Hydrogen (H)

Chemical Properties Overview

Hydrogen occupies a unique position. While it is placed above copper in the electrochemical series (standard electrode potential of H⁺/H₂ is 0.00 V), indicating its ability to displace less reactive metals from acids, it does not fit neatly into the general metal reactivity series.
Electropositive Character: Can lose one electron to form H⁺ (proton), which is characteristic of metals.
Electronegative Character: Can gain one electron to form H⁻ (hydride ion), characteristic of non-metals (e.g., in ionic hydrides like NaH).

Pauling electronegativity: 2.20. This intermediate value allows it to form both ionic (with highly electropositive metals) and covalent bonds (with non-metals).

Diatomic Nature: Exists as H₂ gas, with a very strong H-H single covalent bond (bond dissociation enthalpy = 435.88 kJ/mol).
Inertness at Room Temperature: Due to high bond dissociation enthalpy, H₂ is relatively unreactive at room temperature.
Reactivity at Higher Temperatures/Presence of Catalysts: Becomes highly reactive at elevated temperatures or in the presence of suitable catalysts (e.g., Pt, Pd, Ni).
Reducing Agent: Acts as a powerful reducing agent, especially at higher temperatures, reducing metal oxides and organic compounds.

Hydrogen burns vigorously in air or pure oxygen.

Conditions: Ignites with a pale blue flame when a burning splinter is introduced, or with an electric spark.
Reaction: Forms water. The reaction is highly exothermic. $2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \xrightarrow{\text{Spark/Heat}} 2\text{H}_2\text{O}\text{(l)} + \text{Energy}$
Explosive Mixture: A mixture of hydrogen and oxygen (or air) in certain proportions (e.g., 2:1 by volume H₂:O₂) is highly explosive.

Hydrogen gas (H₂) does not chemically react with water or steam under normal conditions. It is sparingly soluble in water.

The question might implicitly refer to reactions where hydrogen is formed from water/steam, which are common preparation methods, but as a direct reaction of hydrogen with water, there is no significant chemical reaction.

Hydrogen gas (H₂) is generally unreactive with dilute or concentrated acids and bases under normal conditions.

Hydrogen itself is largely non-reactive towards acids and bases because it does not possess acidic or basic functional groups to interact directly in a typical acid-base manner.
Similar to the “Action of Water and Steam” section, if the question implicitly refers to reactions where hydrogen is produced by the action of acids/bases on metals, those are different reactions of metals with acids/bases, yielding hydrogen.

Hydrogen gas is typically identified by its characteristic combustion property.

Procedure: A burning splint is introduced into a test tube containing the gas.
Observation: If the gas is hydrogen, the burning splint is extinguished with a sharp, characteristic “pop” sound. This “pop” is due to the rapid and localized combustion of hydrogen with residual oxygen inside the test tube.
Reaction: $2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \xrightarrow{\text{Heat}} 2\text{H}_2\text{O}\text{(l)}$