Chemical reactions can cause many noticeable changes, and one of the most obvious is a change in color. This happens because of how the different parts, called reactants and products, interact with each other at a tiny level. Let’s explore why some reactions change color while others do not.

Factors That Affect Color Change

1. Chemical Structure: Molecules can absorb and give off light in different ways depending on their structure. When a reaction happens, the structure of the molecules may change, leading to a different color of light being absorbed. This is why we see a color change.

2. pH Changes: Some reactions create acidic or basic solutions, which can change the color of certain indicators. For example:

   • Litmus paper changes color: it turns red in acid (when pH is less than 7) and blue in base (when pH is more than 7).
   • Phenolphthalein is clear in acid but turns pink in basic solutions.

3. Product Formation: Some reactions make colored products. For example, when copper(II) sulfate reacts with sodium hydroxide, it creates a blue solid called copper(II) hydroxide. This shows a clear change in color.

Color Change Statistics

• In solutions, color change often happens because of the amount of colored ions present. For instance, if you increase the amount of potassium permanganate in a solution, it becomes a deeper purple.
• About 30% of chemical reactions studied in high school chemistry show visible color changes. This means that while color changes are common, many reactions don’t change color because of the types of reactants and products involved.

In summary, color changes in chemical reactions are important clues that help us understand what is happening during the reaction.