Problem 33 - Chemistry and Chemical Reactivity

Question

Which of the following insoluble calcium compounds does not dissolve in hydrochloric acid? (a) limestone, \(\mathrm{CaCO}_{3}\) (b) slaked lime, \(\mathrm{Ca}(\mathrm{OH})_{2}\) (c) gypsum, \(\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}\) (d) hydroxyapatite, \(\mathrm{Ca}_{5}(\mathrm{OH})\left(\mathrm{PO}_{4}\right)_{3}\)

Step-by-Step Solution

Answer

Gypsum ( CaSO_4 imes 2H_2O ) does not dissolve in hydrochloric acid.

Step 1: Examine Calcium Carbonate (Limestone)

Calcium carbonate ( CaCO_3 ) reacts with acid such as hydrochloric acid ( HCl ) to produce calcium chloride ( CaCl_2 ), carbon dioxide ( CO_2 ), and water. Hence, limestone dissolves in HCl .

Step 2: Examine Calcium Hydroxide (Slaked Lime)

Calcium hydroxide ( Ca(OH)_2 ) is soluble in water and dissolves in HCl , forming calcium chloride ( CaCl_2 ) and water. Thus, slaked lime dissolves in HCl .

Step 3: Examine Calcium Sulfate Dihydrate (Gypsum)

Calcium sulfate dihydrate ( CaSO_4 imes 2H_2O ) is sparingly soluble in water. When it comes in contact with HCl , it reacts very minimally, thus remaining mostly undissolved.

Step 4: Examine Hydroxyapatite

Hydroxyapatite ( Ca_5(OH)(PO_4)_3 ) slightly dissolves in acidic environments, including HCl , forming various calcium phosphates and chloride. However, its solubility in acid is less than calcium carbonate and calcium hydroxide.

Step 5: Conclusion

Among the given calcium compounds, gypsum ( CaSO_4 imes 2H_2O ) remains largely undissolved in HCl compared to others, which react more vigorously.

Key Concepts

Hydrochloric Acid Reactions

Hydrochloric acid (HCl) is known for its strong acidity and its ability to react with many substances, especially those containing calcium. When insoluble calcium compounds come in contact with HCl, the acid promotes reactions that often lead to the formation of soluble products. For example, calcium carbonate (limestone) reacts with hydrochloric acid to form calcium chloride, carbon dioxide, and water. This reaction can be represented by the equation:\[\text{CaCO}_3 + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{CO}_2 + \text{H}_2\text{O}\]This type of reaction is common for many calcium compounds, which makes hydrochloric acid a useful agent in dissolving particular structures and elements, with some exceptions like gypsum.

Chemical Solubility

The concept of chemical solubility refers to the ability of a substance to dissolve in a solvent, like water or acid. Solubility varies greatly among different compounds and changes with the solvent used. For instance, calcium hydroxide, commonly known as slaked lime, is soluble in water, and when it comes into contact with hydrochloric acid, it dissolves further. The chemical reaction with HCl produces calcium chloride and water:\[\text{Ca(OH)}_2 + 2\text{HCl} \rightarrow \text{CaCl}_2 + 2\text{H}_2\text{O}\]Insoluble compounds, however, react less or very minimally. Understanding the solubility of various compounds helps in determining their behavior in different chemical environments, which is crucial for scientific analysis and industrial applications.

Calcium Sulfate

Calcium sulfate, particularly in the form of calcium sulfate dihydrate known as gypsum, is a unique compound due to its limited solubility in water and acids. Gypsum is sparingly soluble, meaning it dissolves slightly but not completely. When placed in hydrochloric acid, gypsum reacts minimally and remains mostly intact. This behavior contrasts with more soluble calcium compounds, which tend to dissolve much more readily. This quality makes gypsum particularly useful in applications where a stable, minimal dissolution in acidic environments is required, such as in construction material for drywall.

Compound Analysis

Analyzing compounds involves understanding their chemical properties and reactions. It's essential to know how each compound reacts under different conditions such as exposure to acids like HCl. In compound analysis, we might assess how compounds like hydroxyapatite behave. Hydroxyapatite slightly dissolves in hydrochloric acid, forming different calcium phosphates and chlorides. Its reaction is not as vigorous as calcium carbonate or calcium hydroxide, demonstrating a unique level of solubility. By examining these reactions, chemists can predict how these compounds will behave in natural environments or when used in products, making it a key step in both research and practical applications.