Question

Give an example of a chemical reaction for the following situation: A change in colour is observed.

Answer 1

Hint:To answer this question, you must recall various reactions and the changes occurring through the course of the reaction. A chemical reaction is the process in which some substances known as reactants react chemically to form other substances known as the products by some chemical transformations.

Complete answer:
When two or more substances combine to create some new substances, it may have different chemical properties and molecular structure from the original reactants.
Chemical reactions show various characteristics and each chemical reaction shows at least one or more of those characteristics. The characteristics of chemical reactions are evolution of gas, change in temperature, change in color, change in energy, change in state or formation of precipitate.
For a reaction showing colour change the substances absorb and radiate light wavelengths in various ways and thus, showing colour change.
For example, when lead nitrate reacts with potassium iodide, a precipitate of lead iodide is formed along with potassium nitrate. The initial substances, i.e., lead nitrate and potassium iodide are colorless and the precipitate of lead iodide formed is yellow in colour.
Such reactions are the most commonly used for the determination of presence of a certain anion or cation in qualitative analysis if possible.

Note:
Some other characteristics of chemical reactions, like evolution of gas, change in temperature and formation of precipitate can be easily detected physically and are thus also used commonly for identification in the qualitative analysis of various compounds along with the change in colour.