Question

Which of the following statements is false? (1) The metal that is protected by its own oxide layer is aluminium. (2) \Lambdaluminium vessels are casily corroded in coastal areas of sca, because the oxide layer is removed by salts present in sea water. (3) Aluminium vessels should not be washed with washing powders containing washing soda, because it reacts with aluminium to form soluble aluminate. (4) Aluminium dissolves in ammonium hydroxide liberating hydrogen.

Short answer

Statement (4) is false.

Step-by-step solution

Understanding the Statements

Carefully read each statement to understand what it claims about aluminium and its properties.

Verifying the Protection of Aluminium by Oxide Layer

Statement (1) claims aluminium is protected by its oxide layer. Verify this fact. Aluminium indeed forms a protective oxide layer that prevents corrosion.

Examining Aluminium Corrosion in Coastal Areas

Statement (2) mentions aluminium vessels are easily corroded in coastal areas due to the removal of the oxide layer by sea salts. This claim is true as salts can break down the protective layer, increasing corrosion.

Interaction with Washing Powder

Statement (3) states that washing powders with washing soda react with aluminium to form soluble aluminate. Washing soda, which contains sodium carbonate, will indeed react with aluminium to form aluminate, making this statement true.

Reaction with Ammonium Hydroxide

Statement (4) claims that aluminium dissolves in ammonium hydroxide, releasing hydrogen gas. This reaction does not usually occur because the protective oxide layer of aluminium prevents it from reacting under normal conditions. Therefore, this statement is false.

Key concepts

Aluminium Oxide Layer

Aluminium is a widely used metal known for its lightweight and versatile properties. One remarkable feature of aluminium is its ability to protect itself from corrosion. This is achieved through the formation of a thin but effective aluminium oxide layer on its surface.
When aluminium is exposed to air, it reacts with oxygen to form this oxide layer. The formation can be represented by the chemical reaction:
\(4Al + 3O_2 \rightarrow 2Al_2O_3\)
This layer is incredibly effective in preventing further reactions between the underlying aluminium and environmental factors like moisture and air. As a result, the metal is shielded from severe corrosion,
making it a popular choice for many applications, including automotive and aerospace industries.

Corrosion of Aluminium

While the aluminium oxide layer is generally protective, it can be compromised under certain conditions. One notable situation is in coastal areas where sea salt can deteriorate this layer.
The salty and humid environments promote the breakdown of the oxide layer, which accelerates corrosion. The salt acts as an electrolyte, facilitating the corrosive reactions:
\(2Al + 6H_2O + 3O_2 \rightarrow 4Al(OH)_3\)
The removal of the oxide layer makes the aluminium more vulnerable to corrosion.
Without this protection, aluminium surfaces can degrade much faster, leading to structural failures in marine applications, coastal buildings, and other exposed installations.

Reaction with Washing Soda

Aluminium should not be cleaned with washing powders containing washing soda (sodium carbonate). This substance can react with aluminium, corroding the metal surface and forming soluble aluminate.
The chemical reaction can be summarized as follows:
\(2Al + 3Na_2CO_3 + 3H_2O \rightarrow 2NaAlO_2 + 3CO_2 + 3H_2\)
In household cleaning, this reaction is unwanted because it causes surface damage to aluminium utensils and appliances. When cleaning aluminium, it's best to use mild detergents that do not contain sodium carbonate or other strong alkaline substances.

Ammonium Hydroxide Reaction

Ammonium hydroxide (NH₄OH), also known as ammonia solution, generally does not react with aluminium. This is because the protective aluminium oxide layer shield the metal underneath.
For such a reaction to occur, the ammonium hydroxide must be able to penetrate or disrupt the oxide layer. Under normal conditions, this does not happen. The oxide layer remains intact, preventing the aluminium from reacting with the solution.
However, in specific industrial processes where the oxide layer might be removed intentionally,
aluminium could potentially react with concentrated ammonium hydroxide, releasing hydrogen gas in the following manner:
\(2Al + 6NH_4OH \rightarrow 2Al(OH)_3 + 3H_2 + 6NH_3\)
But, in standard scenarios, this reaction is unlikely.
Thus, the statement that aluminium dissolves in ammonium hydroxide liberating hydrogen is generally considered false.