Classification of matter  (Page 5/11)

Activity: using models to represent substances

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.

It is easy to describe elements and mixtures. We simply use the names that we find on the periodic table for elements and we use words to describe mixtures. But how are compounds named? In the example of iron sulphide that was used earlier, which element is named first, and which 'ending' is given to the compound name (in this case, the ending is -ide)?

The following are some guidelines for naming compounds:

1. The compound name will always include the names of the elements that are part of it.
   • A compound of iron ( $\mathrm{Fe}$ ) and sulphur ( $\mathrm{S}$ ) is iron sulph ide ( $\mathrm{FeS}$ )
   • A compound of potassium ( $\mathrm{K}$ ) and bromine ( $\mathrm{Br}$ ) is potassium brom ide ( $\mathrm{KBr}$ )
   • A compound of sodium ( $\mathrm{Na}$ ) and chlorine ( $\mathrm{Cl}$ ) is sodium chlor ide ( $\mathrm{NaCl}$ )
2. In a compound, the element that is on the left of the Periodic Table, is used first when naming the compound. In the example of $\mathrm{NaCl}$ , sodium is a group 1 element on the left hand side of the table, while chlorine is in group 7 on the right of the table. Sodium therefore comes first in the compound name. The same is true for $\mathrm{FeS}$ and $\mathrm{KBr}$ .
3. The symbols of the elements can be used to represent compounds e.g. $\mathrm{FeS}$ , $\mathrm{NaCl}$ , $\mathrm{KBr}$ and $\mathrm{H}{}_2\mathrm{O}$ . These are called chemical formulae . In the first three examples, the ratio of the elements in each compound is 1:1. So, for $\mathrm{FeS}$ , there is one atom of iron for every atom of sulphur in the compound. In the last example ( $\mathrm{H}{}_2\mathrm{O}$ ) there are two atoms of hydrogen for every atom of oxygen in the compound.
4. A compound may contain compound ions . An ion is an atom that has lost (positive ion) or gained (negative ion) electrons. Some of the more common compound ions and their formulae are given below.

   Name of compound ion	Formula
   Carbonate	${\mathrm{CO}}_3^{2-}$
   Sulphate	$\mathrm{SO}{}_4^{2-}$
   Hydroxide	${\mathrm{OH}}^{-}$
   Ammonium	${\mathrm{NH}}_4^{+}$
   Nitrate	${\mathrm{NO}}_3^{-}$
   Hydrogen carbonate	${\mathrm{HCO}}_3^{-}$
   Phosphate	$\mathrm{PO}{}_4^{3-}$
   Chlorate	${\mathrm{ClO}}_3^{-}$
   Cyanide	${\mathrm{CN}}^{-}$
   Chromate	${\mathrm{CrO}}_4^{2-}$
   Permanganate	$\mathrm{MnO}{}_4^{-}$

5. When there are only two elements in the compound, the compound is often given a suffix (ending) of -ide. You would have seen this in some of the examples we have used so far. For compound ions, when a non-metal is combined with oxygen to form a negative ion (anion) which then combines with a positive ion (cation) from hydrogen or a metal, then the suffix of the name will be ...ate or ...ite. ${\mathrm{NO}}_3^{-}$ for example, is a negative ion, which may combine with a cation such as hydrogen ( $\mathrm{HNO}{}_3$ ) or a metal like potassium (KNO ₃ ). The ${\mathrm{NO}}_3^{-}$ anion has the name nitr ate . ${\mathrm{SO}}_3^{2-}$ in a formula is sulph ite , e.g. sodium sulphite ( $\mathrm{Na}{}_2\mathrm{SO}{}_3$ ).
   $\mathrm{SO}{}_4^{2-}$ is sulph ate and $\mathrm{PO}{}_4^{3-}$ is phosph ate .
6. Prefixes can be used to describe the ratio of the elements that are in the compound. You should know the following prefixes: 'mono' (one), 'di' (two) and 'tri' (three).
   • $\mathrm{CO}$ (carbon monoxide) - There is one atom of oxygen for every one atom of carbon
   • $\mathrm{NO}{}_2$ (nitrogen dioxide) - There are two atoms of oxygen for every one atom of nitrogen
   • $\mathrm{SO}{}_3$ (sulphur trioxide) - There are three atoms of oxygen for every one atom of sulphur