75 free MCQs on Hydrogen with worked answers and explanations. The simplest and most abundant element in the universe. Study its unique position in the periodic table, isotopes (protium, deuterium, tritium), properties of water and hydrogen peroxide, and industrial uses.
Take the timed Hydrogen chapterwise test →Below are 75 practice questions on Hydrogen, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Hydrogen notes.
Each water molecule's O-H bonds (covalent, solid) can hydrogen-bond (dashed) to neighbouring molecules, forming an extended network responsible for water's unusually high boiling point and density anomaly.
Q1.
Hydrogen resembles the halogens (Group 17) in which one of these respects?
Answer: A. It is one electron short of a noble-gas shell
Why: Like the halogens, hydrogen is one electron short of the nearest noble-gas configuration (He) and can gain an electron to form the hydride ion H<sup>-</sup>, so it is sometimes placed with Group 17.
Q2.
Which of the following is classified as an electron-deficient hydride?
Answer: B. Diborane, B<sub>2</sub>H<sub>6</sub>
Why: Diborane B<sub>2</sub>H<sub>6</sub> has fewer electrons than needed for normal covalent bonds, making it an electron-deficient hydride of the Group 13 element boron.
Q3.
Which element has the greatest tendency to form covalent molecular hydrides?
Answer: C. Carbon, a p-block non-metal
Why: p-block non-metals such as carbon form covalent molecular hydrides; s-block metals like Na, K and Ca form ionic (saline) hydrides instead.
Q4.
Why is dihydrogen relatively unreactive at ordinary room temperature?
Answer: D. Its H–H bond enthalpy is very high
Why: The strong H–H bond (about 435.9 kJ mol<sup>-1</sup>) must be broken for a reaction, so dihydrogen is comparatively inert unless heated or a catalyst is used.
Q5.
Hydrogen peroxide is stored in wax-lined bottles kept in the dark because:
Answer: A. Light and rough surfaces catalyse its decomposition
Why: H<sub>2</sub>O<sub>2</sub> decomposes to water and oxygen; light, heat and ions present in ordinary glass catalyse this, so it is kept in dark wax-lined or plastic containers.
Q6.
What is the atomic number of hydrogen?
Answer: A. 1
Why: Hydrogen has atomic number 1 (one proton) and is the lightest and most abundant element in the universe.
Q7.
How many isotopes does hydrogen have?
Answer: A. 3 (protium, deuterium, tritium)
Why: Hydrogen has three isotopes: protium (1H, no neutrons), deuterium (2H, 1 neutron), and tritium (3H, 2 neutrons, radioactive).
Q8.
Heavy water is:
Answer: A. D<sub>2</sub>O (deuterium oxide)
Why: Heavy water is D<sub>2</sub>O, where normal hydrogen is replaced by deuterium (2H).
Q9.
Hydrogen gas is produced in the laboratory by:
Answer: A. Reaction of zinc with dilute sulfuric acid
Why: Zn + H<sub>2</sub>SO<sub>4</sub> (dil) → ZnSO<sub>4</sub> + H<sub>2</sub> is the common lab method to produce hydrogen gas.
Q10.
Which process is used industrially to produce hydrogen from natural gas?
Answer: A. Steam reforming
Why: Steam reforming: CH<sub>4</sub> + H<sub>2</sub>O → CO + 3H<sub>2</sub> (catalysed by Ni at high temperature) is the main industrial route.
Q11.
Hydrogen burns in air to produce:
Answer: A. Water
Why: 2H<sub>2</sub> + O<sub>2</sub> → 2H<sub>2</sub>O; hydrogen burns cleanly producing only water.
Q12.
What is the oxidation state of hydrogen in metal hydrides (e.g., NaH)?
Answer: A. -1
Why: In metal hydrides, hydrogen exists as H- (hydride ion) with oxidation state -1.
Q13.
What is the oxidation state of hydrogen in compounds with non-metals (e.g., HCl)?
Answer: A. +1
Why: In most compounds with non-metals, hydrogen has oxidation state +1.
Q14.
Hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) is a powerful:
Answer: A. Oxidising agent
Why: H<sub>2</sub>O<sub>2</sub> is primarily an oxidising agent; it can also act as a reducing agent in reactions with stronger oxidants.
Q15.
