Problem 75 - Chemistry and Chemical Reactivity

Question

You are given mixtures containing the following compounds. Which compound in each pair could be separated by stirring the solid mixture with water? (a) \(\mathrm{NaOH}\) and \(\mathrm{Ca}(\mathrm{OH})_{2}\) (b) \(\mathrm{MgCl}_{2}\) and \(\mathrm{MgF}_{2}\) (c) AgI and KI (d) \(\mathrm{NH}_{4} \mathrm{Cl}\) and \(\mathrm{PbCl}_{2}\)

Step-by-Step Solution

Answer

(a) NaOH, (b) MgCl2, (c) KI, (d) NH4Cl.

Step 1: Identify Solubility Properties

For each pair of compounds, determine the solubility of each in water. Solubility rules can help determine if a compound will dissolve in water. Typically, most sodium, potassium, ammonium, and nitrate salts are soluble. Conversely, many hydroxides, sulfates, and halides of heavier metals are less soluble.

Step 2: Analyze Pair (a)

For the pair (a) \( \mathrm{NaOH} \) and \( \mathrm{Ca(OH)}_{2} \):\ \( \mathrm{NaOH} \) is highly soluble in water, dissolving readily. \ \( \mathrm{Ca(OH)}_{2} \) has limited solubility but is also sparingly soluble. NaOH will dissolve completely, allowing for potential separation.

Step 3: Analyze Pair (b)

For the pair (b) \( \mathrm{MgCl}_{2} \) and \( \mathrm{MgF}_{2} \):\ \( \mathrm{MgCl_{2}} \) is highly soluble in water, while \ \( \mathrm{MgF_{2}} \) has low solubility. Stirring the mixture with water will dissolve \( \mathrm{MgCl_{2}} \), which allows for the separation of the two compounds.

Step 4: Analyze Pair (c)

For the pair (c) AgI and KI: \( \ \mathrm{KI} \) is highly soluble in water, while \( \mathrm{AgI} \) is highly insoluble. Stirring in water will dissolve \( \mathrm{KI} \), enabling separation from \( \mathrm{AgI} \).

Step 5: Analyze Pair (d)

For the pair (d) \( \mathrm{NH}_{4} \mathrm{Cl} \) and \( \mathrm{PbCl}_{2} \):\ \( \mathrm{NH}_{4} \mathrm{Cl} \) is highly soluble in water, whereas \( \mathrm{PbCl}_{2} \) is less soluble, especially in cold water. \ \( \mathrm{NH}_{4} \mathrm{Cl} \) will dissolve, facilitating separation.

Key Concepts

Solubility Rules

Understanding solubility rules is like unlocking a secret code in chemistry. Solubility determines how well a substance will dissolve in a solvent, such as water. Knowing these rules can save you a lot of trouble when trying to figure out which substances can be separated using water. Here are some handy guidelines:

• Most sodium (Na), potassium (K), and ammonium (NH₄) salts are soluble.
• Nitrate (NO₃) salts are typically soluble.
• Chlorides (Cl^-), bromides (Br^-), and iodides (I^-) are generally soluble, except when paired with silver (Ag⁺), lead (Pb²⁺), or mercury (Hg₂²⁺).
• Most hydroxides (OH^-) and sulfates (SO₄^2-) have varying solubility based on the metal they are paired with.

Knowing these rules helps predict whether a given compound in a mixture will dissolve when stirred with water. This is crucial in determining if a chemical separation is possible.

Chemical Separation

Chemical separation involves breaking down a mixture into its individual components. In the context of solubility, it means using a solvent, such as water, to select which components will dissolve and which will not. By understanding the solubility properties of each compound, you can strategically choose a method to separate them.For instance, consider the pair (b), \( \mathrm{MgCl}_{2} \) and \( \mathrm{MgF}_{2} \):

• \( \mathrm{MgCl_{2}} \) dissolves readily in water.
• \( \mathrm{MgF_{2}} \) barely dissolves, making it insoluble in practical terms.

When stirred in water, \( \mathrm{MgCl_{2}} \) can completely dissolve, allowing you to filter out the remaining solid \( \mathrm{MgF_{2}} \). This process exemplifies how solubility can guide chemical separation strategies.

Solubility Properties

Solubility properties are unique to each compound. These properties determine how a substance interacts with a solvent at a given temperature and pressure. Factors such as the ionic charge, size of the ions, and lattice energy contribute to a substance's solubility.For example, within the pair (c),- **\( \mathrm{KI} \)**: Being a common salt, it dissolves easily due to the presence of potassium and iodide ions, which are readily solvated by water.- **\( \mathrm{AgI} \)**: Highly insoluble because silver iodide forms a strong ionic lattice that water cannot easily break.Understanding these properties allows you to predict which substances can be isolated by stirring them with water, affirming that a solid understanding of solubility properties is crucial for effective mixture separation.

Mixture Separation

Mixture separation is a fundamental process in chemistry used to isolate desired substances from a group. This is achieved by exploiting differences in solubility. When you have a mixture and want to separate it, a solvent can be used to select which components will stay dissolved and which will remain as solids.Take, for instance, the pair (d),\( \mathrm{NH}_{4} \mathrm{Cl} \) and \( \mathrm{PbCl}_{2} \):

• \( \mathrm{NH}_{4} \mathrm{Cl} \) dissolves readily in water due to its solubility properties.
• \( \mathrm{PbCl}_{2} \), however, is less soluble, especially in cold water.

By making this distinction, \( \mathrm{NH}_{4} \mathrm{Cl} \) can be removed from the mixture, leaving \( \mathrm{PbCl}_{2} \) as a solid. This technique allows for effective separation and is frequently used in laboratories to purify substances or simply to extract one component of a mixture for further study.