular) hydrides; and many transition metals trap hydrogen inside their crystal lattice to give metallic hydrides. That last type is how a very large volume of hydrogen can be stored in a small metal block.

Definition

The three classes of hydride, by the partner element:

Ionic (saline) hydrides — hydrogen plus a highly reactive metal (alkali and alkaline-earth metals, e.g. NaH, CaH₂); the metal donates an electron and hydrogen becomes the H⁻ ion.
Covalent (molecular) hydrides — hydrogen plus a non-metal (e.g. CH₄, NH₃, H₂O, HCl); shared electron pairs, often gases or volatile liquids.
Metallic (interstitial / non-stoichiometric) hydrides — hydrogen absorbed into the lattice of a transition metal (e.g. palladium); the hydrogen sits between the metal atoms, so the formula is not a whole-number ratio. A very large volume of hydrogen can be stored this way.

Syngas (synthesis gas) is a separate fact: it is a mixture of carbon monoxide (CO) and hydrogen (H₂) — never nitrogen oxides.

Hydride type	Forms with	Example	Key point
Ionic / saline	Reactive metals (alkali, alkaline-earth)	NaH, CaH₂	Contains the H⁻ ion
Covalent / molecular	Non-metals	CH₄, NH₃, H₂O, HCl	Shared electron pairs
Metallic / interstitial	Transition metals (e.g. palladium)	PdH₍ₓ₎	Stores a very large volume of hydrogen; non-stoichiometricQ Large-volume hydrogen storage = non-stoichiometric (interstitial) hydrides, NOT hydrogen peroxide or simple hydrides.

Reactive metal → ionic; non-metal → covalent; transition metal → metallic (the storage type).

Practice this conceptself-check · 5 quick reps

From the bank · past-year question

Example 2Hydrogen and WaterHARD

A very large volume of hydrogen can be accommodated by making

[Q88 · Sep · 2019]

Syngas is CO + H₂, never NO₂ + H₂

Synthesis gas (syngas) is a mixture of carbon monoxide and hydrogen. A statement that syngas is a mixture of NO₂ and H₂ is the false one the bank plants.

Storage is interstitial hydrides, not H₂O₂

Large volumes of hydrogen are accommodated by non-stoichiometric (interstitial) hydrides of transition metals like palladium — not by hydrogen peroxide and not by simple alkali-metal hydrides.

Drill 1 more on types of hydrides and hydrogen storage

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

Physical properties of dihydrogen5 rows

Property	Value / fact
Colour	Colourless (also odourless, tasteless)Q Hydrogen gas is colourless — a coloured-flame or coloured-gas option is always wrong.
Density vs air	Lighter than air (lightest of all gases)
Solubility in water	Almost insoluble
Molecular form	Diatomic, H₂ (dihydrogen)
Reactivity at room temperature	Inert — strong H–H bond, about 436 kJ/mol

Lightest, colourless, insoluble, diatomic, and unreactive until heated.

Types of hydrides and hydrogen storage3 rows

Hydride type	Forms with	Example	Key point
Ionic / saline	Reactive metals (alkali, alkaline-earth)	NaH, CaH₂	Contains the H⁻ ion
Covalent / molecular	Non-metals	CH₄, NH₃, H₂O, HCl	Shared electron pairs
Metallic / interstitial	Transition metals (e.g. palladium)	PdH₍ₓ₎	Stores a very large volume of hydrogen; non-stoichiometricQ Large-volume hydrogen storage = non-stoichiometric (interstitial) hydrides, NOT hydrogen peroxide or simple hydrides.

Reactive metal → ionic; non-metal → covalent; transition metal → metallic (the storage type).

Watch out for (3)

Hydrogen is colourless, not pale-blue
→ Physical properties of dihydrogen
Syngas is CO + H₂, never NO₂ + H₂
→ Types of hydrides and hydrogen storage
Storage is interstitial hydrides, not H₂O₂
→ Types of hydrides and hydrogen storage

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