What is a locating agent?

• A. A chemical sprayed on chromatograms to reveal colourless substances
• B. A solvent for chromatography
• C. A catalyst for reactions
• D. A reagent that speeds up chromatography

Answer: (A)

A locating agent is a chemical used to reveal colourless spots on a chromatogram after development. Many substances that are separated in chromatography don’t have any color, so they’re invisible under normal light. Spraying or dipping the plate with a locating agent causes those spots to develop a visible colour (or fluorescence), allowing you to see and compare the different components.

This is why it’s the best answer: the purpose of a locating agent is specifically visualization, not movement or speeding up the process. A solvent helps move substances through the stationary phase to achieve separation, but it doesn’t make colourless spots visible. A catalyst speeds up a chemical reaction, which is not the role here. A reagent that speeds up chromatography would imply accelerating the separation itself, which isn’t what a locating agent does.