Pure Substances

Definition of Pure Substances

A pure substance is made up of only one type of particle. This means it has a uniform and consistent composition throughout. Because of this, pure substances have fixed melting and boiling points 6 they change state at specific temperatures.

Pure substances cannot be separated into other substances by physical methods such as filtration, distillation, or chromatography. This is because they are not mixtures but single substances with consistent properties.

Examples of Pure Substances

Pure substances include:

• Elements: Substances made of only one type of atom. For example, pure copper (Cu), which is a metallic element, or pure oxygen (O₂), which is a molecule made of two oxygen atoms.
• Compounds: Substances made of two or more elements chemically combined in fixed proportions. For example, pure water (H₂O) or pure carbon dioxide (CO₂).

Both elements and compounds have fixed melting and boiling points because their particles are arranged in a consistent way.

Properties of Pure Substances

Pure substances have a consistent chemical composition, meaning every particle is identical. This leads to characteristic physical properties such as:

• Fixed melting point: The temperature at which the solid turns into a liquid is sharp and precise.
• Fixed boiling point: The temperature at which the liquid turns into a gas is also sharp and precise.
• Consistent density, colour, and hardness: These properties do not vary within a pure sample because the particles are uniformly arranged and identical.

These properties help to identify pure substances and distinguish them from mixtures.

For example, pure water melts at exactly 06C and boils at exactly 1006C at normal atmospheric pressure. If the water contains impurities, these temperatures change and the melting and boiling points occur over a range.

Distinguishing Pure Substances from Mixtures

Pure substances differ from mixtures in several ways:

• Melting and boiling points: Pure substances have sharp, fixed melting and boiling points. Mixtures melt and boil over a range of temperatures.
• Homogeneity: Pure substances are homogeneous 6 the same throughout. Mixtures can be heterogeneous, with visibly different parts.
• Separation: Pure substances cannot be separated by physical methods. Mixtures can be separated by physical methods such as filtration, distillation, or chromatography.

For example, salt dissolved in water forms a mixture. The solution does not have a fixed boiling point; instead, boiling occurs over a range. The salt can be separated by evaporating the water, a physical method.

In contrast, pure sodium chloride (table salt) has a fixed melting point of 8016C and cannot be separated into simpler substances by physical means.

For instance, if a sample melts over a range from 956C to 1056C, it is likely impure or a mixture. A pure substance would melt sharply at a single temperature.

Learning Example: Identifying a Pure Substance by Melting Point

Suppose you have a solid that melts exactly at 1566C. You know the pure substance melts at 1566C. This sharp melting point suggests the solid is pure.

If the solid melted between 1506C and 1606C instead, the melting point range would indicate impurities or a mixture.