If a solution turns blue litmus red, its pH is likely to be:
6
Understanding pH and Litmus Paper
The question asks about the likely pH of a solution that turns blue litmus paper red. This observation is a classic test used to identify the nature of a solution, specifically whether it is acidic, basic (alkaline), or neutral.
The Role of Litmus Paper
Litmus paper is a common indicator derived from lichens. It is used to test the acidity or basicity of a substance. It comes in two main colors: red litmus paper and blue litmus paper.
- Acidic Solutions: Acidic solutions turn blue litmus paper red.
- Basic (Alkaline) Solutions: Basic solutions turn red litmus paper blue.
- Neutral Solutions: Neutral solutions do not change the color of either red or blue litmus paper.
The pH Scale
The pH scale is a numerical scale used to specify the acidity or basicity of an aqueous solution. The scale typically ranges from 0 to 14.
- Solutions with a pH less than 7 are acidic.
- Solutions with a pH exactly 7 are neutral.
- Solutions with a pH greater than 7 are basic or alkaline.
The pH value is inversely related to the concentration of hydrogen ions (\(\text{H}^+\)). A lower pH indicates a higher concentration of \(\text{H}^+\) ions and greater acidity.
Connecting Litmus Test and pH
We are given that the solution turns blue litmus red. According to the properties of litmus paper, a solution that turns blue litmus red is acidic. Based on the pH scale, an acidic solution has a pH less than 7.
Therefore, if a solution turns blue litmus red, its pH must be a value less than 7.
Analyzing the Options
Let's examine the given options in light of our understanding:
| Option | pH Value | Nature of Solution (based on pH) | Does it turn blue litmus red? |
|---|---|---|---|
| 1 | 9 | Basic (\(pH > 7\)) | No (turns red litmus blue) |
| 2 | 8 | Basic (\(pH > 7\)) | No (turns red litmus blue) |
| 3 | 6 | Acidic (\(pH < 7\)) | Yes |
| 4 | 7 | Neutral (\(pH = 7\)) | No (no color change) |
From the analysis, only a solution with pH 6 is acidic (\(6 < 7\)), which is consistent with turning blue litmus paper red. The other options represent basic or neutral solutions, which would not turn blue litmus red.
Thus, a solution that turns blue litmus red is likely to have a pH of 6 among the given options.
Summary of Litmus Paper Color Changes and pH
| Solution Type | pH Range | Effect on Blue Litmus | Effect on Red Litmus |
|---|---|---|---|
| Acidic | \(pH < 7\) | Turns Red | Stays Red |
| Neutral | \(pH = 7\) | Stays Blue | Stays Red |
| Basic (Alkaline) | \(pH > 7\) | Stays Blue | Turns Blue |
Revision Table: pH and Litmus Test
Here’s a quick recap of the relationship between pH and litmus paper color changes:
- Blue litmus turns red → Acidic solution → pH < 7
- Red litmus turns blue → Basic solution → pH > 7
- No change in either litmus → Neutral solution → pH = 7
Additional Information: Acid-Base Indicators
Litmus paper is just one type of acid-base indicator. Indicators are substances that change color depending on the pH of the solution they are in. Different indicators change color at different pH ranges. Some other common indicators include:
- Phenolphthalein: Colorless in acidic solutions (pH < 8.2), pink in basic solutions (pH > 8.2).
- Methyl Orange: Red in acidic solutions (pH < 3.1), yellow in basic solutions (pH > 4.4), orange in between.
- Universal Indicator: A mixture of indicators that shows a range of colors over a wide pH range, allowing for estimation of the actual pH value, not just whether it's acidic, basic, or neutral.
Understanding how indicators work is crucial for determining the approximate pH of solutions in the lab.
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