Question

Suppose you are given a sample of a homogeneous liquid. What would you do to determine whether it is a solution or a pure substance?

Short answer

To determine whether a homogeneous liquid is a solution or a pure substance, follow these steps: 1. Observe the liquid's appearance for consistency. 2. Heat the liquid and observe its boiling behavior, with a pure substance exhibiting a sharp boiling point and a solution having a boiling point range. 3. Examine residues after boiling, with no residues potentially indicating a pure substance and residues suggesting a solution. 4. Measure the refractive index; a constant value implies a pure substance, while varying values indicate a solution. 5. Perform a chromatography test to identify the components, with one component being a pure substance and multiple components indicating a solution.

Step-by-step solution

Observe the liquid sample

Look at the liquid and observe its appearance. A consistent or uniform appearance usually indicates a homogeneous substance, which could be a solution or a pure substance. If visible separation occurs, it is not a homogeneous substance and thus not a sample we are interested in.

Boil the liquid

Heat the liquid gently to observe its boiling behavior. A pure substance will typically have a sharp boiling point, meaning that it will boil and turn into a vapor at a specific temperature. A solution, however, will have a boiling point range, meaning that it will start boiling at a particular temperature and continue boiling over a range of temperatures, as different components vaporize at different temperatures.

Observe the residues

After boiling the liquid, observe the residues left behind. If there are no residues and the liquid completely evaporates (assuming it is not a volatile pure substance), it may be a pure substance. If there are residues left behind after boiling, it is likely a solution because the solute is left behind after the solvent has evaporated.

Conduct a refractive index test

Measure the refractive index of the liquid using a refractometer. A pure substance has a constant refractive index value, whereas the refractive index of a solution can vary depending upon the concentrations of the solutes dissolved in the solvent. If the refractive index value matches the known values of a pure substance, it can be identified as a pure substance. Otherwise, it's likely a solution.

Perform a chromatography test

Use a chromatography test (like paper chromatography or thin-layer chromatography) to analyze the components present in the liquid sample. If the test shows the presence of only one component, it is a pure substance. If the test shows the presence of multiple components, it is a solution. By following these steps, one can confidently determine whether a given homogeneous liquid is a solution or a pure substance.