How can a coloured reaction product reveal the presence of a chemical substance?

A coloured reaction product reveals the presence of a chemical substance through selective chemical reactions and the principles of light absorption.

Qualitative Identification

In classical qualitative analysis, specific reagents are added to an analyte to induce a reaction that produces a distinctive, visible colour. For example, copper(II) reacts with ammonia to form a deep blue complex. Similarly, the disappearance of a characteristic colour can signal a reaction, such as when bromine’s red-brown colour fades upon reacting with carbon-carbon double bonds. These visual changes allow chemists to deduce the identity of the substance present.1

Quantitative Measurement

Beyond simple identification, the intensity of a colour can be used to determine the concentration of a substance through colorimetry or spectrophotometry. These methods rely on Beer’s law, which states that the amount of light absorbed by a solution is directly proportional to the concentration of the absorbing substance. By measuring the light transmitted through a sample, scientists can precisely quantify substances, often at extremely low concentrations.23

The colour itself arises from chromophores—specific groups of atoms within a molecule that absorb energy from light at certain wavelengths, reflecting or transmitting the remaining visible light.4