Mixtures, compounds and elements  (Page 5/8)

We can also use symbols to represent elements, mixtures and compounds. The symbols for the elements are all found on the periodic table. Compounds are shown as two or more element names written right next to each other. Subscripts may be used to show that there is more than one atom of a particular element. (e.g. $\mathrm{H}{}_2\mathrm{O}$ or $\mathrm{NaCl}$ ). Mixtures are written as: a mixture of element (or compound) A and element (or compound) B. (e.g. a mixture of $\mathrm{Fe}$ and $\mathrm{S}$ ).

One way to think of mixtures and compounds is to think of buildings. The building is a mixture of different building materials (e.g. glass, bricks, cement, etc.). The building materials are all compounds. You can also think of the elements as Lego blocks. Each Lego block can be added to other Lego blocks to make new structures, in the same way that elements can combine to make compounds.

Activity: using models to represent substances

• Hydrogen
• Oxygen
• Nitrogen
• Neon
• Sodium chloride (salt, $\mathrm{NaCl}$ )
• Potassium permanganate ( ${\mathrm{KMnO}}_4$ )
• Water ( ${\mathrm{H}}_2\mathrm{O}$ )
• Iron sulphide ( $\mathrm{FeS}$ )

Elements, mixtures and compounds

1. In the following table, tick whether each of the substances listed is a mixture or a pure substance . If it is a mixture, also say whether it is a homogeneous or heterogeneous mixture.

   Substance	Mixture or pure	Homogeneous or heterogeneous mixture
   fizzy colddrink
   steel
   oxygen
   iron filings
   smoke
   limestone ( ${\mathrm{CaCO}}_3$ )

2. In each of the following cases, say whether the substance is an element, a mixture or a compound.
   1. $\mathrm{Cu}$
   2. iron and sulphur
   3. $\mathrm{Al}$
   4. $\mathrm{H}{}_2\mathrm{SO}{}_4$
   5. $\mathrm{SO}{}_3$

Giving names and formulae to substances

Think about what you call your friends. Their full name is like the substances name and their nickname is like the substances formulae. Without these names your friends would have no idea which of them you are referring to. In the same way scientists like to have a consistent way of naming things and a short way of describing the thing being named. This helps scientists to communicate efficiently.