Define elements, compounds and mixtures, distinguish atoms, molecules and ions, and explain how a compound differs from a mixture

What this dot point is asking

The syllabus wants you to define an element, a compound, and a mixture, to tell apart atoms, molecules, and ions, and to explain clearly how a compound differs from a mixture of the same elements. This is the language that the rest of chemistry is built on, so getting the definitions exactly right matters. The key idea is the difference between substances that are just mixed together and substances whose atoms are chemically joined.

The answer

Elements

An element is a substance made of only one type of atom. It cannot be broken down into anything simpler by chemical means. There are over a hundred elements, listed in the Periodic Table, such as oxygen, copper, and carbon. Each has its own symbol, such as $\text{O}$, $\text{Cu}$, and $\text{C}$.

Compounds

A compound is a substance made of two or more different elements chemically combined in a fixed ratio. The atoms are joined by chemical bonds, and the compound has its own new properties that are different from the elements it is made from. Examples are water, $\text{H}_2\text{O}$, carbon dioxide, $\text{CO}_2$, and sodium chloride, $\text{NaCl}$.

Mixtures

A mixture contains two or more substances that are simply mixed together but not chemically combined. The parts keep their own properties and can be separated by physical methods such as filtering or distilling. Air, sea water, and a mixture of iron and sulfur powders are all mixtures.

Atoms, molecules and ions

These three words describe the particles:

• An atom is the smallest particle of an element, such as a single oxygen atom.
• A molecule is two or more atoms chemically joined together, such as $\text{O}_2$ or $\text{H}_2\text{O}$.
• An ion is an atom (or group of atoms) that has gained or lost electrons, so it carries a charge, such as $\text{Na}^{+}$ or $\text{Cl}^{-}$.

Examples in context

Example 1. Water is not just hydrogen plus oxygen. Hydrogen and oxygen are both gases, and oxygen helps things burn. Yet water, the compound made when they combine, is a liquid that puts fires out. This shows how a compound has completely new properties, unlike a mixture where each part keeps its own.

Example 2. Separating a mixture proves it is not a compound. Sea water can be separated into pure water and salt by distillation, a physical method, which shows it is a mixture. If it were a compound, no amount of boiling or filtering would split it, and a chemical reaction would be needed instead.

Try this

Q1. State whether each of the following is an element, a compound, or a mixture: copper, carbon dioxide, air. [3 marks]

• Cue. Copper is an element (one type of atom), carbon dioxide is a compound (carbon and oxygen joined), and air is a mixture (gases not joined).

Q2. Explain why oxygen gas, $\text{O}_2$, is described as an element but is made of molecules. [2 marks]

• Cue. It contains only one type of atom (oxygen), so it is an element; the atoms are joined in pairs, so the particles are molecules, but they are still all the same element.

Q3. Give two differences between a compound and a mixture of the same two elements. [2 marks]

• Cue. A compound has its elements chemically joined in a fixed ratio with new properties and needs a chemical reaction to separate; a mixture is unjoined, can be any ratio, keeps each element's properties, and separates physically.