The purest form of water is:
Answer: A. Distilled water
Why: Distilled water has most dissolved impurities removed and is considered very pure for laboratory use.
Q16.
Hard water contains dissolved salts of:
Answer: A. Calcium and magnesium
Why: Hard water contains dissolved Ca<sup>2+</sup> and Mg<sup>2+</sup> salts (bicarbonates, sulfates, chlorides) that prevent lathering with soap.
Q17.
Temporary hardness of water is caused by:
Answer: A. Ca(HCO<sub>3</sub>)<sub>2</sub> and Mg(HCO<sub>3</sub>)<sub>2</sub>
Why: Temporary hardness is caused by bicarbonates of calcium and magnesium, which can be removed by boiling.
Q18.
Permanent hardness of water is caused by:
Answer: A. CaSO<sub>4</sub>, MgSO<sub>4</sub>, CaCl<sub>2</sub>, MgCl<sub>2</sub>
Why: Permanent hardness is due to sulfates and chlorides of Ca and Mg, which cannot be removed by boiling.
Q19.
Which method removes both temporary and permanent hardness?
Answer: A. Ion exchange method
Why: Ion exchange resins replace Ca<sup>2+</sup> and Mg<sup>2+</sup> ions with Na+ (or H+), removing all types of hardness.
Q20.
H<sub>2</sub>O<sub>2</sub> decomposes on exposure to light to give:
Answer: A. H<sub>2</sub>O and O<sub>2</sub>
Why: 2H<sub>2</sub>O<sub>2</sub> → 2H<sub>2</sub>O + O<sub>2</sub>; this is why H<sub>2</sub>O<sub>2</sub> is stored in dark/coloured bottles.
Q21.
Hydrogen is used as a fuel because:
Answer: A. It has very high calorific value and burns cleanly
Why: Hydrogen has the highest energy density per unit mass and produces only water on combustion.
Q22.
Which type of hydride is formed by alkali metals?
Answer: A. Ionic (saline) hydrides
Why: Alkali metals (Group 1) react with H<sub>2</sub> to form ionic hydrides containing H- ions (e.g., NaH, LiH).
Q23.
Water has an unusually high boiling point (100°C) due to:
Answer: A. Extensive hydrogen bonding
Why: Water molecules form strong intermolecular hydrogen bonds, requiring extra energy to vaporise.
Q24.
The dihydrogen molecule (H<sub>2</sub>) has a bond order of:
Answer: A. 1
Why: H<sub>2</sub> has two electrons in the bonding MO and none in antibonding: bond order = (2-0)/2 = 1.
Q25.
Which of the following is NOT an isotope of hydrogen?
Answer: A. Helium, the noble gas with two protons and two neutrons in its common isotope
Why: Helium (He) is a separate element. The three isotopes of hydrogen are protium (H), deuterium (D), and tritium (T).
Q26.
In 2MnO<sub>4</sub><sup>-</sup> + 5H<sub>2</sub>O<sub>2</sub> + 6H<sup>+</sup> → 2Mn<sup>2+</sup> + 5O<sub>2</sub> + 8H<sub>2</sub>O, hydrogen peroxide behaves as a:
Answer: B. Reducing agent, its oxygen going from -1 to 0
Why: Here H<sub>2</sub>O<sub>2</sub> reduces MnO<sub>4</sub><sup>-</sup> to Mn<sup>2+</sup>; its own oxygen is oxidised from -1 to 0 (O<sub>2</sub>), so it acts as a reducing agent.
Q27.
A hydrogen peroxide sample marked '10 volume' means that:
Answer: C. 1 mL gives 10 mL of O<sub>2</sub> at STP on decay
Why: Volume strength is the volume of O<sub>2</sub> (at STP) liberated per unit volume of solution; '10 volume' gives 10 mL of O<sub>2</sub> per mL of the solution.
Q28.
The catalytic reaction CO + 2H<sub>2</sub> → CH<sub>3</sub>OH is used industrially to manufacture:
Answer: D. Methanol
Why: Catalytic combination of CO and H<sub>2</sub> (synthesis gas) yields methanol, an important industrial hydrogenation of carbon monoxide.
Q29.
Assertion: The two O–H bonds of H<sub>2</sub>O<sub>2</sub> do not lie in one plane. Reason: It has an open-book (non-planar) structure.
