In acid–base chemistry, the oxide of which of the following elements forms a basic oxide that will turn red litmus paper blue when dissolved in water?

Introduction / Context:
Metal and nonmetal oxides often behave differently when dissolved in water. Many metal oxides form basic solutions, while many nonmetal oxides form acidic solutions. Litmus paper is a simple indicator used to identify whether a solution is acidic or basic: red litmus turns blue in a basic solution and blue litmus turns red in an acidic solution. This question tests your knowledge of which element forms an oxide that is basic and therefore capable of turning red litmus paper blue when dissolved in water.

Given Data / Assumptions:

• The elements listed are magnesium, phosphorus, sulphur, and carbon.
• We consider their common oxides and how those oxides behave in water.
• Red litmus paper turning blue indicates a basic or alkaline solution.
• We assume simple, well known oxides such as magnesium oxide, phosphorus pentoxide, sulphur dioxide, and carbon dioxide.

Concept / Approach:
Magnesium is a metal, and its oxide, magnesium oxide, is a basic oxide. When magnesium oxide reacts with water, it forms magnesium hydroxide, which is slightly soluble and produces a basic solution. This basic solution can turn red litmus paper blue. On the other hand, phosphorus, sulphur, and carbon are nonmetals whose common oxides, such as phosphorus pentoxide, sulphur dioxide, sulphur trioxide, and carbon dioxide, form acids when dissolved in water. These acidic solutions turn blue litmus red rather than red litmus blue. Therefore, among the options given, the oxide of magnesium is the basic one that will turn red litmus blue.

Step-by-Step Solution:
Step 1: Recall that metal oxides are usually basic, while many nonmetal oxides are acidic. Step 2: Identify magnesium as a metal whose oxide, magnesium oxide, reacts with water to form magnesium hydroxide, a base. Step 3: Recognise that phosphorus forms oxides such as phosphorus pentoxide, which react with water to form phosphoric acids. Step 4: Note that sulphur forms sulphur dioxide and sulphur trioxide, which dissolve in water to produce sulphurous acid and sulphuric acid, both acidic. Step 5: Remember that carbon dioxide dissolves in water to form carbonic acid, giving an acidic solution. Step 6: Conclude that only magnesium oxide produces a basic solution that can turn red litmus blue, and select magnesium as the correct answer.

Verification / Alternative check:
Experimental observations confirm that a suspension of magnesium oxide or magnesium hydroxide in water is basic and can cause red litmus paper to change to blue. Solutions of phosphoric, sulphurous, sulphuric, and carbonic acids clearly show acidic behaviour and turn blue litmus red. General chemistry rules summarise that basic oxides typically come from metals, especially metals from groups one and two, while acidic oxides generally come from nonmetals such as carbon, sulphur, and phosphorus. This pattern supports the conclusion that magnesium oxide is the basic oxide among those implied in the options.

Why Other Options Are Wrong:
Phosphorus oxides, such as P4O10, form phosphoric acids when added to water and therefore lead to acidic solutions, not basic ones. Sulphur oxides produce sulphurous and sulphuric acids, again creating acidic conditions. Carbon dioxide forms carbonic acid in water, which is weakly acidic but still turns blue litmus red rather than red litmus blue. These nonmetal oxides do not yield basic solutions and thus cannot be responsible for turning red litmus paper blue.

Common Pitfalls:
Students may sometimes forget the general rule linking metal oxides with basic behaviour and nonmetal oxides with acidic behaviour, leading them to guess incorrectly. Another pitfall is confusion between amphoteric oxides, such as those of aluminium and zinc, and strictly basic or acidic oxides. To avoid these errors, remember the simple guideline that most group one and group two metal oxides are basic, and common nonmetal oxides, especially of carbon, sulphur, and phosphorus, are acidic. Using this rule in exam questions makes it easier to select the correct answer quickly.

Final Answer:
The oxide that forms a basic solution and turns red litmus blue is the oxide of magnesium, forming basic magnesium oxide.