Question

Assume that you have three white solids: \(\mathrm{NaCl}, \mathrm{KCl}\), and \(\mathrm{MgCl}_{2}\). What tests could you do to tell which is which?

Short answer

Perform a flame test; NaCl gives a yellow flame, KCl gives a lilac flame, and MgCl_2 does not change the flame color.

Step-by-step solution

Solubility Test

Dissolve a small amount of each white solid in water. - NaCl and KCl will dissolve easily in water since both are highly soluble. - MgCl_2 is very soluble in water and will also dissolve quickly. - Note: Solubility test alone will not distinguish between the three salts as they all dissolve.

Flame Test

Perform a flame test for each dissolved solution. - Dip a clean wire loop into each solution and place it in the flame of a Bunsen burner. - Observe the color of the flame: - NaCl will produce a yellow flame. - KCl will produce a lilac-colored flame. - MgCl_2 will not produce a characteristic flame color due to Mg's emission spectrum not being visible.

Precipitation Test with Silver Nitrate

Add a few drops of silver nitrate (\( ext{AgNO}_3\)) solution to each dissolved sample:- NaCl, KCl, and MgCl_2 will all produce a white precipitate of \( ext{AgCl}\).- Distinguishing between them requires further testing.

Conductivity Test

Measure the electrical conductivity of each solution:- NaCl and KCl solutions will have high conductivity due to the presence of sodium (\( ext{Na}^+\)) and potassium (\( ext{K}^+\)) ions.- MgCl_2 will exhibit higher conductivity due to magnesium ions (\( ext{Mg}^{2+}\)), which provide two charges per ion, increasing ionic mobility.

Key concepts

Solubility Test

The solubility test is a simple and effective way to determine how well substances dissolve in a given solvent, often water. When dealing with salts like sodium chloride (NaCl), potassium chloride (KCl), and magnesium chloride (MgCl₂), understanding solubility is important, although all these salts dissolve well in water.

Here's how this test works:

• Take a small amount of each white solid.
• Add each to separate containers filled with water.
• Stir until no more solid dissolves.

In this case, NaCl, KCl, and MgCl₂ are all highly soluble and dissolve completely, making them indistinguishable by this test alone. However, it's good to start with solubility to signal which salts might dissolve quickly in moderate conditions.

Flame Test

The flame test is a handy technique for identifying metal ions based on the color they emit when heated. This occurs because the heat excites the electrons, causing them to emit light of specific wavelengths.

To conduct a flame test:

• Take a clean wire loop and dip it into one of the dissolved salt solutions.
• Place the wet wire loop into the flame of a Bunsen burner.
• Observe and note the flame color.

With our specific salts:

• NaCl will burn with a bright yellow flame, a distinct color due to sodium ions.
• KCl will produce a lilac or light violet flame, which helps distinguish potassium ions.
• MgCl₂ will not generate a characteristic color, making it less useful for this salt.

Thus, flame tests are great for spotting Na+ and K+ ions but less effective with Mg²+ ions due to their minimal visible emission.

Precipitation Reaction

A precipitation reaction involves the formation of an insoluble compound from the mixture of two soluble salts. In identifying chloride salts, you would add silver nitrate (AgNO₃) to the solution, which can cause a white precipitate of silver chloride (AgCl) to form.

Here's the step-by-step process:

• Add a few drops of silver nitrate solution to each of the dissolved salt solutions.
• Look for the formation of a white precipitate.

For NaCl, KCl, and MgCl₂:

• All solutions will produce a white precipitate of AgCl.

Since all three salts react similarly with AgNO₃, the precipitation test itself is not adequate for distinguishing these specific salts alone. Nonetheless, it confirms the presence of chloride ions.

Conductivity Test

Conductivity tests are used to measure the ability of an aqueous solution to conduct electricity, which depends on the presence of ions.

The steps include:

• Insert conductive probes into each salt solution.
• Measure the conductivity using a conductivity meter.

With NaCl and KCl solutions, you'd expect high conductivity due to their abundance of sodium (Na⁺) and potassium (K⁺) ions. However, MgCl₂ shines in this test.

• MgCl₂ solution will have the highest conductivity, owing to its ions, Mg²+ and Cl⁻, providing more charge and enhancing ionic mobility.

Therefore, measuring conductivity can crucially differentiate MgCl₂ from the other salts, as its ions confer the highest charge per ion ratio.