Hydrogen is used for all of the following except extinguishing fire. Hydrogen gas itself is highly flammable and can even ignite explosively in the presence of air, oxygen or any other oxidizing gas. It does not work as an extinguisher and, in fact, can even make a fire worse by adding fuel to the fire. The other options listed are all valid uses of hydrogen. It is used in the manufacturing of ammonia, the synthesis of hydrochloric acid, the conversion of coal to petrol and in the manufacture of margarine.