A chromatogram that shows only one peak indicates the sample is:

• A. A pure substance
• B. A mixture
• C. An alloy
• D. A solvent

Answer: (A)

Seeing a single peak means there is only one component in the sample. In chromatography, each different substance interacts with the stationary phase and travels at a different rate with the mobile phase, so it shows up as its own peak. If the sample were a mixture, you’d expect multiple peaks, one for each component present. So a single peak is the sign of a pure substance. A solvent is just the liquid used to carry the sample, and an alloy is a solid metal mixture that chromatography isn’t used to distinguish in this context; a mixture would not give a single peak.