Reaction with litmus paper
Hydrogen is neutral to litmus. When we introduce wet blue and red litmus paper into a jar of hydrogen, no change of color is noticed.
Combustibility
Pure hydrogen burns quietly in air or oxygen with a pale blue flame to give water and large amount of heat. However, hydrogen mixed with a little air, when ignited starts burning with a "pop" sound or explosion. If a mixture of 2 volumes of hydrogen and 1 volume of oxygen is ignited, a rather loud explosion occurs. If a pinch of well-divided platinum is added to this mixture, the mixture explodes instantly.
The following experiment is carried out to show the reactivity of hydrogen with oxygen. A jet of hydrogen is burnt along with oxygen within a large inverted funnel as shown in figure 9.9.
The vapour formed is passed through the Leibig's condenser. It gets condensed to form a colorless, odourless, tasteless and neutral liquid. This collected in a beaker. When a few drops of this liquid are added to white anhydrous copper sulphate, the latter turns blue. This proves that the liquid is water.
Remember :
Water can be considered as an oxide of hydrogen, namely dihydrogen monoxide.
Though hydrogen is combustible, it does not support combustion. A burning splinter introduced deep into a jar of hydrogen is put off, though the gas itself catches fire and burns at the mouth of the jar (Fig.9.10).
Combination with non-metals
Hydrogen combines with non-metals, to give the respective hydrides.
- With oxygen
Hydrogen burns in oxygen to form water. Instead of igniting the mixture, if a small quantity of well divided platinum is added to the mixture of the two gases, they instantly combine to form water. The platinum acts as a catalyst.
- With chlorine
Hydrogen combines with chlorine to form hydrogen chloride, in the presence of sunlight. In diffused sunlight, the reaction is slow. The reaction is extremely slow in the dark.
If a burning jet of hydrogen is introduced into a jar of chlorine, hydrogen will continue to burn in chlorine, and the greenish yellow color of chlorine gradually disappears, due to the formation of colorless hydrogen chloride. (Fig.9.11).
- With sulphur
When hydrogen is passed through boiling sulphur, a gas with an unpleasant odour of rotten eggs is evolved. When the gas is tested with lead acetate paper, the paper turns black. This shows the presence of hydrogen sulphide (Fig.9.12).
- With nitrogen
Since nitrogen is a less reactive gas, it needs the presence of catalysts and promoters and corresponding suitable conditions of temperature, pressure, to combine with hydrogen. Under these conditions it reacts to form ammonia.
Remember :
A promoter is a substance that increases the efficiency of a catalyst.
Reaction with metals
As hydrogen behaves like a metal, it usually does not combine with metals. However, highly electro-positive metals like potassium, sodium and calcium combine with hydrogen, to form their respective hydrides. In these hydrides, hydrogen has a negative valency (H-1).
Remember :
On the other hand, when hydrogen combines with non-metals, it forms compounds, which on dissolving in water, produce positively charged hydrogen ions (H+) e.g., hydrogen sulphide, hydrogen chloride etc.
Action with metallic oxides - As a reducing agent
Hydrogen acts as a reducing agent when acting with some metallic oxides. A reaction in which oxygen is taken away from a compound is known as 'Reduction'. When passed over heated metallic oxides, hydrogen reduces the oxides to the respective metals by removing oxygen.
A combustion tube with a wide diameter is taken. Three crucibles, with small amounts of copper oxide, lead oxide and ferric oxide respectively, are placed in the combustion tube. The tube is then strongly heated and hydrogen passed through it. All the three oxides get reduced to the respective metals (Fig. 9.13).
However, oxides of potassium, sodium, calcium, magnesium and aluminium do not lose their oxygen in this manner.
