Problem 26 - Chemistry The Molecular Science

Question

For each substance, what ions are present in an aqueous solution? (a) \(\mathrm{CaI}_{2}\) (b) \(\mathrm{Mg}_{3}\left(\mathrm{PO}_{4}\right)_{2}\) (c) NiS (d) \(\mathrm{MgBr}_{2}\)

Step-by-Step Solution

Answer

(a) \( \text{Ca}^{2+} \) and \( 2 \text{I}^- \); (b) \( 3 \text{Mg}^{2+} \) and \( 2 \text{PO}_4^{3-} \); (c) No ions; (d) \( \text{Mg}^{2+} \) and \( 2 \text{Br}^- \).

Step 1: Identify the compound

For each given compound, identify its chemical formula and the elements present. Each compound consists of positive and negative ions.

Step 2: Determine the ions for \( \text{{CaI}}_2 \)

Calcium iodide, \( \text{{CaI}}_2 \), dissociates in water to form calcium ions \( \text{{Ca}}^{2+} \) and iodide ions \( \text{{I}}^- \). Since there are two iodide ions for every calcium ion, the ions present are \( \text{{Ca}}^{2+} \) and \( 2 \text{{I}}^- \).

Step 3: Determine the ions for \( \text{{Mg}}_3(\text{{PO}}_4)_2 \)

Magnesium phosphate, \( \text{{Mg}}_3(\text{{PO}}_4)_2 \), when dissociated in water, forms magnesium ions and phosphate ions. It dissociates into \( 3 \text{{Mg}}^{2+} \) ions and \( 2 \text{{PO}}_4^{3-} \) ions.

Step 4: Determine the ions for NiS

Nickel sulfide, NiS, is insoluble in water and does not dissociate into ions in an aqueous solution. Therefore, there are no ions present in solution.

Step 5: Determine the ions for \( \text{{MgBr}}_2 \)

Magnesium bromide, \( \text{{MgBr}}_2 \), dissociates in water to form magnesium ions \( \text{{Mg}}^{2+} \) and bromide ions \( \text{{Br}}^- \). Since there are two bromide ions for every magnesium ion, the ions present are \( \text{{Mg}}^{2+} \) and \( 2 \text{{Br}}^- \).

Key Concepts

Aqueous Solution Chemistry

When a substance dissolves in water, it results in an aqueous solution. This is a liquid solution with water as the solvent. Many ionic compounds, when added to water, separate into their individual ions and become part of the liquid phase. This process is crucial in many fields such as biological functions, chemical reactions, and environmental processes.

In aqueous solutions, the ions are completely surrounded by water molecules. This occurs due to the polarity of water, which helps stabilize the charged particles. Aqueous solutions freely allow ions to move, making them conductive for electric current.

• Water molecules surround cations by orienting their negatively charged part, oxygen, towards the ion.
• For anions, they orient their positively charged parts, the hydrogen atoms.

Understanding how substances behave in aqueous solutions is key to predicting reactions, including precipitation, gas formation, and acid-base reactions.

Ion Dissociation

Ion dissociation refers to the separation of ions that occurs when an ionic compound dissolves in water. Depending on the compound's nature, it can dissociate completely or partially. Most strong electrolytes, like NaCl or MgBr\(_2\), dissociate completely.

When an ionic compound dissolves, it dissociates into its respective cations and anions. For example, when CaI\(_2\) is dissolved, it splits into Ca\(^{2+}\) and I\(^-\) ions.

• Each dissociated ion interacts with water molecules.
• The process increases the number of particles in the solution, impacting properties like boiling point and conductivity.

This dissociation is a physical change, as the ionic compound's chemical identity remains the same. Understanding dissociation helps determine how substances will interact in various reactions and environments.

Solubility Rules in Chemistry

Solubility rules help predict whether a compound will dissolve in water and to what extent. Certain compounds, like those containing alkali metal ions or ammonium ions, are generally soluble in water.

Other general rules include:

• Sulfates are mostly soluble, except for barium, calcium, and lead sulfate.
• Halides are usually soluble, except when paired with silver, lead, or mercury.

Understanding these rules is fundamental for predicting the outcome of reactions. For example, Nickel sulfide (NiS) is known to be insoluble due to its lack of dissociation in water. This identification helps chemists decide whether a precipitation reaction will occur and which ions will remain in solution.

Solubility rules are essential tools in laboratory settings and industries for efficient decision-making and successful experimental outcomes.