Problem 22 - 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 soft-drink preservative) (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4}\) (used as an emulsifier in the manufacture of pasteurized cheese)

Step-by-Step Solution

Answer

(c) \(\mathrm{Na}_2\mathrm{HPO}_4\) gives a basic solution when dissolved in water.

Step 1: Identifying the Nature of Each Compound

Understand that when a compound dissolves in water, it can produce a solution that is acidic, basic, or neutral depending on the ions it releases. Review the chemical composition of each compound to determine the presence of ions that could make the solution basic.

Step 2: Analyze NaNO₃

Sodium nitrate (\(\mathrm{NaNO}_3\)) is a salt formed from a strong acid (\(\mathrm{HNO}_3\)) and a strong base (\(\mathrm{NaOH}\)). Solutions of such salts are typically neutral, as neither ion affects the pH of the solution significantly.

Step 3: Analyze NaC₆H₅CO₂

Sodium benzoate (\(\mathrm{NaC}_6\mathrm{H}_5\mathrm{CO}_2\)) is the sodium salt of benzoic acid. Sodium ions (\(\mathrm{Na}^+\)) do not affect pH, but the benzoate ion (\(\mathrm{C}_6\mathrm{H}_5\mathrm{CO}_2^-\)) can react with water to form a slightly basic solution. This is because it comes from the relatively weak acid, benzoic acid.

Step 4: Analyze Na₂HPO₄

Sodium hydrogen phosphate (\(\mathrm{Na}_2\mathrm{HPO}_4\)) can donate a hydrogen ion \(\mathrm{H}^+\) when dissolved in water, forming \(\mathrm{PO}_4^{3-}\), which can react to form \(\mathrm{OH}^-\) ions, making the solution basic. This compound acts as a base because the \(\mathrm{HPO}_4^{2-}\) ion is the conjugate base of a weak acid.

Step 5: Conclusion and Answer Selection

Among the given options, only sodium benzoate (\(\mathrm{NaC}_6\mathrm{H}_5\mathrm{CO}_2\)) and sodium hydrogen phosphate (\(\mathrm{Na}_2\mathrm{HPO}_4\)) can make the solution basic. Select the option that best matches the criteria of forming a basic solution in water.

Key Concepts

Food Additives

Food additives are substances added to food to enhance its flavor, appearance, or shelf life. These compounds can range from natural to synthetic and are used to preserve the freshness and quality of food products.
For example, sodium benzoate (found in soft drinks) and sodium nitrate (used in meat preservation) are common food additives.
However, beyond their primary function, these additives can interact with water to form various solutions.
By understanding the chemical behavior of these additives, we can infer how they will affect the flavor and longevity of food, as well as their potential health implications.

Sodium Salts

Sodium salts are a category of salt compounds that contain sodium ions \(\mathrm{Na}^+\).
They are widely used in both industrial and food applications due to their versatility and stability.
Let's take a closer look:

• Sodium Nitrate \( (\mathrm{NaNO}_{3}) \): Primarily used as a preservative, it dissolves in water without altering pH significantly, producing a neutral solution.
• Sodium Benzoate \( (\mathrm{NaC}_{6} \mathrm{H}_{5} \mathrm{CO}_{2}) \): Used to inhibit microbial growth in foods and beverages, this salt can lead to a slightly basic solution when dissolved in water.
• Sodium Hydrogen Phosphate \( (\mathrm{Na}_{2} \mathrm{HPO}_{4}) \): Utilized as an emulsifier, it undergoes ionization to produce a basic aqueous solution, thanks to its conjugate base properties.

Each sodium salt contributes uniquely to the pH of aqueous solutions, which can impact both the food's chemistry and its preservation methods.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons from acids to bases. They are essential in understanding the nature of solutions formed by various compounds.
In such reactions:

• An acid donates a proton, while a base accepts it.
• The strength of an acid or base is determined by their ability to dissociate or ionize in water.
• A solution's pH depends on the relative strengths of its ions.

For instance, sodium benzoate forms a basic solution because its benzoate ion reacts with water, accepting a proton and producing hydroxide ions \( (\mathrm{OH}^{-}) \).
Similarly, sodium hydrogen phosphate dissociates to form ions that lead to a basic solution. By reviewing the acid-base properties of these compounds, students can better predict the behavior of food additives in water.