Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved in the above reactions as elements, compounds or mixtures, with justification.
The substances involved in the reactions can be classified as follows:
Elements: Iron and magnesium are elements because they cannot be broken down into simpler substances. They are the building blocks of matter. Oxygen is also an element that exists as a gas at room temperature and supports combustion.
Compounds: Iron oxide and magnesium oxide are compounds because they are formed when elements combine chemically in fixed ratios. Iron oxide is formed by the chemical combination of iron and oxygen, while magnesium oxide is formed by the chemical combination of magnesium and oxygen. These compounds have properties different from their constituent elements.
Mixtures: Moist air is a mixture because it contains water vapour mixed with air (which itself is a mixture of nitrogen, oxygen, and other gases). The components of moist air are not chemically combined and can be separated by physical methods.
Explanation
The chapter defines elements as 'substances that cannot be further broken down into simpler substances' and compounds as substances 'formed when different elements combine in fixed ratios.' Mixtures are defined as substances where 'two or more substances are mixed, where each substance retains its properties.' The components of a mixture 'do not react chemically with each other.'