Question

Freshly prepared solutions containing iodide ion are colorless, but over time they usually turn yellow. Describe a plausible chemical reaction (or reactions) to account for this observation.

Short answer

Iodide ion (I-) in the solution gets oxidized over time to form iodine (I2), which is yellow. This is why the colorless solution turns yellow over time.

Step-by-step solution

Identifying the initial substance

Identify the initial substance in the solution which is iodide ion (I-). This is a colorless solution.

Recognizing the change over time

Recognize that overtime, the solution changes its color to yellow. This indicates that a chemical reaction is taking place.

Identifying the product of the reaction

Based on the color change, identify the product of the reaction. Yellow color indicates that the iodide ion (I-) is getting oxidized to iodine (I2). The balanced oxidation reaction is: 2I-(aq) -> I2(s)

Key concepts

Color Change in Chemical Reactions

When you observe a color change in a chemical solution, it signals a chemical reaction. In the case of iodide ions turning yellow, the initial solution starts as clear and colorless. This change in color happens because substances can transform when they react with each other. As these reactions occur, new substances form, often with different colors.
The yellow color we see is a visual indicator of the presence of iodine. This is because the iodide ions undergo a transformation. As they change, they display different light-absorbing and reflecting properties.

• Color changes can help us identify new substances.
• These transformations are clues to underlying chemical processes.

Understanding color change helps students connect visual cues to chemical principles.

Oxidation Reactions

Oxidation reactions are a key part of chemistry where atoms lose electrons. In the oxidation of iodide ions, the iodide ( I^- ) gives up electrons and becomes iodine ( I_2 ). This reaction can be simplified as: [ 2I^{- } ightarrow I_{2} ]
Oxidation is always paired with reduction, where another substance gains electrons. However, in many reactions like this, the focus is on the substance losing electrons.

• Oxidation involves electron loss, leading to a chemical transformation.
• In our example, iodide ions undergo oxidation to form iodine.

Understanding oxidation helps students explain why substances change during reactions.

Iodine Formation

Iodine formation is the result of the iodide ions undergoing oxidation. As I^- ions lose electrons, they bond together to create I_2 , which is iodine in its elemental form.
This process changes the solution from colorless to yellow due to iodine's distinct color. Unlike iodide ions, iodine absorbs and reflects light differently, which results in the visible color change.

• Iodide ions bond to form neutral iodine molecules.
• The reaction's color change is due to iodine's presence.

Understanding how and why iodine forms helps students see the connection between reactions and their visible signs.