Question

Which of the following substances are likely to be soluble in water? (a) \(\mathrm{ZnS}\) (b) \(\mathrm{Au}_{2}\left(\mathrm{CO}_{3}\right)_{3}\) (c) \(\mathrm{PbCl}_{2}\) (d) \(\mathrm{Na}_{2} \mathrm{~S}\)

Short answer

Only \( \text{Na}_2\text{S} \) is likely soluble in water.

Step-by-step solution

Understanding Solubility Rules

To determine solubility, we need to apply general solubility rules for ionic compounds. In water, compounds containing alkali metal ions and ammonium are generally soluble. Nitrates, bicarbonates, and chlorates are soluble. Exceptions exist for sulfates, carbonates, and other rules need to be considered.

Examine Each Substance

Let's consider each compound separately to apply the solubility rules.

Substance (a): ZnS

Zinc sulfide (\( ZnS \)) is a sulfide. Sulfides are generally insoluble except those of alkali metals, ammonium, and a few others, which do not include zinc. Hence, \( ZnS \) is likely insoluble in water.

Substance (b): Au2(CO3)3

Gold(III) carbonate \( Au_2(CO_3)_3 \) involves a carbonate. Carbonates are generally insoluble except those of alkali metals and ammonium. Since gold is not an exception, this compound is likely insoluble in water.

Substance (c): PbCl2

Lead(II) chloride \( PbCl_2 \) is a chloride. Chlorides are generally soluble except for those of lead, silver, and mercury. Hence, \( PbCl_2 \) is an exception and tends to be only slightly soluble in water.

Substance (d): Na2S

Sodium sulfide \( Na_2S \) contains sodium, which is an alkali metal. Compounds with alkali metals are generally soluble. Therefore, \( Na_2S \) is soluble in water.

Conclusion

Based on solubility rules, among the given substances, only \( Na_2S \) is likely to be soluble in water while the others are not.

Key concepts

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons between each other, resulting in positively charged ions (cations) and negatively charged ions (anions). These oppositely charged ions are held together by strong electrostatic forces known as ionic bonds. Ionic compounds typically form crystalline structures and are often hard and brittle.

The formation of ionic compounds is especially common between metals and nonmetals. Metals tend to lose electrons, becoming cations, while nonmetals tend to gain those electrons to become anions. This electron exchange creates stable ionic compounds like sodium chloride (NaCl) or zinc sulfide (ZnS).

• Characteristics of Ionic Compounds:
  - High melting and boiling points
  - Conduct electricity when melted or dissolved in water
  - Generally soluble in water, but with exceptions

Water Solubility

Water solubility refers to the ability of a substance to dissolve in water. Solubility is influenced by the intermolecular forces between the solute (the substance being dissolved) and the solvent (water, in this case).

For ionic compounds, solubility in water often results from the interaction between the ions and water molecules. Water is a polar molecule, meaning it has a partial positive charge at one end and a partial negative charge at the other. This polarity allows water molecules to surround and separate the ions in an ionic compound, leading to solubility.

• Factors Affecting Solubility:
  - Temperature: Solubility typically increases with temperature
  - Nature of the solute and solvent: "Like dissolves like" (polar solvents dissolve polar solutes)
  - The specific ions involved: Some ions interact more strongly with water than others

Solubility Exceptions

Although many ionic compounds dissolve in water, there are notable exceptions based on solubility rules. These rules help predict whether a compound is soluble or insoluble. Some of the general rules to remember include:

• Compounds containing alkali metal ions (like Na⁺, K⁺) and ammonium (NH₄⁺) are typically soluble.
• Nitrates (NO₃^-), bicarbonates (HCO₃^-), and chlorates (ClO₃^-) are generally soluble.
• Most sulfates (SO₄^2-) are soluble, but those of barium (Ba²⁺), lead (Pb²⁺), and calcium (Ca²⁺) have exceptions.
• Carbonates (CO₃^2-) and phosphates (PO₄^3-) are usually insoluble, except with alkali metals or ammonium.
• Sulfides (S^2-) are generally insoluble, except for those of the alkali metals and ammonium.
• Chlorides (Cl^-) are mostly soluble, though exceptions include silver chloride (AgCl) and lead(II) chloride (PbCl₂).

These exceptions are vital when predicting the solubility of different substances.

Chemistry Education

Understanding solubility rules is crucial in chemistry education. These rules serve as foundational knowledge for predicting reactions and outcomes in aqueous solutions. Learning how to identify soluble and insoluble compounds aids students in predicting precipitate formation in chemical reactions.

Solubility concepts are commonly introduced in high school chemistry, where students learn through practical experiments and theoretical lessons. In more advanced studies, students explore the quantitative aspects of solubility, like solubility product constants (K_sp) and their calculations.

• Skills Acquired from Learning Solubility:
  - Ability to predict reaction products
  - Understanding reaction mechanisms and equilibria
  - Developing problem-solving skills in chemistry
  - Applying solubility principles to real-world situations

Incorporating exercises that involve identifying soluble and insoluble substances, like the given example, helps reinforce these critical skills.