Why can oil prevent reactive metal to react, and how is oil useful?
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Reactive metals of group I and group II form different kinds of oxide and hydroxide when they come in contact with oxygen (or air) and water (or moisture present in air). Reactivity increases as one goes down the group. Thus, for group I metals, caesium is the most reactive and lithium is the least. Lithium has the uniqueness in that it also reacts with the nitrogen in the air to form lithium nitride. Therefore, these metals need to be stored out of contact with air to prevent their oxidation. Reactive metals also have the dangerous tendency of catching fire in contact with moisture, because of instantaneous ignition of the produced hydrogen, utilizing the heat of the parent reaction:
2Na + 2H_2O --> 2NaOH + H_2(g)
Depending on how far down the group a metal is placed, different means of protection is adopted to prevent direct contact of the metal surface with air, viz.
a) Highly reactive metals rubidium and caesium are normally stored in sealed glass tubes in vacuum or in an inert atmosphere to prevent air getting at them.
b) Reactive metals like lithium, sodium and potassium are stored in oil (usually kerosene or paraffin oil). Lithium has a very low density and floats on the oil, but there will be enough oil coating it to give it some protection, which is sufficient given its low reactivity. Sodium and potassium submerges in the oil and remains completely sealed from air. The metal is taken out of oil, and wiped dry when it is used.
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