Question

How can we distinguish: (A) I \(_{2}\) solution (B) NaI solution (C) \(\mathrm{Br}_{2} / \mathrm{H}_{2} \mathrm{O}\) solution (D) Na metal

Short answer

To distinguish between the given solutions and metal: 1. Observe color and state of matter: Iodine (I₂) solution is brown or purple, Sodium iodide (NaI) solution is colorless, Bromine (Br₂) in water (H₂O) is orangish or reddish-brown, and Sodium (Na) metal is silvery-white. 2. Perform a chemical test with silver nitrate (AgNO₃) solution: Only Sodium iodide (NaI) solution reacts, forming a yellow precipitate of silver iodide (AgI). 3. Perform a chemical test with chlorine (Cl₂) gas: Iodine (I₂) solution and Sodium iodide (NaI) solution show color changes, while Bromine (Br₂) in water (H₂O) and Sodium (Na) metal do not react measurably.

Step-by-step solution

Observe color and state of matter

First, look at the color and state of each solution. Here are some distinguishing characteristics: (A) Iodine (I₂) solution: Iodine in water appears brown, and in some organic solvents it appears purple. (B) Sodium iodide (NaI) solution: This solution is a colorless, aqueous liquid. (C) Bromine (Br₂) in water (H₂O): This solution is typically an orangish or reddish-brown liquid. (D) Sodium (Na) metal: Sodium is a soft, silvery-white metal at room temperature.

Perform a chemical test with silver nitrate (AgNO₃) solution

Add a few drops of silver nitrate (AgNO₃) solution to a small portion of each solution. Observe any change in the solution, such as precipitation or color change. (A) Iodine (I₂) solution: No visible reaction is expected since AgNO₃ does not react with iodine. (B) Sodium iodide (NaI) solution: A yellow precipitate of silver iodide (AgI) forms immediately due to the following reaction: \[ \mathrm{NaI} + \mathrm{AgNO}_3 \rightarrow \mathrm{AgI} \downarrow + \mathrm{NaNO}_3 \] (C) Bromine (Br₂) in water (H₂O): No visible reaction is expected since AgNO₃ does not react with bromine. (D) Sodium (Na) metal: No visible reaction is expected since AgNO₃ is not reactive with the metal in its solid state.

Perform a chemical test with chlorine (Cl₂) gas

Carefully introduce chlorine (Cl₂) gas to a small portion of each solution in a well-ventilated area: (A) Iodine (I₂) solution: The iodine will be displaced by chlorine, and the color will change from brown or purple to a lighter color. The reaction is: \[ \mathrm{I}_2 + \mathrm{Cl}_2 \rightarrow \mathrm{2ICl} \] (B) Sodium iodide (NaI) solution: The iodide will be displaced by chlorine, and a brown color of iodine will be observed. The reaction is: \[ 2\mathrm{NaI} + \mathrm{Cl}_2 \rightarrow \mathrm{I}_2 + \mathrm{2NaCl} \] (C) Bromine (Br₂) in water (H₂O): Since chlorine is a more reactive halogen than bromine, chlorine will displace bromine from the solution, producing a color change from reddish-brown to lighter. The reaction is: \[ \mathrm{2Br} + \mathrm{Cl}_2 \rightarrow \mathrm{2ClBr} \] (D) Sodium (Na) metal: No measurable reaction will occur.

Key concepts

Understanding Iodine Solution

Iodine solution is an intriguing chemical substance due to its noticeable color changes depending on the medium it is dissolved in. When iodine is mixed with water, it typically appears as a brown solution. However, if you dissolve iodine in certain organic solvents, it takes on a purple hue. These color variations make iodine solution distinct and relatively easy to identify visually in a laboratory setting.
Additionally, when performing chemical tests with iodine solution, such as introducing silver nitrate ( AgNO₃ ) or chlorine ( Cl₂ ) gas, you may notice certain characteristics. For instance, silver nitrate does not react visibly with iodine because there's no precipitation or observable color shift. On the other hand, if chlorine gas is mixed with an iodine solution, the chlorine displaces iodine resulting in a lighter color due to the formation of iodine monochloride ( ICl ). This displacement reaction highlights chlorine's higher reactivity compared to iodine.

Exploring Sodium Iodide Solution

Sodium iodide solution ( NaI ) has its own set of properties that help distinguish it from other solutions. This solution usually appears colorless when in its aqueous form, making it deceptive in terms of visual identification. A practical method to differentiate sodium iodide involves conducting a chemical test with silver nitrate ( AgNO₃ ).
When silver nitrate is added to sodium iodide, you will notice the immediate formation of a yellow precipitate known as silver iodide ( AgI ). This reaction occurs because iodide ions react with silver ions to form the insoluble compound. The reaction can be represented as: \[ \mathrm{NaI} + \mathrm{AgNO}_3 \rightarrow \mathrm{AgI} \downarrow + \mathrm{NaNO}_3 \]
Introducing chlorine gas to a sodium iodide solution results in chlorine displacing iodide ions, which then forms iodine and sodium chloride. This causes the solution to exhibit a brown color due to newly formed iodine, confirming the presence of sodium iodide through a visible chemical change.

Unveiling Bromine Water

Bromine water is typically an orangish or reddish-brown solution, created when bromine ( Br₂ ) dissolves in water ( H₂O ). This visually distinctive color sets it apart from colorless or differently colored solutions. In chemical testing, bromine water can demonstrate its unique reactivity, or rather, lack thereof.
When silver nitrate is added to bromine water, no visible reaction occurs. This is because silver ions do not react with bromine in water to form a precipitate or a noticeably different color. However, introducing chlorine gas into bromine water results in an interesting reaction due to chlorine’s higher reactivity. Chlorine displaces bromine from the solution, leading to a lighter color as bromine is effectively displaced from the water. The chemical reaction can be exemplified as:
\[ \mathrm{2Br}_2 + \mathrm{Cl}_2 \rightarrow \mathrm{2ClBr} \] This is useful for chemical analysis in determining the composition of an unknown solution containing halogens.

Properties of Sodium Metal

Sodium metal is an element characterized by its distinctive physical appearance and chemical properties. At room temperature, sodium appears as a silvery-white metal that is soft enough to be cut with a knife. Its softness and color can help easily distinguish it from other metals or solutions.
In terms of chemical reactions, sodium metal is relatively reactive, especially with water and other substances. However, when carrying out tests with silver nitrate ( AgNO₃ ) , you won’t observe any visible reaction since silver nitrate is not reactive with sodium metal in its solid state. Furthermore, introducing chlorine gas to sodium metal will not result in a measurable reaction. Sodium does react vigorously with other substances like water where it forms sodium hydroxide and hydrogen gas, but in these particular analytical tests with AgNO₃ and Cl₂ , no significant change to note occurs.