Pure Substances

The Chemistry of "Pure"

If you buy a carton of "pure orange juice" from the supermarket, a chemist would tell you it is completely impure. In everyday language, "pure" simply describes a substance in its natural state with nothing extra added.

However, in chemistry, a pure substance is defined strictly as a single element or compound that has not been mixed with any other substance. Pure water, for example, contains only $H_2O$ molecules and absolutely nothing else. By contrast, orange juice is a mixture, as it consists of water, sugars, and acids that are physically mixed together but not chemically combined.

Distinguishing Purity Using Temperature

You can easily test if a substance is chemically pure by measuring its exact melting and boiling points.

Pure substances melt and boil at single, fixed, specific, and sharp melting points. Because every atom or molecule in a pure substance is identical, the forces between them require the exact same amount of thermal energy to break.

Mixtures, however, contain multiple components. Because each component has its own unique melting and boiling point, mixtures melt and boil over a range of temperatures rather than at one sharp point.

The Effect of Impurities

When an impurity is added to a pure substance, it physically disrupts the regular arrangement of atoms or molecules (the crystal lattice) in the solid state.

This disruption weakens the forces of attraction between the particles. Because these forces are weaker, less energy is required to overcome them. Therefore, impurities will always lower (depress) the melting point and cause the solid to melt over a broader range of temperatures. Conversely, impurities will increase (elevate) the boiling point.

Feature	Pure Substance	Impure Substance / Mixture
Melting Point	Single, fixed temperature (sharp)	Melts over a range of temperatures
Boiling Point	Single, fixed temperature	Boils over a range of temperatures
Heating Curve	Shows a horizontal plateau (flat section) during state change	Shows a sloped or rising line during state change
Comparison to Data	Matches official literature data exactly	Melting point is lower than literature data

Worked Example: Analysing Melting Point Data

A student tests a sample of sulfur using a melting point apparatus. Pure sulfur melts at $114^\circ\text{C}$. The student's sample begins to melt at $103^\circ\text{C}$ and finishes melting at $112^\circ\text{C}$. Explain whether the sample is a pure substance or a mixture.

Step 1: Identify the starting data and reference values.

• Reference melting point of pure sulfur = $114^\circ\text{C}$.
• Sample melting data = $103^\circ\text{C}$ to $112^\circ\text{C}$.

Step 2: Compare the sample data to the pure reference.

• The sample's melting point is lower than the pure substance's melting point ($112^\circ\text{C}$ is less than $114^\circ\text{C}$).
• The sample melts over a $9^\circ\text{C}$ range ($103\text{--}112^\circ\text{C}$) rather than at a single temperature.

Step 3: State your conclusion and causal reasoning.

• The sample is a mixture (impure). This is because impurities disrupt the regular particle arrangement, lowering the melting point and causing it to melt over a range of temperatures instead of a sharp point.