Option 1: This statement is correct. Dihydrogen (H2) is indeed lighter than air and insoluble in water. This property is why it tends to rise and disperse in the atmosphere.

Option 2: This statement is correct. At room temperature, dihydrogen (H2) is inert because it has a high H-H bond dissociation enthalpy. This means that the bond between the two hydrogen atoms in a dihydrogen molecule is very strong and requires a significant amount of energy to break.

Option 3: This statement is correct. When dihydrogen (H2) reacts with alkali metals at high temperatures, it forms metal hydrides. This reaction is highly exothermic and releases a large amount of energy.

Option 4: This statement is incorrect. A mixture of NO2 (nitrogen dioxide) and H2 (dihydrogen) gas is not known as Syngas. Syngas, also known as synthesis gas, is a different gas mixture composed of carbon monoxide (CO) and hydrogen (H2). Syngas is commonly used in industrial processes as a feedstock for the production of various chemicals and fuels.

Therefore, the correct answer is option 4. Alert -