The Classification of Matter: Pure Substances, Mixtures, and Solutions

Matter: Pure Substances and Mixtures

Matter can be classified into pure substances and mixtures. Pure substances have characteristic properties that do not change under the same conditions of pressure and temperature. They can be identified according to their characteristic properties, such as density and melting point.

Properties of Matter

• Characteristic Properties: These serve to identify substances and do not depend on the amount of the substance.
• Non-Characteristic Properties: These serve to measure or describe substances but not to identify them. They depend on the amount of the substance.

Pure Substances

A pure substance, or chemical substance, is a sample of matter that has a definite chemical composition and cannot be separated into other kinds of matter by any physical or mechanical process.

Elements

An element is a pure substance made up of only one kind of atom, with all atoms having the same atomic number. Examples include hydrogen, oxygen, nitrogen, and carbon.

Compounds

A compound is a pure substance formed by the chemical combination of atoms from two or more elements linked by chemical bonds. It can be decomposed into simpler substances by ordinary chemical means.

Molecules

A molecule is formed when two or more atoms are chemically combined.

Mixtures

A mixture is a sample of matter formed by two or more different components that are not chemically bound. There are two kinds of mixtures:

Heterogeneous Mixtures

Heterogeneous mixtures are those whose composition and physical properties vary from one point to another within the material.

Homogeneous Mixtures

Homogeneous mixtures are those in which the composition and physical properties are the same throughout because their constituents are uniformly mixed. They do not display a sharp melting point; instead, they melt over a range of temperatures.

Solutions and Colloids

Solutions

Homogeneous mixtures are also called solutions. A common type of solution is an alloy, which is a homogeneous mixture of two or more metals, or a metal and another substance. Examples include:

• Steel: Iron and carbon
• Bronze: Copper and tin
• Brass: Copper and zinc

Colloids

Colloids are heterogeneous mixtures that can be difficult to recognize at first glance because the suspended particles are very small. Examples include:

• Fog
• Gelatin
• Milk
• Smoke
• Soap
• Bath gel
• Mayonnaise

Differences Between Solutions and Colloids

The main differences between solutions and colloids are:

• The particle size of the dispersed phase is greater in a colloid than in a solution.
• A colloidal mixture can be identified by shining a beam of light through it. If the mixture is a colloid, the beam of light will be scattered by the suspended particles and become visible. This is known as the Tyndall effect.

Emulsions

A particular type of colloid is an emulsion, which is a mixture of two or more liquids that are normally immiscible.