Answer: A. Both true and the reason explains it
Why: H<sub>2</sub>O<sub>2</sub> has a non-planar open-book structure in which the two O–H bonds lie in different planes, so the reason correctly explains the assertion.
Q30.
Which statement about the three isotopes of hydrogen is correct?
Answer: B. Protium, deuterium, tritium have 0,1,2 neutrons
Why: Protium, deuterium and tritium all have one proton but 0, 1 and 2 neutrons respectively; protium is by far the most abundant isotope.
Q31.
The water-gas shift reaction is:
Answer: A. CO + H<sub>2</sub>O → CO<sub>2</sub> + H<sub>2</sub>
Why: The water-gas shift reaction (CO + H<sub>2</sub>O → CO<sub>2</sub> + H<sub>2</sub>) increases H<sub>2</sub> yield from syngas by converting CO with steam.
Q32.
Which compound acts as both oxidising and reducing agent?
Answer: A. H<sub>2</sub>O<sub>2</sub>
Why: H<sub>2</sub>O<sub>2</sub> can oxidise (it gains electrons) or reduce (it donates electrons to a stronger oxidant like KMnO<sub>4</sub>).
Q33.
The concentration of H<sub>2</sub>O<sub>2</sub> is expressed in:
Answer: A. Volume strength (V10, V20) based on O<sub>2</sub> liberated
Why: '10 volume' H<sub>2</sub>O<sub>2</sub> means 1 L of the solution liberates 10 L of O<sub>2</sub> at STP.
Q34.
Bleaching by H<sub>2</sub>O<sub>2</sub> is due to its:
Answer: A. Oxidising action (releases nascent oxygen)
Why: H<sub>2</sub>O<sub>2</sub> bleaches by releasing nascent oxygen that oxidises coloured compounds, destroying chromophore groups.
Q35.
The reaction H<sub>2</sub>O<sub>2</sub> + KI → I<sub>2</sub> + KOH + H<sub>2</sub>O demonstrates H<sub>2</sub>O<sub>2</sub> acting as:
Answer: A. Oxidising agent (oxidises I- to I<sub>2</sub>)
Why: H<sub>2</sub>O<sub>2</sub> oxidises I- (from KI) to I<sub>2</sub>, itself being reduced to water. This is its oxidising action.
Q36.
Why does ice float on water?
Answer: A. Water at 4°C is densest; ice has open hydrogen-bonded lattice making it less dense than liquid water
Why: In ice, hydrogen bonds create an open tetrahedral lattice with more space between molecules than in liquid water, making ice less dense.
Q37.
The softening of water by Clark's method involves adding:
Answer: A. Slaked lime Ca(OH)<sub>2</sub> to remove temporary hardness by precipitating CaCO<sub>3</sub>
Why: Clark's method adds just enough lime to convert soluble bicarbonates to insoluble carbonates: Ca(HCO<sub>3</sub>)<sub>2</sub> + Ca(OH)<sub>2</sub> → 2CaCO<sub>3</sub> + 2H<sub>2</sub>O.
Q38.
The Permutit process uses which substance to soften water?
Answer: A. Sodium aluminium silicate (zeolite/permutit) ion exchanger
Why: Permutit (zeolite, Na2Al2Si2O8) exchanges Na+ ions for Ca<sup>2+</sup> and Mg<sup>2+</sup> hardness ions, softening the water.
Q39.
Interstitial hydrides are formed by:
Answer: A. d and f block transition metals (e.g., Pd, La) absorbing H<sub>2</sub> into lattice gaps
Why: Transition and lanthanide metals can absorb large amounts of H<sub>2</sub> into interstitial (gap) sites in their crystal lattices.
Q40.
The Haber process produces ammonia from:
Answer: A. N<sub>2</sub> and H<sub>2</sub> (using Fe catalyst, high pressure, 400-500°C)
Why: The Haber process: N<sub>2</sub> + 3H<sub>2</sub> ⇌ 2NH<sub>3</sub> (Fe catalyst, 150-200 atm, 400-500°C).
Q41.
What is syngas?
Answer: A. A mixture of CO and H<sub>2</sub> produced from steam reforming of hydrocarbons
Why: Synthesis gas (syngas) is a CO/H<sub>2</sub> mixture used as feedstock for methanol, ammonia, and Fischer-Tropsch fuels.
