Quick questions on Elements, compounds and mixtures explained: O-Level Chemistry

An element is a substance made of only one kind of atom. It cannot be broken down into anything simpler by chemical means. The roughly 100 elements are listed in the Periodic Table; examples are oxygen, iron, carbon and copper. An element may exist as single atoms (helium) or as molecules of identical atoms (oxygen, $\text{O}_2$).

A compound is a substance in which two or more different elements are chemically combined in a fixed ratio. Forming a compound is a chemical change, and the compound has its own new properties, different from the elements that made it. Water ($\text{H}_2\text{O}$), carbon dioxide ($\text{CO}_2$) and sodium chloride ($\text{NaCl}$) are compounds. Because the elements are bonded together, a compound can only be separated into its elements by a chemical reaction, not by physical methods.

A mixture contains two or more substances (elements or compounds) that are not chemically combined. The substances keep their own properties and can be present in any ratio. Because nothing is bonded, a mixture can be separated by physical means such as filtration, distillation or a magnet. Air, sea water and a solution of salt in water are mixtures.

State what is meant by an element. [1 mark]

Give two ways a compound differs from a mixture of the same elements. [2 marks]

Classify sea water and explain your answer. [2 marks]