Classifying Matter

In short: All matter is either a pure substance or a mixture. Pure substances are elements (one kind of atom) or compounds (elements chemically joined in a fixed ratio), while mixtures are physically combined in any ratio and can be separated by physical methods.

The big picture

All matter can be sorted with a few questions.

Pure substances

A pure substance has a fixed composition. Every part of it is exactly the same, and it has a fixed melting and boiling point.

• An element is a pure substance made of only one type of atom. It cannot be broken down into simpler substances by physical or chemical means. Examples: gold (Au), oxygen (${\mathrm{O}}_2$), carbon (C).
• A compound is a pure substance made of two or more different elements chemically joined in a fixed ratio. Examples: water (${\mathrm{H}}_2\mathrm{O}$), table salt (NaCl), sugar (${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}$).

A compound can only be split into its elements by a chemical reaction. Passing electricity through water splits it into hydrogen and oxygen gas.

A compound behaves nothing like the elements it is made from. Sodium is an explosive metal and chlorine is a poisonous gas, but sodium chloride is the salt on your chips.

Mixtures

A mixture contains two or more substances that are physically combined, not chemically joined. The amounts can vary, and each substance keeps its own properties. Mixtures can be separated by physical methods.

Solutions

In a solution, one substance (the solute) is dissolved in another (the solvent).

$$\text{solute}+\text{solvent}\to \text{solution}$$

In salt water, salt is the solute and water is the solvent. Water dissolves so many things that it is called the universal solvent.

Solutions are not only liquids:

Properties of matter

A property is a feature that describes a substance.

• A physical property can be observed or measured without changing what the substance is: colour, smell, state, density, melting point, boiling point, hardness, conductivity, solubility.
• A chemical property describes how a substance reacts to become a new substance: flammability, reacting with acid, rusting, reacting with water.

"Water boils at 100 °C" is a physical property. "Sodium reacts violently with water" is a chemical property.

Intensive and extensive properties

• An intensive property does not depend on how much you have: density, colour, melting point, boiling point. A gold ring and a gold bar have the same density.
• An extensive property does depend on the amount: mass, volume, length, total energy.

Intensive properties are useful for identifying a substance, because they are the same for any size sample.

Quick summary

• Matter is either a pure substance or a mixture.
• Pure substances are elements (one type of atom) or compounds (elements chemically joined in a fixed ratio).
• Mixtures are physically combined, can be in any ratio and can be separated physically.
• Homogeneous mixtures (solutions) look the same throughout. Heterogeneous mixtures do not.
• A solution is a solute dissolved in a solvent.
• Physical properties are observed without changing the substance. Chemical properties describe how it reacts.
• Intensive properties do not depend on amount. Extensive properties do.

Quiz