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Which of the following acts as a pickling agent?
A. \[{\text{HN}}{{\text{O}}_3}\]
B. \[{\text{HCl}}\]
C. \[{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\]
D. \[{\text{HN}}{{\text{O}}_2}\]

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Last updated date: 22nd Jun 2024
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Answer
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Hint: During production of metals, many impurities or oxide layers are left on the surface of metal. These can be efficiently cleaned if strong acids are used.

Complete step by step solution: On large scales, some metals are refined or produced at extreme high temperatures. This leaves a layer of oxide layer on the surface of metal which decolourises the metal. This layer is called a scale. In order to remove this, the metal surface is treated with a copper solution which contains strong acids like \[{{\text{H}}_2}{\text{S}}{{\text{O}}_4}\]. This helps in removing the scale and gives the metal surface a clean finish.

Hence, option C is the correct answer.

Additional information: Pickling is a process used to treat metal surfaces to remove the impurities on them. It generally uses a copper solution containing strong acids. This complete solution is called pickle liquor.
The primary acid used in steelmaking is hydrochloric acid, although sulfuric acid was previously more common. Hydrochloric acid is more expensive but it pickles much faster while minimizing base metal loss.
This process is also used in jewellery making to remove the copper oxide layer that results from heating copper. A diluted sulfuric acid pickling bath is traditionally used.
However, this process also has certain disadvantages. The regents used are extremely difficult to handle since they are highly corrosive in nature. Also, the hydrogen released from the acids makes the metal brittle.

Note: Even though sulphuric acid is majorly used, the reagent changes as per the requirements. This process is also called acid cleaning.