Question

Classify the following aqueous solutions as strongly acidic, weakly acidic, neutral, weakly basic, or strongly basic: (a) stomach acid, \(\mathrm{pH} 1.5\) (b) vinegar, \(\mathrm{pH} 3.2\) (c) shampoo, \(\mathrm{pH} 7.0\) (d) ammonia, \(\mathrm{pH} 11.1\)

Short answer

(a) Strongly acidic, (b) Weakly acidic, (c) Neutral, (d) Weakly basic.

Step-by-step solution

Understanding pH Scale

The pH scale ranges from 0 to 14 and classifies solutions based on acidity or basicity. A pH below 7 indicates an acidic solution, a pH of 7 is neutral, and a pH above 7 is basic. The closer the pH is to 0, the stronger the acid, and the closer it is to 14, the stronger the base.

Classify Stomach Acid

Stomach acid has a pH of 1.5. Since this value is well below 7 and close to 0, it is classified as a strongly acidic solution.

Classify Vinegar

Vinegar has a pH of 3.2. Although acidic, this pH is higher than that of stomach acid, meaning it is less acidic. Therefore, it is classified as a weakly acidic solution.

Classify Shampoo

Shampoo has a pH of 7.0. This is the neutral value on the pH scale, meaning the solution is neither acidic nor basic. Hence, shampoo is classified as a neutral solution.

Classify Ammonia

Ammonia has a pH of 11.1, which is above 7, indicating a basic solution. Because it is significantly above 7 but not extremely close to 14, it is classified as a weakly basic solution.

Key concepts

Understanding Acidic Solutions

Acidic solutions are liquids that have a pH value below 7. This means they contain a higher concentration of hydrogen ions (H\(^+\)). The more hydrogen ions present, the lower the pH level and the stronger the acidity.
Hence, solutions with pH values closer to 0 are considered strongly acidic.

• An example of a strongly acidic solution is stomach acid, with a pH of 1.5. This value indicates a very high concentration of hydrogen ions, which helps in digesting food.
• On the other hand, vinegar with a pH of 3.2, is also acidic but not as harsh as stomach acid. It is classified as weakly acidic because it has fewer hydrogen ions compared to stronger acids.

When dealing with acids, it is important to understand their reactivity and how they behave in chemical reactions.

Defining Neutral Solutions

Neutral solutions have a distinct position on the pH scale at exactly 7. This means they have an equal number of hydrogen ions (H\(^+\)) and hydroxide ions (OH\(^-\)).
As a result, these solutions are not acidic nor basic and exhibit balanced characteristics.

• The most common example of a neutral solution is pure water, which is the benchmark for neutrality.
• Another example, as given in the exercise, is shampoo. With its pH set at 7, it is designed to be gentle and non-irritating to both your skin and hair.

It is helpful to remember that any solution aiming for neutrality will try to maintain this balance of ions, making it stable and non-reactive under normal conditions.

Identifying Basic Solutions

Basic solutions are those with a pH greater than 7. These solutions contain more hydroxide ions (OH\(^-\)) than hydrogen ions (H\(^+\)). As pH values move closer to 14, the solution becomes more strongly basic.
This is because hydroxide ions are increasing in concentration, making the solution more alkaline.

• Ammonia, with a pH of 11.1, serves as an excellent example of a basic solution. It is considered weakly basic as its pH is not as close to 14, which means it is less alkaline than something like bleach, which is strongly basic.

Basic solutions are commonly used in cleaning products because their alkalinity helps break down oils and grease effectively.