Question

Which is insoluble in water? (a) \(\mathrm{CaF}_{2}\) (b) \(\mathrm{H}_{2} \mathrm{O}\) (c) \(\mathrm{HgCl}_{2}\) (d) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\)

Short answer

Calcium fluoride ( ext{CaF}_2) is insoluble in water.

Step-by-step solution

Understanding Solubility in Water

To identify which substance is insoluble in water, we must examine the general rules of solubility for ionic compounds. In water, most nitrates ( ext{NO}_3^-), and halides (except those of Ag, Pb, and Hg) are soluble. Insolubility often applies to certain salts such as fluorides, carbonates, and phosphates of certain cations.

Reviewing Each Option

- ext{CaF}_2: Calcium fluoride is generally insoluble in water due to the strong attraction between its ions which are not easily separated by water molecules. - ext{H}_2 ext{O}: Water is the solvent itself and fully miscible with itself. - ext{HgCl}_2: Although some mercury compounds are insoluble, ext{HgCl}_2 is slightly soluble in water. - ext{Ca(NO}_3 ext{)}_2: Calcium nitrate is soluble in water because all nitrates are soluble.

Choosing the Correct Answer

From the options, ext{CaF}_2 is known to be insoluble in water. It does not dissolve readily due to its specific ionic lattice structure, which requires a significant amount of energy to break apart, energy that water does not provide.

Key concepts

Insoluble Compounds

Understanding which compounds are insoluble in water is important in chemistry. Compounds that do not dissolve well in water are termed insoluble. These compounds do not dissociate into ions effectively when mixed with water. Insoluble compounds are often found in various fields including cooking, geology, and pharmaceuticals.
Common examples include certain salts like carbonates, phosphates, and sulfides. Around specific cations, such as calcium and magnesium, these salts form strong ionic bonds that water molecules cannot break easily.
For instance, calcium carbonate (CaCO extsubscript{3}) is known for being insoluble because the energy required to dissociate its ions exceeds the energy provided by water molecules trying to hydrate them.
Recognizing insoluble compounds helps in predicting and explaining phenomena such as why certain mixtures cloud up or why some solutions remain clear.

Ionic Compounds Solubility

The solubility of ionic compounds in water is determined by their ability to dissociate into ions when mixed with water. Solubility rules help predict whether a compound will dissolve in water.
Many ionic compounds are soluble, but some exceptions exist based on the cations and anions involved.

• Salts containing nitrate (NO extsubscript{3} extsuperscript{-}), acetate (CH extsubscript{3}COO extsuperscript{-}), and most alkali metal ions (like sodium and potassium) are generally soluble.
• Halide salts, such as chlorides, bromides, and iodides, are soluble except when paired with silver, lead, and mercury.
• Sulfates are usually soluble, but exceptions include those of barium, calcium, and lead.

Solubility can vary with temperature and pressure, and knowing these rules is crucial for experiments involving solution reactions, manufacturing, and industrial processes.

Calcium Fluoride

Calcium fluoride (CaF extsubscript{2}) is a classic example of an insoluble ionic compound. Unlike many other ionic salts, calcium fluoride does not dissolve easily in water.
The ionic bonds between calcium ions (Ca extsuperscript{2+}) and fluoride ions (F extsuperscript{-}) are particularly strong, forming a stable lattice structure. This structure is difficult for water to penetrate and break apart.
As a result, CaF extsubscript{2} remains largely intact in aqueous environments, appearing as a precipitate if mixed with water. This property makes it useful in various applications where stability is needed, such as in toothpaste for fluoride addition and in optical components for lenses.
Understanding the solubility of calcium fluoride and similar compounds is important for fields that deal with water purification, chemical synthesis, and the production of materials where insolubility might be desirable or problematic.