Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

At \(25^{\circ} \mathrm{C}\) the \(\mathrm{pH}\) value of a solution is 6, the solution is (a) neutral (b) acidic (c) alkaline (d) basic

Step-by-Step Solution

Answer

The solution is acidic.

Step 1: Understanding pH Scale

The pH scale is used to determine how acidic or basic a solution is. The scale ranges from 0 to 14, with 7 being neutral. Values less than 7 indicate acidity, and values greater than 7 indicate alkalinity (basicity).

Step 2: Reviewing pH of the Given Solution

In the problem, the pH value of the solution is given as 6. This value is less than 7.

Step 3: Comparing pH with Scale Range

Since the pH value of 6 is less than 7, it falls in the acidic range of the pH scale.

Step 4: Conclusion Based on pH Value

Based on the pH value of 6, the solution is acidic, which corresponds to option (b).

Key Concepts

Acidic Solutions

Acidic solutions have a pH value less than 7. They are often associated with sour tastes, such as those found in citrus fruits like lemons and oranges. The presence of more hydrogen ions (H extsuperscript{+}) than hydroxide ions (OH extsuperscript{-}) in a solution leads to acidity.

Some common examples of acidic solutions include:

• Vinegar, which contains acetic acid
• Lemon juice, rich in citric acid
• Gastric acid, primarily made up of hydrochloric acid in the stomach

The strength of an acid in a solution is determined by how completely it ionizes. Strong acids, like hydrochloric acid, dissociate entirely in water, producing a higher concentration of hydrogen ions. This makes the pH value drop significantly. Conversely, weak acids, like acetic acid, only partially ionize, resulting in a pH value closer to 7.

To measure the pH of an acidic solution, one can use pH paper, pH meters, or indicators like phenolphthalein, which changes color based on the acidity level.

Alkaline Solutions

Alkaline solutions, also known as basic solutions, have a pH value greater than 7. These solutions are characterized by a bitter taste and a slippery feel. They contain more hydroxide ions (OH extsuperscript{-}) than hydrogen ions (H extsuperscript{+}).

Examples of alkaline solutions include:

• Sodium hydroxide (lye), commonly used in soap making
• Ammonia, found in many household cleaning products
• Milk of magnesia, used as a laxative and antacid

The strength of an alkaline solution is determined by how fully it dissociates in water to release hydroxide ions. Strong bases, like sodium hydroxide, completely ionize, resulting in a pH significantly above 7.

Understanding the behavior of alkaline solutions is crucial in various applications, from cooking and cleaning to industrial processes. They are identified using similar methods as acids, such as pH meters and indicators like litmus paper, which turns blue in basic solutions.

Neutral Solutions

Neutral solutions have a pH of exactly 7, meaning there is a perfect balance between hydrogen ions (H extsuperscript{+}) and hydroxide ions (OH extsuperscript{-}) in the solution. Pure water is the most common example of a neutral solution at 25°C.

Neutral solutions are characterized by being neither acidic nor basic, making them important in maintaining balance in both natural and artificial systems.

In biology, maintaining a neutral pH is crucial for processes like enzyme activity, as many enzymes function optimally at or near pH 7. In the lab, neutral pH conditions are often required for chemical reactions that could be affected by acidity or alkalinity.

To verify if a solution is neutral, a pH meter or neutral litmus paper can be used, which remains an unchanged color in these kinds of solutions. Understanding the nature of neutral solutions is essential for a wide range of scientific disciplines and practical applications.