Pure Substances: Elements and Compounds

1What does "pure" really mean?

A bottle of mineral water in a Kigali shop says "pure natural water". Turn the bottle round and the label lists what is dissolved in it: calcium, magnesium, sodium, hydrogencarbonate and more, in milligrams per litre. So is the water pure or not? In everyday speech, "pure" means "clean, with nothing harmful added". To a chemist it means something much stricter: one substance only.

2Experiment: is it pure?

Pure substances melt and boil at exact temperatures; impurities change these temperatures. Heat 50 cm³ of distilled water and 50 cm³ of water in which two spoonfuls of salt have been dissolved, in two beakers on tripods, and read a thermometer in each every minute. In a school in Kigali, the distilled water boils at a steady $95{}^{\circ }\mathrm{C}95\backslash ,^\backslash circ\backslash mathrm\{C\}$. The salty water does not start to boil until about $97{}^{\circ }\mathrm{C}97\backslash ,^\backslash circ\backslash mathrm\{C\}$, and its temperature creeps upwards as it boils, because the salt becomes more concentrated as the water boils away.

Figure 1.1: A pure solid melts at one exact temperature; an impure one melts lower and over a range of temperatures.

3Elements and compounds

There are two kinds of pure substance (Figure 1.2).

Figure 1.2: Two elements, a compound and a mixture, as particle diagrams.

Table 1.1: Some common elements

Table 1.2: Some common compounds