Which One Of The Following Will Turn Red Litmus Blue

Which One of the Following Will Turn Red Litmus Blue? Understanding pH and Indicators

The question, "Which one of the following will turn red litmus blue?" probes our understanding of acids, bases, and pH indicators. To answer it effectively, we need to delve into the fundamental concepts of pH, the nature of acids and bases, and how litmus paper functions as a simple yet powerful indicator. This article will explore these concepts in detail, offering a comprehensive explanation accessible to everyone, from beginners to those seeking a deeper understanding.

Understanding pH: The Acid-Base Scale

The pH scale is a logarithmic scale that measures the acidity or basicity (alkalinity) of a solution. It ranges from 0 to 14, with 7 representing neutral. Solutions with a pH less than 7 are considered acidic, while those with a pH greater than 7 are basic or alkaline. The lower the pH value, the stronger the acid; the higher the pH value, the stronger the base. Each whole number change in pH represents a tenfold change in the concentration of hydrogen ions (H⁺). For example, a solution with a pH of 3 is ten times more acidic than a solution with a pH of 4, and one hundred times more acidic than a solution with a pH of 5.

Key characteristics of the pH scale:

• pH 0-3: Strongly acidic (e.g., battery acid, stomach acid)
• pH 4-6: Weakly acidic (e.g., vinegar, orange juice)
• pH 7: Neutral (e.g., pure water)
• pH 8-10: Weakly alkaline (e.g., baking soda solution, seawater)
• pH 11-14: Strongly alkaline (e.g., drain cleaner, lye)

Acids and Bases: A Deeper Dive

Acids and bases are fundamental chemical substances with distinct properties. Acids are substances that donate protons (H⁺ ions) when dissolved in water, increasing the concentration of H⁺ ions. Bases, on the other hand, accept protons or release hydroxide ions (OH⁻ ions) when dissolved in water, decreasing the concentration of H⁺ ions. This proton exchange is crucial in many chemical reactions.

Several theories help us understand acids and bases:

• Arrhenius Theory: Defines acids as substances that produce H⁺ ions in water and bases as substances that produce OH⁻ ions in water.
• Brønsted-Lowry Theory: Broadens the definition, stating that acids are proton donors and bases are proton acceptors. This theory extends beyond aqueous solutions.
• Lewis Theory: The most general theory, defining acids as electron-pair acceptors and bases as electron-pair donors.

Litmus Paper: A pH Indicator

Litmus paper is a common pH indicator, a substance that changes color depending on the pH of the solution it's in contact with. Litmus paper is made from a mixture of different dyes extracted from lichens. Red litmus paper turns blue in the presence of a base (alkaline solution), while blue litmus paper turns red in the presence of an acid. This color change is a qualitative test, indicating whether a solution is acidic or basic but not providing the precise pH value.

How Litmus Paper Works:

The color change in litmus paper is due to the change in the molecular structure of the dye molecules upon interaction with H⁺ or OH⁻ ions. In acidic solutions (high H⁺ concentration), the dye molecules exist in a specific form that absorbs light differently, resulting in a red color. In basic solutions (low H⁺ concentration, high OH⁻ concentration), the dye molecules transform into a different form that absorbs light differently, leading to a blue color. This color change is reversible; the litmus paper can be reused after rinsing.

Identifying Substances That Turn Red Litmus Blue

Now, let's address the original question: which substances turn red litmus blue? The answer is any substance that is basic or alkaline, meaning it has a pH greater than 7. Here are some examples:

• Sodium hydroxide (NaOH): A strong base commonly known as lye or caustic soda. It readily dissociates in water, releasing hydroxide ions (OH⁻) that drastically increase the pH.

• Potassium hydroxide (KOH): Similar to sodium hydroxide, it's a strong base and will cause a dramatic color change in red litmus paper.

• Ammonia (NH₃): A weak base, it dissolves in water to form ammonium hydroxide (NH₄OH), which slightly increases the pH, causing a change from red to blue, although possibly a less intense blue compared to strong bases.

• Sodium carbonate (Na₂CO₃): Also known as washing soda, it's a weak base that will turn red litmus paper blue.

• Sodium bicarbonate (NaHCO₃): Baking soda, a weak base, will also show a change, though possibly a less pronounced one.

• Many household cleaners: Many commercial cleaning products contain alkaline substances that will readily turn red litmus blue. Always exercise caution when handling cleaning solutions and refer to the product's safety data sheet.

• Solutions of metal hydroxides: Metal hydroxides such as calcium hydroxide (Ca(OH)₂) and magnesium hydroxide (Mg(OH)₂) are bases that turn red litmus blue. The strength of the base will affect the intensity of the color change.

• Certain salts: Some salts formed from the reaction between a strong base and a weak acid can hydrolyze in water to produce basic solutions that turn red litmus blue. Examples include sodium acetate (CH₃COONa) and sodium cyanide (NaCN). However, this reaction is more complex than the simple dissociation of strong bases.

Beyond Litmus: Other pH Indicators

While litmus paper is a useful indicator, it only provides a qualitative assessment of pH. More precise pH measurements require other methods, such as:

• pH meters: Electronic devices that provide a precise numerical reading of the pH.
• Universal indicator: A mixture of several indicators that displays a range of colors across the pH scale, providing a more precise estimate than litmus paper.
• Other specific indicators: Many other indicators exist, each with its specific color change range. Examples include methyl orange, phenolphthalein, and bromothymol blue, each exhibiting unique color changes within specific pH ranges. These indicators are often used in titrations to determine the endpoint of a neutralization reaction.

Practical Applications of pH Indicators

Understanding pH and the use of indicators has numerous practical applications in various fields:

• Chemistry: In analytical chemistry, indicators are essential for titrations, determining the concentration of unknown solutions through controlled neutralization reactions.

• Medicine: Monitoring blood pH is crucial for diagnosing and treating various health conditions. Maintaining the proper pH balance is vital for bodily functions.

• Agriculture: Soil pH influences nutrient availability and plant growth. Farmers utilize pH indicators to manage soil conditions for optimal crop yields.

• Environmental monitoring: Monitoring the pH of water bodies is important for assessing water quality and protecting aquatic life. Acid rain, for instance, can drastically lower the pH of lakes and rivers, harming aquatic ecosystems.

• Food industry: Controlling pH is important in food processing and preservation. The pH of many food products affects their taste, texture, and shelf life.

Conclusion

Determining which substances turn red litmus blue hinges on understanding the nature of acids and bases and their interaction with pH indicators. Red litmus paper turns blue in the presence of bases, indicating a pH greater than 7. Numerous substances, from strong alkalis like sodium hydroxide to weaker bases like baking soda and ammonia, will cause this change. This understanding of pH and indicators has far-reaching applications across various scientific disciplines and everyday life, underscoring the importance of mastering this fundamental chemical concept. This knowledge is not merely about turning a piece of paper a different color; it's the key to understanding many critical processes in our world. From the health of our bodies and the environment to industrial processes and agricultural practices, the concept of pH and its measurement are fundamentally important.