Question

Give reasons :
(i) Conc. $HNO_{3}$ can be transported in Aluminium container.
(ii) A mixture of dilute NaOH and Aluminium pieces is used to open drain.
(iii) Graphite is used as lubricant.

Answer 1

(i) A thin protective layer of aluminium oxide is formed on the surfaces of the aluminium containers due to the chemical reaction taking place between Aluminium and Concentrated ${ HNO }_{ 3 }$. Now this oxide layer prevents further reaction thereby making aluminium passive.  

$2Al\quad +6HNO_{ 3 }+\longrightarrow \underbrace { { Al }_{ 2 }{ O }_{ 3 }(alumina) }_{ a\quad protective\quad layer } \quad +{ 6NO }_{ 2 }+{ 3H }_{ 2 }O$

(ii) When sodium hydroxide is made to react with aluminium, it forms sodium tetrahydroxoaluminate (III). During this reaction, hydrogen is also liberated. The pressure of this hydrogen gas helps in opening the blocked drains.

$2NaOH+2Al+{ 2H }_{ 2 }O\longrightarrow 2NaAlO_{ 2 }+{ 3H }_{ 2 }\uparrow$

(iii) The structure of graphite is a layered one. The Van-der-Waal's forces that hold the layers together are weak. This will lead to slipping over one another. This is the reason why graphite acts as a lubricant.