Q42.
The bond dissociation energy of H<sub>2</sub> is approximately:
Answer: A. 436 kJ/mol
Why: The H-H bond dissociation energy is about 436 kJ/mol, relatively strong for a single bond.
Q43.
H<sub>2</sub>O<sub>2</sub> structure has a dihedral angle of about 111° in the gas phase because:
Answer: A. Lone pair repulsion on oxygen atoms causes non-planar structure
Why: Each oxygen in H<sub>2</sub>O<sub>2</sub> has two lone pairs; their repulsion twists the molecule into a non-planar 'book' structure.
Q44.
Nascent hydrogen is:
Answer: A. Highly reactive atomic hydrogen formed in situ (not H<sub>2</sub> gas)
Why: Nascent hydrogen is freshly generated atomic H (before it dimerises to H<sub>2</sub>), highly reactive due to unpaired electron.
Q45.
The ortho and para forms of dihydrogen differ in:
Answer: A. Relative nuclear spin orientation of the two H nuclei (parallel vs antiparallel)
Why: ortho-H<sub>2</sub> has parallel proton spins (triplet); para-H<sub>2</sub> has antiparallel spins (singlet). They have slightly different physical properties.
Q46.
H<sub>2</sub>O<sub>2</sub> acts as a reducing agent when it reacts with:
Answer: A. Acidified KMnO<sub>4</sub> (a stronger oxidising agent)
Why: When a stronger oxidant like acidified KMnO<sub>4</sub> reacts with H<sub>2</sub>O<sub>2</sub>, H<sub>2</sub>O<sub>2</sub> reduces MnO<sub>4</sub><sup>-</sup> by donating electrons: 2KMnO<sub>4</sub> + 5H<sub>2</sub>O<sub>2</sub> + 3H<sub>2</sub>SO<sub>4</sub> → 2MnSO<sub>4</sub> + 5O<sub>2</sub> + K<sub>2</sub>SO<sub>4</sub> + 8H<sub>2</sub>O.
Q47.
What percentage concentration is commercial H<sub>2</sub>O<sub>2</sub>?
Answer: A. 3% (household), 30% (laboratory), 90%+ (industrial/rocket fuel)
Why: H<sub>2</sub>O<sub>2</sub> is sold in different concentrations: 3% (antiseptic), ~6-9% (hair bleaching), ~30% (lab), >90% (propellant).
Q48.
Calgon (sodium hexametaphosphate) softens water by:
Answer: A. Forming soluble complexes with Ca<sup>2+</sup> and Mg<sup>2+</sup> ions (sequestration)
Why: Calgon sequestrates Ca<sup>2+</sup> and Mg<sup>2+</sup> by forming stable soluble complexes, preventing them from causing hardness.
Q49.
Liquid hydrogen is used as:
Answer: A. Rocket fuel (cryogenic propellant)
Why: Liquid H<sub>2</sub> is used as cryogenic rocket fuel; it has the highest specific impulse of any chemical propellant.
Q50.
The dielectric constant of water (80) is much higher than most solvents because:
Answer: A. Strong hydrogen bonding creates a highly ordered, polar medium that stabilises ions effectively
Why: The high dielectric constant of water means it strongly diminishes the electrostatic attraction between ions, making it an excellent ionic solvent.
Q51.
A hydrogen peroxide solution is labelled '20 volume'. Its percentage strength (w/v) is closest to:
Answer: C. About 6 percent
Why: Percentage strength (w/v) = volume strength / 5.6, so 20/5.6 is roughly 3.6, giving about 6% H<sub>2</sub>O<sub>2</sub> expressed as grams per 100 mL.
Q52.
H<sub>2</sub>O<sub>2</sub> liberates iodine from acidified KI yet decolourises acidified KMnO<sub>4</sub>. This dual behaviour shows that it:
Answer: D. Can act as both oxidant and reductant
Why: With KI it oxidises I<sup>-</sup> to I<sub>2</sub> (oxidant); with KMnO<sub>4</sub> it reduces Mn(VII) to Mn(II) (reductant). So H<sub>2</sub>O<sub>2</sub> is both an oxidising and a reducing agent.
Q53.
