Litmus paper is a simple yet effective pH indicator that helps determine whether a substance is acidic or basic. It changes color depending on the pH level of the solution it comes into contact with. This method is widely used in laboratories, schools, and even household applications. But how does litmus paper work? What causes the color change? In this topic, we will explore the science behind litmus paper color change, its uses, and its limitations.
What Is Litmus Paper?
Litmus paper is a natural pH indicator made from paper treated with a mixture of dyes extracted from lichens. It is available in two types:
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Red Litmus Paper – Used to test for bases (alkaline substances).
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Blue Litmus Paper – Used to test for acids.
When exposed to a solution, the litmus paper undergoes a chemical reaction that results in a color change, indicating whether the substance is acidic or basic.
How Litmus Paper Works
Litmus paper functions based on the acid-base reaction. The dyes in the paper respond to the concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻) in the solution.
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In Acidic Solutions (pH < 7)
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Blue litmus paper turns red.
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Red litmus paper remains red.
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In Basic (Alkaline) Solutions (pH > 7)
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Red litmus paper turns blue.
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Blue litmus paper remains blue.
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In Neutral Solutions (pH = 7)
- There is no color change in either red or blue litmus paper.
Common Examples of Litmus Paper Reactions
To better understand how litmus paper reacts, let’s look at some everyday substances and their effects on litmus paper.
1. Acidic Substances (pH < 7)
These substances turn blue litmus paper red:
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Lemon Juice (Citric Acid)
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Vinegar (Acetic Acid)
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Soft Drinks (Carbonic Acid & Phosphoric Acid)
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Stomach Acid (Hydrochloric Acid)
2. Basic (Alkaline) Substances (pH > 7)
These substances turn red litmus paper blue:
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Baking Soda (Sodium Bicarbonate)
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Soapy Water (Alkaline Detergents)
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Bleach (Sodium Hypochlorite)
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Ammonia Solution
3. Neutral Substances (pH = 7)
These substances do not change the color of litmus paper:
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Pure Water
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Salt Solution (Sodium Chloride in Water)
Why Does Litmus Paper Change Color?
The color change occurs due to a chemical reaction between the litmus dye and the H⁺ or OH⁻ ions in the solution.
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Acids increase H⁺ ions, which react with the blue dye in the litmus paper, turning it red.
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Bases increase OH⁻ ions, which react with the red dye in the litmus paper, turning it blue.
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Neutral substances have an equal balance of H⁺ and OH⁻ ions, so no reaction occurs.
Limitations of Litmus Paper
While litmus paper is a quick and easy pH test, it has some limitations:
1. It Only Indicates Acidity or Alkalinity
Litmus paper does not provide an exact pH value; it only shows whether a substance is acidic, basic, or neutral.
2. It May Not Work on Certain Substances
Some strong chemicals can bleach litmus paper, removing its color and making it ineffective.
3. It Can Be Affected by Environmental Factors
Humidity, temperature, and contaminants can affect the accuracy of the test.
Alternatives to Litmus Paper
For more accurate pH testing, consider these alternatives:
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pH Strips – Provide a color-based pH scale reading.
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pH Meters – Electronic devices that give precise numerical pH values.
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Universal Indicator Solutions – Liquid indicators that show different colors for different pH levels.
Litmus paper is a simple and effective tool for identifying whether a substance is acidic, neutral, or basic. The color change reaction is based on the interaction between litmus dye and hydrogen or hydroxide ions. While useful for quick tests, it has limitations and may not always provide precise pH values. Understanding how litmus paper works can help in various applications, from science experiments to household testing.
