Problem 83 - 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: Solubility in Water

To separate compounds by dissolving one in water, we need to determine which compound is soluble in water. Check the solubility guidelines for each compound.

Step 2: Analyzing Pair (a)

For NaOH and Ca(OH)2: NaOH is highly soluble in water, while Ca(OH)2 is only slightly soluble. Therefore, NaOH can be separated from Ca(OH)2 by dissolving it in water.

Step 3: Analyzing Pair (b)

For MgCl2 and MgF2: MgCl2 is soluble in water, while MgF2 is insoluble. Thus, MgCl2 can be separated from MgF2 by dissolving it in water.

Step 4: Analyzing Pair (c)

For AgI and KI: AgI is insoluble in water, whereas KI is soluble. Therefore, KI can be separated from AgI by dissolving KI in water.

Step 5: Analyzing Pair (d)

For NH4Cl and PbCl2: NH4Cl is soluble in water, while PbCl2 is sparingly soluble. Hence, NH4Cl can be separated from PbCl2 by dissolving it in water.

Key Concepts

Separation Techniques

Separation techniques allow us to isolate components from a mixture, which is particularly useful in chemical processes. One common technique is dissolution, where compounds are separated based on their ability to dissolve in a specific solvent, such as water. This approach is effective when substances have different solubility in water. For instance, if one compound in a mixture readily dissolves in water and the other does not, we can use water to selectively dissolve the soluble compound. Other separation methods include:

• Filtration: used to separate solids from liquids.
• Distillation: takes advantage of differences in boiling points.
• Centrifugation: separates based on density.
• Chromatography: used for separating substances based on movement through a medium.

Each technique has its own advantages and applications, but the right choice of method depends on the specific physical and chemical properties of the substances involved.

Inorganic Compounds

Inorganic compounds are often mineral-based and are not associated with living organisms. They can be salts, oxides, silicates, and more, comprising a large variety of chemicals. Many of these compounds are characterized by ionic bonds formed between metals and nonmetals. Examples of inorganic compounds include:

• Sodium hydroxide (\( \mathrm{NaOH} \)
• Calcium hydroxide (\( \mathrm{Ca(OH)}_{2} \)
• Magnesium chloride (\( \mathrm{MgCl}_{2} \)
• Ammonium chloride (\( \mathrm{NH}_{4} \mathrm{Cl} \)

In contrast to organic compounds, inorganic compounds often exhibit high melting and boiling points, and they are good conductors of electricity when dissolved in water due to the formation of ions.

Chemical Solubility

Chemical solubility refers to the maximum amount of a substance that can dissolve in a solvent at a given temperature. It's a critical concept in chemistry because it determines whether a compound can be used for reactions in a particular solution. Factors affecting solubility include:

• Temperature: Generally, solubility increases with temperature for solids.
• Nature of solute and solvent: Like dissolves like; polar solvents dissolve polar substances.
• Pressure: This mainly affects the solubility of gases.

For example, in our exercise:- \( \mathrm{NaOH} \) is highly soluble in water.- \( \mathrm{MgCl}_{2} \) is soluble, whereas \( \mathrm{MgF}_{2} \) is not, illustrating the varied solubility even among similar compounds. Knowing the solubility helps chemists predict how substances will react when mixed with solvents.

Mixture Separation

Mixture separation is an essential process in both chemical laboratories and industries. A mixture consists of two or more substances that are not chemically bonded, and physical methods can often separate the components. These methods rely on differences in physical properties like solubility, boiling point, or magnetic properties.In the context of the given exercise:

• Sodium hydroxide (\( \mathrm{NaOH} \)) can be separated from calcium hydroxide (\( \mathrm{Ca(OH)}_{2} \)) using water, as it dissolves readily.
• Ammonium chloride (\( \mathrm{NH}_{4} \mathrm{Cl} \)) can be separated from lead chloride (\( \mathrm{PbCl}_{2} \)) due to its higher solubility.

Understanding the principles of mixture separation is crucial for purifying substances and recovering materials from complex matrices.

Solubility Guidelines

Solubility guidelines provide general rules about the solubility of various compounds in water. These guidelines are essential for predicting whether a chemical will dissolve or form a precipitate when combined with water.Some basic solubility guidelines are:

• Most nitrates (\( \mathrm{NO}_{3}^{-} \)) are soluble.
• Alkali metal salts and \( \mathrm{NH}_{4}^{+} \) salts are generally soluble.
• Chlorides (\( \mathrm{Cl}^{-} \)), bromides (\( \mathrm{Br}^{-} \)), and iodides (\( \mathrm{I}^{-} \)) are soluble, except with \( \mathrm{Ag}^{+} \), \( \mathrm{Hg}_2^{2+} \), and \( \mathrm{Pb}^{2+} \).
• Sulfates (\( \mathrm{SO}_{4}^{2-} \)) are soluble, with exceptions like \( \mathrm{BaSO}_{4} \) and \( \mathrm{PbSO}_{4} \).

By understanding these guidelines, chemists can effectively plan experiments and predict the outcomes of reactions involving solutions.