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 sample is a solution or a pure substance, perform the following tests: 1) Observe the sample's physical properties; 2) Determine the boiling point; 3) Determine the freezing point; 4) Measure the refractive index using a refractometer; 5) Compare the results to known values of pure substances; and 6) Confirm your findings using additional tests such as solubility tests, chemical tests, or chromatography. By analyzing the results of these tests, you can differentiate between a solution and a pure substance.

Step-by-step solution

Observe the sample

Carefully examine the homogeneous liquid sample. Pay close attention to its color, smell, and consistency. A pure substance will generally have a consistent appearance, while a solution may have varying attributes. However, this observation alone may not be enough to determine whether it is a solution or a pure substance as some solutions may appear to be a single substance.

Determine the boiling point

Perform a boiling point test on the liquid sample. Heat the sample in a controlled environment using a thermometer to monitor the temperature. Record the boiling point. A pure substance will have a specific and constant boiling point, while a solution may have a boiling point that varies based on the ratio of the substances within the solution.

Determine the freezing point

Perform a freezing point test on the sample. Cool the sample in a controlled environment using a thermometer to monitor the temperature. Record the freezing point. A pure substance will have a specific and constant freezing point, while a solution may have a freezing point that varies based on the ratio of the substances within the solution.

Measure the refractive index

Using a refractometer, measure the refractive index of the liquid sample. The refractive index is a property that can help distinguish between a pure substance and a solution, as the refractive index of a solution will be different from that of a pure substance.

Compare the results to known values

Compare the boiling point, freezing point, and refractive index results with known values for the pure substances. If the measured values correspond to a known pure substance, the sample is likely a pure substance. However, if the measured values do not match any known pure substance and show significant variation from the known values, the sample is likely a solution.

Confirm your findings

To further confirm your findings, you can perform additional tests such as solubility tests, chemical tests, or chromatography. These tests can help identify the presence of other substances in the sample, which would indicate that it is a solution rather than a pure substance. By following these steps and carefully analyzing the results, we can determine whether a given homogeneous liquid sample is a solution or a pure substance.

Key concepts

Boiling Point Determination

The boiling point is a crucial property used to identify substances. When you boil a substance, it's the temperature at which it changes from a liquid to a gas. This can tell us a lot about its composition.
For pure substances, the boiling point is specific and doesn't vary, given constant pressure. This means you'll observe a steady temperature until the substance fully vaporizes.

• Use a thermometer to monitor the temperature of the liquid while heating it.
• Pure substances, like water, will have a precise boiling point, such as 100°C at sea level.
• Solutions, on the other hand, may display a boiling point elevation.
• The boiling point may change depending on the substances mixed within.

Such information helps differentiate between a solution and a pure substance with precision.

Freezing Point Determination

Just like boiling point, the freezing point is another fundamental characteristic of a substance. It's the temperature at which a substance changes from a liquid to a solid.
For pure substances, the freezing point is as consistent as their boiling point. It remains unchanged under fixed pressure conditions.

• Cool the liquid while monitoring the temperature carefully.
• Observe when the liquid starts to solidify.
• Pure substances have a definitive freezing point; water freezes at 0°C.
• Solutions may exhibit a freezing point depression, altering the temperature.

Identifying these changes can signal the presence of multiple components in a solution rather than in a pure substance.

Refractive Index Measurement

The refractive index is an optical property that helps identify substances through their interaction with light. Essentially, it's how much light bends when entering the substance.
Using a refractometer can give us precise measurements of this property.

• The refractive index for pure substances is distinct and measurable under specific conditions.
• A pure liquid will show a steady, known value that's consistent in scientific tables.
• Solutions usually have a refractive index that differs from their individual components.
• This makes it a useful tool for distinguishing mixtures or pure substances.

By comparing it to known values, you can decide whether the sample aligns with a pure substance or a complex solution.

Identification of Substances

Identifying substances goes beyond just measuring physical properties like boiling or freezing points. It involves comparing these measurements with known standards to classify a sample accurately.
In scientific practice, the objective is to match the observed data with existing data for pure substances.

• Compare boiling and freezing points with standard values.
• Check the refractive index against known substances.
• If values match, the sample is likely pure.
• Discrepancies may suggest the sample is a mixture or solution.

This analytical process aids in confirming whether your sample is a known pure substance or a complex solution. It's a fundamental step in scientific experimentation and quality control.

Solubility Test

Solubility tests are an excellent follow-up when your initial observations aren't conclusive, especially after measuring boiling or freezing points and refractive index.
These involve checking how well a substance dissolves in a particular solvent and can reveal more about the sample's nature.

• Test the substance with different solvents to observe its solubility.
• Pure substances might have predictable solubility patterns.
• Mixtures or solutions could show more complex solubility behavior.
• Observe for complete or partial dissolution and any precipitate formation.

Such results can confirm or negate earlier findings, providing a comprehensive understanding of whether you're dealing with a mixture or single, pure substance.