This chapter's contents refer to the material covered in Topic 1 of the IB Chemistry Specification.

SPREAD 1: Writing formulae

You can't begin a chemistry course until you have these basic ideas sorted out. Simplistically there are three types of substance: element, compound and mixture.

Elements are substances made of only one type of atom. Elements cannot be broken down into anything simpler that will have a form of chemical existence. They behave differently from their constituent elements. For example, table salt (sodium chloride) is a relatively benign white solid that isn't toxic, whereas the elements sodium and chlorine are both very dangerous, as we will see.

Compounds are substances made of two or more elements chemically bonded together (see Chapter 4). Compounds can be written in the form of chemical formulae. This shows the fixed ratio of elements in the compound. For example, in magnesium carbonate, the formula is MgCO3— one magnesium, one carbon and three oxygens.

What is the difference between copper sulfide and copper sulfate?

In simple terms “ide” can be taken to mean combined in a compound, so copper sulfide is just copper and sulfur CuS. “Ate” can be taken to mean “with oxygen as well,” so copper sulfate is CuSO4. The exact nature of “ate” will be discussed further in the chapter on bonding.

Mixtures are elements or compounds (or both) chemically uncombined. Mixtures that contain chemicals in the same chemical state are called homogeneous (from the Greek hom.— the same, genou.— combining from). A good example is air. Heterogenous mixtures contain chemicals in different states (heter.— different). An example would be sea water. As the components in a mixture are not chemically combined they retain their individual characteristics.

Substances can occur in three states: solid (s), liquid (l) and gas (g). If you have a solution (a mixture of a substance dissolved in water) then it is called aqueous (aq) (from the Latin via French for water). These bracketed signs should appear next to any chemical formulae in equations.