Why is a chromatogram run in the dark?

A chromatogram is run in the dark primarily to preserve the pigments from fading. Many pigments that are used in chromatography can be sensitive to light, which can lead to degradation or alteration of their chemical structure when exposed to light over time. This degradation can cause a loss of color intensity or even a complete change in the pigment, affecting the results of the chromatography. By conducting the experiment in darkness, the integrity of the pigments is maintained, allowing for a more accurate analysis of the results once they are visualized or analyzed.

The other options do not accurately reflect the primary reason for running a chromatogram in the dark. While increasing the size of the spots, preventing overheating, or enhancing color contrast might be considerations in different contexts, they do not address the specific concern of pigment preservation as effectively as the correct choice does.