CHEMISTRY [2 Mark Question Answer]

Apparatus for laboratory preparation of hydrogen should be airtight and away from a naked flame because a mixture of hydrogen and air explodes violently when brought near a flame.

Hydrogen is combustible, but it does not support combustion. So, the candle burns in air or oxygen when brought near the mouth of a jar containing hydrogen but is extinguished when pushed inside the jar as the supply of oxygen is cut off.

Hydrogen is collected by the downward displacement of air because of

1. It is insoluble in water.
2. It forms an explosive mixture with air.

Hydrogen is evolved when dilute HCl reacts with magnesium which is placed above hydrogen in the activity series. However, this does not occur for metals below hydrogen such as mercury and silver. This is because only metals that are more reactive than hydrogen can displace it from HCl.

Dilute sulphuric acid.
Conc. nitric acid, even in its diluted form, is not used in the preparation of hydrogen from metals because it is a powerful oxidising agent and oxygen formed due to its decomposition oxidises hydrogen to give water, thus defeating the purpose of the reaction.
Conc.sulphuric acid is not used in the preparation of hydrogen as it will produce sulphur dioxide.

In the free state, hydrogen is found in traces in the earth's crust and atmosphere. Volcanic gases contain 0.025%, the earth's crust 0.98%, the earth's atmosphere 0.01% and the atmosphere of the Sun and stars 1.1%.

Potassium and sodium react violently with acid. Hence, potassium and sodium are not used for reaction with dilute hydrochloric acid or dilute sulphuricacid in the laboratory preparation of hydrogen.

Zinc granules are preferred over pure zinc in the lab preparation of hydrogen because the impurity present in granulated zinc is copper, whose catalysing effect speeds up the rate of the reaction.

In the free state, hydrogen is found in traces in the earth's crust and atmosphere.
In the combined state, plant and animal tissues are made of compounds of hydrogen with carbon, oxygen, and nitrogen.

Hydrogen is the first element in the periodic table. Its atomic number is 1, and it has only one electron in its valence shell. So, it belongs to the first group and the first period of the periodic table.