An aqueous solution that turns blue litmus paper red is classified as which type of substance?

Introduction / Context:
This simple chemistry question concerns acid base indicators, specifically litmus paper. Blue and red litmus papers are common tools used in laboratories and classrooms to identify whether a solution is acidic or basic. Understanding these colour changes is a foundation for more advanced acid base concepts.

Given Data / Assumptions:

• We are told that blue litmus paper turns red when dipped into a solution.
• We need to classify the solution as acidic, basic, alkaline, neutral or something else.
• We assume that the solution is aqueous and that litmus behaves in its usual way.
• Alkali refers to a soluble base, usually a hydroxide of an alkali metal or alkaline earth metal.

Concept / Approach:
Litmus is a natural dye used as an indicator. In acidic solutions, blue litmus turns red, and in basic or alkaline solutions, red litmus turns blue. Neutral solutions usually do not change the colour of litmus paper. Therefore, if blue litmus paper becomes red upon immersion, the solution must be acidic in nature. This rule is widely used as a quick and qualitative test of acidity or basicity in introductory chemistry.

Step-by-Step Solution:
1) Recall the basic rule: acids turn blue litmus red, and bases turn red litmus blue.2) The question states that blue litmus paper is converted into red when placed in the solution.3) This colour change matches the behaviour of an acidic solution, not of a base or an alkali.4) Neutral salt solutions such as sodium chloride solution typically leave litmus unchanged, so they do not fit the described behaviour.5) Organic solvents like alcohols often have little effect on litmus in the absence of water and are not classified simply as acid or base on this test.6) Therefore, the solution that turns blue litmus red must be an acid.

Verification / Alternative check:
In basic laboratory practice, tests with both red and blue litmus papers are often used. An unknown acidic solution will turn blue litmus red, but leaves red litmus unchanged. Conversely, a basic solution will turn red litmus blue, but leave blue litmus unchanged. Neutral solutions leave both colours unchanged. Since only the behaviour of blue litmus turning red is specified, this clearly corresponds to an acidic solution according to standard indicator rules.

Why Other Options Are Wrong:
Base: A base would cause the opposite effect, turning red litmus paper blue rather than blue litmus red.
Alkali: An alkali is a soluble base, so it shares the same indicator behaviour as bases and would not turn blue litmus red.
Neutral salt solution: Neutral solutions do not change the colour of either blue or red litmus paper in a noticeable way.
Organic solvent: Many organic solvents do not affect litmus strongly and are not simply categorised as acids or bases based on a litmus test alone.

Common Pitfalls:
Students sometimes remember only part of the litmus rules and confuse which colour change indicates acid or base. A helpful memory aid is the phrase "acids are red," reminding you that acidic solutions will ultimately cause red colour on litmus. Another pitfall is assuming that terms base and alkali mean different indicator behaviour, but in practice both bases and alkalis turn red litmus blue. Paying attention to which litmus colour is changing and in which direction is key to answering such questions correctly.

Final Answer:
A solution that turns blue litmus paper red is an acid.