Question

An ionic compound \(\mathrm{X}\) is only slightly soluble in water. What test would you employ to show that the compound does indeed dissolve in water to a certain extent?

Short answer

Use a conductivity test to show the ionic compound partially dissolves, as increased conductivity indicates dissolved ions.

Step-by-step solution

Understanding Solubility

An ionic compound being slightly soluble means that only a small amount of the compound will dissolve in water to form ions. To confirm this, we need to find evidence of ions in the solution.

Utilizing Conductivity Test

One way to test for dissolved ions is to use a conductivity test. When an ionic compound dissolves, it dissociates into ions, and these ions can carry an electric current in the solution.

Set Up the Experiment

Prepare a conductivity meter and two beakers, one with pure water and another with the water whose solubility you want to test by adding some of the ionic compound.

Perform the Conductivity Test

First, test the conductivity of pure water (it should show very low conductivity). Then, test the solution containing the ionic compound. An increase in conductivity indicates the presence of dissolved ions.

Analyze Results

Compare the conductivity readings. If the solution with the compound has higher conductivity than pure water, it confirms that the ionic compound dissolves to some extent in the water.

Key concepts

Ionic Compound

Ionic compounds are fascinating substances made up of positively and negatively charged ions held together by electrostatic forces. These ions form a crystalline structure and are generally composed of metals bonded with non-metals. Here's how it works:

• Metals tend to lose electrons, forming positive ions, known as cations,
• while non-metals gain electrons, forming negative ions, known as anions.

For example, sodium chloride (NaCl), a common table salt, is an ionic compound where sodium ions (Na⁺) and chloride ions (Cl⁻) bond together. This bond creates a solid crystal lattice, giving ionic compounds high melting points and making them soluble in polar solvents like water.

Water Solubility

Solubility is the ability of a substance to dissolve in a solvent, such as water, to form a solution. When we say an ionic compound is slightly soluble in water, it means only a minimal amount of the compound will dissolve. This behavior depends on several factors:

• The strength of the ionic bonds in the compound,
• Temperature of the water,
• The polarity of water, which effectively attracts and pulls apart the ions.

In our exercise, even if the ionic compound does not completely dissolve, a small amount may still form ions in water, suggesting some degree of solubility.

Dissolved Ions

When an ionic compound dissolves in water, it separates into its individual ions. These dissolved ions are what allow the solution to conduct electricity. This dissociation is the reason ionic compounds are excellent electrolytes. In a water solution:

• The positive and negative ions disperse and move freely,
• allowing them to interact with water molecules and conduct an electric current.

Therefore, the presence of dissolved ions in water is a key indicator that a compound has indeed dissolved to some extent, despite its limited solubility.

Conductivity Meter

A conductivity meter is a crucial tool for testing the presence of ions in a solution. It measures electrical conductivity to determine if and how many ions are present. The principle is simple: the more ions that are dissolved, the higher the conductivity. To utilize this tool:

• First, check the conductivity of pure water, which should have minimal conductivity due to only a few self-ionizing water molecules,
• then compare it with a solution containing the ionic compound.

An increase in conductivity in this test indicates that the ionic compound has dissolved, releasing ions into the water. This method provides a clear visual confirmation of solubility, making the conductivity meter an essential instrument for such experiments.