Many metals, such as aluminium and zinc, react with aqueous sodium hydroxide under suitable conditions. In such reactions, which gas is evolved as a product along with soluble complex salts?

Introduction / Context:
Certain metals show amphoteric behaviour, meaning they can react with both acids and bases. Metals such as aluminium and zinc react with strong bases like sodium hydroxide to produce soluble complexes and a gas. This question asks which gas is evolved during such reactions. Understanding these reactions reinforces knowledge about metal activity, hydrogen evolution, and the behaviour of amphoteric metals.

Given Data / Assumptions:

• We are considering the reaction of metals like aluminium or zinc with aqueous sodium hydroxide solution.
• The reaction occurs in the presence of water and often with heating.
• A gas is evolved as one of the products.
• Soluble complex salts such as sodium aluminate or sodium zincate are also formed.

Concept / Approach:
When active metals react with water or aqueous alkalies, they can displace hydrogen from water molecules, producing hydrogen gas. Aluminium reacts with sodium hydroxide and water to form sodium aluminate and hydrogen gas. Similarly, zinc forms sodium zincate and hydrogen gas. The balanced equations show hydrogen gas as a clear product. Oxygen gas is not liberated in these reactions, and sodium metal or chlorine gas are not produced because the reaction conditions and reactants do not support such changes.

Step-by-Step Solution:

Verification / Alternative check:
Practical laboratory experiments demonstrate that when aluminium foil or zinc granules are added to warm sodium hydroxide solution, bubbles of gas are evolved. This gas can be tested by bringing a burning splint near the mouth of the test tube; a characteristic pop sound confirms the presence of hydrogen gas. Textbooks describing the behaviour of amphoteric metals with alkalies consistently show hydrogen gas as the gaseous product. These observations support the conclusion that hydrogen, not oxygen or other gases, is produced.

Why Other Options Are Wrong:

• Oxygen gas: Oxygen is not liberated in these reactions; instead, the metal is oxidised and hydrogen is reduced from water.
• Sodium vapour: Sodium is present as sodium ions in solution and does not vaporise as metal under these conditions.
• Water vapour only: Although heating may produce some water vapour, the key chemical gaseous product is hydrogen.
• Chlorine gas: Chlorine is not part of the reactants and is not produced in reactions involving sodium hydroxide and metals like aluminium or zinc.

Common Pitfalls:
Students sometimes confuse these reactions with electrolysis or other processes where oxygen might be produced. Another confusion is to think that because sodium hydroxide is involved, sodium metal might appear as a product, which does not happen in aqueous solution. The key is to remember that active metals reacting with water or alkalies can displace hydrogen, leading to hydrogen gas evolution. Recognising the typical pop test for hydrogen helps in remembering the correct gas.

Final Answer:
When metals such as aluminium or zinc react with aqueous sodium hydroxide, they evolve Hydrogen gas as a product.