ABSTRACT
The notion of a pure sample, as used earlier, is one which is very important in chemistry and might seem unproblematic in teaching because purity, things being pure, is familiar to students from their everyday lives. A student might expect their orange juice to be pure— but orange juice is a complex mixture of different substances: water, citric acid, ascorbic acid, various sugars and amino acids, polyphenols, and compounds of potassium and phosphorus. Water that might be considered pure enough for cleaning windows or irrigating farmland may not be considered pure enough to drink. There is another important complication when teaching about the structure of liquid water at submicroscopic levels, and this is that even the idea that water is molecular is a simplification. The more philosophical approach is to consider that all the diverse material in the universe is composed of a limited number of basic kinds of stuff.
