1-4 Elements And Compounds

IV. Elements and compounds

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Topic

IV.

Elements and compounds

Reading Assignment

Integrated Chemistry Today (2nd Ed.), L.H.M Chung, Book 1A, pg 62–67

Objectives

1.4 – recognise that oxygen, silicon and aluminium are the three most abundant elements on the earth's crust – recognise the abundance of oxygen in elemental form on earth is unique amongst the planets in the solar system – recognise that silicon and aluminium exit in nature in combined forms called compounds and thus the elements are less readily available for use – recognise that most substances are compounds or their mixtures – describe the differences between compounds and mixture of elements

Notes

IV. Elements and compounds A. Natural abundance of different elements In the universe, hydrogen is the most abundant element, it accounts for over 99% of the mass of the universe. e.g. the whole mass of sun is mainly made of hydrogen.

Earth Earth is very special in the universe. It contains only very few hydrogen. Oxygen is the most abundant element on the earth. Free oxygen can be found in air in form of O2 molecule. Oxygen is also a major composition of water (H2O) and rock (SiO2). Although there are plenty of aluminium and silicon on the earth, the pure aluminium and silicon never exists in nature. Aluminium metal is extracted from an ore called bauxite (aluminium oxide, Al2O3) and silicon is extracted from silicon(IV) oxide (SiO2) which is the major composition of rock. B. Existence of element in elemental form and combined form Only very few elements exist in pure and uncombined form. Gold is one of them. And this is why gold is one of the firstly discovered elements in history. For the other elements, they must be chemically extracted from the compound and physically purified.

IV. Elements and compounds

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C. Element, compound and mixture Element –

A substance contains only one kind of atoms. e.g. Aluminium is an element because it contains only aluminium atom. Compound – A substance which is pure and contains more than one kind of atom chemically combined together. e.g. Water (H2O) is a compound because it consists of hydrogen atoms and oxygen atoms with the ratio of 2 : 1. Mixture – A substance contains more than one pure substance which are not chemically combined together. e.g. sea water is a mixture. It can be separated into water and salt by distillation.

Compounds constant composition e.g. in water (H2O), the ratio of H atom to O atom is 2 to 1.

Composition

Composition by mass

Melting point Separation of constituents Heat change of formation Properties

constant composition e.g. in water, the mass of H atom to O atom is 1 : 16 and the number ratio is 2 : 1, therefore, the mass of hydrogen to oxygen in water would always be 1 : 8. a sharp melting point because it is pure. constituents can only be separated by chemical method (e.g. electrolysis) usually heat is evolved or absorbed upon formation properties of the compound is so different from the constituent elements

Example of Iron(II) sulphide Compound of iron(II) sulphide

Glossary

Mixture of elements variable composition (no formula can be assigned.) e.g. different stainless steels contain different amount of iron and carbon. variable composition

a wide melting range because it is not pure. constituents can be separated by physical method (e.g. distillation, filtration) no heat change involved. individual elements retain their own properties.

Composition Separation

ratio of Fe atom to S atom is 1 : 1. cannot be separated easily.

Heat change of formation Arrangement of atoms

heat is evolved.

Mixture of iron filing and sulphur powder no fixed composition can be separated by a piece of magnet or by decantation because of the difference in density in water. no heat change involved.

iron and sulphur atoms are rearranged and combine together

iron and sulphur atoms separate from each other

bauxite

silicon(IV) oxide

compound

mixture

IV. Elements and compounds

Past Paper Questions 98 44 B

44 Upon heating, a mixture of iron and sulphur gives a black substance. Which of the following statements concerning the black substance are correct ? (1) It is insoluble in water. (2) It can be attracted by a bar magnet. (3) It reacts with dilute hydrochloric acid to give a gas with a pungent smell. A. (1) and (2) only B. (1) and (3) only C. (2) and (3) only D. (1), (2) and (3)

99 1 B

1

Which of the following diagrams can represent a mixture of two compounds ? (In these diagrams and represent a nitrogen atom and an oxygen atom respectively.)

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