Deuterium oxide (D<sub>2</sub>O) is preferred over ordinary water as a moderator in nuclear reactors because:
Answer: A. It slows neutrons while absorbing very few
Why: D<sub>2</sub>O moderates fast neutrons effectively while having a very low neutron-absorption cross-section, letting more neutrons sustain the chain reaction than ordinary water would.
Q54.
Reactions using deuterium generally proceed more slowly than with ordinary hydrogen because:
Answer: B. D–X bonds have lower zero-point energy and break slower
Why: Heavier deuterium gives a lower zero-point vibrational energy, so the D–X bond is effectively stronger and harder to break, slowing reactions in which that bond is broken (kinetic isotope effect).
Q55.
In C + H<sub>2</sub>O → CO + H<sub>2</sub> (steam on hot coke), the CO is removed to get pure dihydrogen by:
Answer: C. Shifting CO with steam to CO<sub>2</sub>, then absorbing it
Why: In the water-gas shift CO + H<sub>2</sub>O reacts over iron chromate to give CO<sub>2</sub> + H<sub>2</sub>; the CO<sub>2</sub> is then scrubbed out under pressure, leaving pure dihydrogen.
Q56.
Tritium is used in which applications?
Answer: A. Nuclear fusion fuel and luminescent markers (radioactive beta emitter)
Why: Tritium (3H, T) is a radioactive hydrogen isotope used in thermonuclear fusion (D-T reaction) and in self-luminous watch dials and exit signs.
Q57.
The kinetic isotope effect (KIE) in chemistry means:
Answer: A. Reactions with lighter isotopes (H) are faster than with heavier ones (D) because zero-point energy is higher for C-H
Why: Primary KIE: C-H bonds break faster than C-D bonds because C-H has higher zero-point energy, lower activation barrier.
Q58.
In the hydrogen economy concept, hydrogen is produced by:
Answer: A. Electrolysis of water using renewable electricity (green hydrogen)
Why: Green hydrogen is produced by electrolysis of water powered by renewable energy (solar/wind), with zero carbon emissions.
Q59.
The anomalous properties of water compared to H<sub>2</sub>S, H<sub>2</sub>Se, H<sub>2</sub>Te are due to:
Answer: A. Hydrogen bonding in water raises its boiling point far above the trend expected from molecular weight
Why: Extrapolating the boiling points of H<sub>2</sub>S (-61°C), H<sub>2</sub>Se (-41°C), H<sub>2</sub>Te (-2°C) would predict H<sub>2</sub>O to boil at about -80°C. Hydrogen bonding raises it to 100°C.
Q60.
What is the role of palladium in hydrogen technology?
Answer: A. Pd absorbs up to 900 times its own volume of H<sub>2</sub> (hydrogen storage) and catalyses hydrogenation
Why: Palladium uniquely dissolves large amounts of hydrogen into its lattice (forming PdHx), useful for purification and storage. It also catalyses hydrogenation reactions.
Q61.
The Born-Oppenheimer approximation applied to H<sub>2</sub><sup>+</sup> (simplest molecular ion) allows:
Answer: A. Separation of nuclear and electronic motion to solve the Schrodinger equation exactly
Why: H<sub>2</sub><sup>+</sup> has only one electron and two nuclei; with the Born-Oppenheimer approximation (fixed nuclei), it has an exact quantum mechanical solution.
Q62.
Why does H<sub>2</sub>O<sub>2</sub> have a higher boiling point (150°C) than water (100°C) despite having similar molecular weight?
Answer: A. Both O atoms in H<sub>2</sub>O<sub>2</sub> form hydrogen bonds (more H-bond donors and acceptors per molecule)
Why: H<sub>2</sub>O<sub>2</sub> has two O-H bonds and two oxygen lone pairs, enabling more extensive hydrogen bonding than water (one O, two O-H).
Q63.
What is 'hydrogen embrittlement'?
Answer: A. Absorption of H<sub>2</sub> into metal lattice weakens metallic bonds, making the metal brittle and prone to cracking
Why: Hydrogen embrittlement occurs when atomic H diffuses into metal (e.g., steel), occupying voids and creating internal pressure that causes cracking.
Q64.
The para-hydrogen to ortho-hydrogen ratio at very low temperature approaches:
Answer: A. 100% para-H<sub>2</sub> (para is lower energy at low T)
Why: para-H<sub>2</sub> (antiparallel nuclear spins) is the lower energy ground state; at near 0 K, essentially all H<sub>2</sub> is para form.
