Exercise 2 solution

(a) Sea water: mixture. It contains water and dissolved salts and other things, and the proportions vary from place to place (no fixed composition).

(b) Oxygen gas: element. It is made of one kind of atom (the Chemical substance article's definition of an element). Chapter 5 shows the atoms are joined in pairs, but it is still one element.

(c) Table salt: compound. It is made of two elements, sodium and chlorine, chemically combined with fixed composition (the compound definition and the law of constant composition). The composition of the two elements is from general knowledge, not from the article.

(d) Air: mixture. It contains several gases, such as nitrogen and oxygen, that are not chemically combined. The gases are from general knowledge.

(e) Iron filings stirred into sulfur powder: mixture. The two stay separate substances and can be separated again with a magnet, which is the article's own example. If the mixture were heated until it reacted, a compound would form, but stirring is a physical process.