Problem 26 - Chemistry and Chemical Reactivity

Question

Which of the following common food additives would give a basic solution when dissolved in water? (a) \(\mathrm{NaNO}_{3}\) (used as a meat preservative) (b) \(\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}\) (sodium benzoate; used as a softdrink preservative) (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) (used as an emulsifier in the manufacture of pasteurized cheese)

Step-by-Step Solution

Answer

The additives that form a basic solution are (b) and (c): sodium benzoate and sodium hydrogen phosphate.

Step 1: Identify the Nature of Each Compound

First, we need to identify the type of anions in each compound. Anions from weak acids will make the solution basic when the compound is dissolved in water. (a) \(\mathrm{NaNO}_{3}\) contains the nitrate ion, \(\mathrm{NO}_{3}^{-}\), from the strong acid \(\mathrm{HNO}_{3}\), so it will not make the solution basic. (b) \(\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}\) contains the benzoate ion, \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}^{-}\), from the weak acid benzoic acid, suggesting it may form a basic solution. (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) contains the hydrogen phosphate ion, \(\mathrm{HPO}_{4}^{2-}\), which is derived from the weak acid \(\mathrm{H}_{2}\mathrm{PO}_{4}^{-}\), suggesting it may also form a basic solution.

Step 2: Evaluate the Hydrolysis of Anions

Let's evaluate how the identified anions react in water to determine if they create a basic solution.(a) \(\mathrm{NO}_{3}^{-}\) does not hydrolyze in water as it is the conjugate base of a strong acid. (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}^{-}\) can accept a proton from water, resulting in hydroxide \(\mathrm{OH^{-}}\), making the solution basic. (c) \(\mathrm{HPO}_{4}^{2-}\) can also accept a proton from water, forming \(\mathrm{OH^{-}}\), which makes the solution basic.

Step 3: Conclusion

Based on the hydrolysis analysis, options (b) and (c) form basic solutions when dissolved in water. Therefore, \(\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}\) and \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) are the additives that form basic solutions when dissolved.

Key Concepts

Basic Solutions in Chemistry

In chemistry, basic solutions are those that have a pH greater than 7. This occurs when the solution has more hydroxide ions (\(\text{OH}^{-}\)) than hydrogen ions (\(\text{H}^{+}\)). When a substance that contains ions from weak acids is dissolved in water, the solution can become basic.

• Substances that form basic solutions are known as bases.
• An important characteristic of bases is their ability to accept protons (\(\text{H}^{+}\)) from water molecules.
• This increases the concentration of hydroxide ions in the solution.

An example of a compound that forms a basic solution is sodium benzoate. When it dissolves in water, it generates hydroxide ions, increasing the pH above 7.8. As students study basic solutions, it's key to remember that the pH scale only indicates the concentration of hydrogen ions, and a higher pH means a lower concentration of these ions.

Hydrolysis of Anions

Hydrolysis is a process where ions in water can react with water molecules to form different ions, altering the pH of the solution. Typically, the anions from weak acids have a tendency to hydrolyze in water.

• Anions from weak acids can capture hydrogen ions from water, generating more hydroxide ions.
• The formation of hydroxide ions due to hydrolysis means that the solution becomes more basic.
• This process can shift the neutral pH (around 7) higher, indicating the presence of more \(\text{OH}^{-}\) ions.

For instance, benzoate ions and hydrogen phosphate ions are examples of anions that undergo hydrolysis. These ions react with water to release hydroxide ions, making the resultant solution basic. This highlights the importance of understanding anion behavior in predicting solution pH.

Chemistry of Preservatives

Preservatives are substances added to food products to prevent spoilage from microorganisms, oxidation, or other chemical changes. These substances are vital in extending the shelf life of food products.

• They work by inhibiting the growth of bacteria, yeast, and molds.
• Preservatives can either be synthetic or naturally derived, with applications across various food products.
• Their effectiveness often depends on their chemical properties and how they interact with other ingredients.

From the examples given, sodium benzoate is a common preservative used in soft drinks. It works effectively over a wide pH range but is most efficient below pH 4. This is because the undissociated form of the acid (benzoic acid) is more effective at penetrating microbial cell walls, inhibiting their growth. Understanding how preservatives function helps in determining how food chemistry affects health and safety.

Sodium Benzoate

Sodium benzoate is a widely used food preservative, recognized for its role in maintaining the freshness of many products. Chemically, it is the sodium salt of benzoic acid. When added to water, sodium benzoate dissociates into sodium and benzoate ions.

• Sodium benzoate is frequently found in acidic foods such as soft drinks, salad dressings, and jams.
• In water, the benzoate ion can accept a proton from water, leaving a hydroxide ion (\(\text{OH}^{-}\)) free in the solution.
• This reaction is what makes the solution slightly basic as hydroxide ions increase the pH.

The acid-benzoate equilibrium is a functional attribute allowing sodium benzoate to operate effectively as a preservative in diverse culinary contexts. It highlights how an understanding of specific preservatives can help mitigate food spoilage, ensuring longer shelf life and safer consumption.