Q65.
Hydrogen fuel cells generate electricity by:
Answer: A. Electrochemical oxidation of H<sub>2</sub> at anode, reduction of O<sub>2</sub> at cathode, water as byproduct
Why: In a PEM fuel cell: H<sub>2</sub> → 2H+ + 2e- (anode); O<sub>2</sub> + 4H+ + 4e- → 2H<sub>2</sub>O (cathode). Electrical energy from chemical energy without combustion.
Q66.
What is the significance of the 21 cm hydrogen emission line in astronomy?
Answer: A. Spin-flip transition of electron in hydrogen; used to map galactic structure and detect neutral hydrogen clouds
Why: The 21 cm line arises from the hyperfine transition in ground-state H (electron spin flip); it penetrates dust clouds, enabling radio mapping of galaxies.
Q67.
In which reaction does H<sub>2</sub>O<sub>2</sub> act as an oxidising agent in acidic medium?
Answer: A. H<sub>2</sub>O<sub>2</sub> + 2Fe<sup>2+</sup> + 2H+ → 2Fe<sup>3+</sup> + 2H<sub>2</sub>O
Why: H<sub>2</sub>O<sub>2</sub> oxidises Fe<sup>2+</sup> to Fe<sup>3+</sup> in the Fenton reaction; this demonstrates its oxidising role in acidic solution.
Q68.
Which condition favours formation of ionic hydrides over covalent hydrides?
Answer: A. Low electronegativity and large size of the metal (strong electropositive character)
Why: Ionic hydrides form when electropositive metals (Li, Na, Ca) transfer electrons completely to H; covalent hydrides form when electronegativity difference is small.
Q69.
The dielectric constant of heavy water (D<sub>2</sub>O) compared to ordinary water (H<sub>2</sub>O):
Answer: A. Slightly lower (~78 vs 80) because D<sub>2</sub>O hydrogen bonds are slightly stronger making it less polarisable
Why: D<sub>2</sub>O has slightly stronger O-D...O hydrogen bonds than O-H...O, making the structure more rigid and slightly less polarisable.
Q70.
Hydrogen can exist as a metallic solid under extreme pressure. This was first observed in:
Answer: A. Theoretical prediction; metallic H may exist in gas giant planet cores (Jupiter)
Why: Metallic hydrogen is predicted to form above 400 GPa; experimental evidence is disputed. It likely exists in Jupiter's core.
Q71.
The equilibrium 2H<sub>2</sub>O(l) ⇌ H<sub>3</sub>O<sup>+</sup>(aq) + OH-(aq) has Kw = 10-14 at 25°C. At 37°C (body temperature), Kw is:
Answer: A. Greater than 10-14 (autodissociation is endothermic, favoured by higher T)
Why: Water autoionisation is endothermic; increasing temperature shifts equilibrium right, increasing Kw above 10-14.
Q72.
In terms of bond enthalpy, the reaction H<sub>2</sub> → 2H requires approximately 436 kJ/mol. This means:
Answer: A. H<sub>2</sub> is thermally stable; significant energy must be supplied to create atomic hydrogen
Why: The high H-H bond dissociation enthalpy means H<sub>2</sub> is very stable thermally; atomic H is highly reactive and rare under normal conditions.
Q73.
Why is water considered a universal solvent?
Answer: A. High polarity, small size, and ability to hydrogen bond with solutes enables it to dissolve many ionic and polar covalent substances
Why: Water's strong polarity and hydrogen-bonding ability stabilise ions (via hydration) and polar solutes, dissolving more substances than any other solvent.
Q74.
Proton transfer reactions (Bronsted acid-base) are fast because:
Answer: A. Protons (H+) are very small and have no electrons, so tunnelling and fast diffusion in water occur via the Grotthuss mechanism
Why: In water, proton transfer occurs via the Grotthuss mechanism: cooperative H-bond network rearrangements relay protons faster than normal diffusion.
Q75.
The H–H bond enthalpy (436 kJ/mol) is one of the highest for a single bond, which explains why dihydrogen is:
Answer: B. relatively inert at room temperature
Why: The very strong H–H bond makes dihydrogen kinetically unreactive at room temperature; reactions need heat, light or a catalyst to break the bond.