CHAPTER 4Elements, Compounds and Mixtures

© 2013 Marshall Cavendish International (Singapore) Private Limited

4.1 Elements

4.2 Compounds

4.3 Mixtures

Elements, Compounds and Mixtures

Chapter 4

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Learning Outcomes

• define an element;

• state the chemical symbols for a number of elements.

At the end of this section, you should be able to:

4.1 Elements

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It is a pure substance that cannot be broken down into two or more simpler substances by chemical processes.

4.1 Elements

What is an Element?

Sugar is not an element.

When heated, it breaks down into carbon and water vapour.

Examples: Carbon, copper, oxygen

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Is Water an Element?

Water is not an element.

It can be broken down into hydrogen and oxygen.

Hydrogen and oxygen are elements.

4.1 Elements

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Chemical Symbols of Elements

• Chemists use chemical symbols to represent elements.

• Each symbol is unique, consisting of one or two letters.

4.1 Elements

The names and symbols of all known elements are found in Periodic Table.

Element Symbol

calcium Ca

carbon C

hydrogen H

iron Fe

Element Symbol

mercury Hg

neon Ne

silicon Si

sodium Na

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Element Symbol

Aluminium

Magnesium

Lead

Copper

Nitrogen

Neon

Bromine

Sulfur

Using the Periodic Table, find the symbols of the following elements.

4.1 Elements

Al

Mg

Pb

Cu

N

Ne

Br

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Classification of Elements

Based on their properties, elements are classified into:

4.1 Elements

• metals; e.g. sodium, magnesium, calcium

• non-metals; e.g. hydrogen, carbon, chlorine

• metalloids. e.g. boron, silicon, germanium, arsenic

(Metalloids have properties of metals and non-metals.)

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Differences Between Metals and Non-Metals

4.1 Elements

Properties Metals Metalloids Non-metals

Appearance Shiny (lustrous) Shiny (lustrous)

Dull (non-lustrous)

Physical state at r.t.p.

Mostly solids (except mercury)

Solids Gases, volatile liquids or solids

Melting and boiling points

High (except for sodium, potassium and mercury)

High Low (except for carbon and silicon)

Heat and Electrical Conductivity

Good Moderate Poor (except carbon in the form of graphite and diamond) 9

Differences Between Metals and Non-Metals

4.1 Elements

Properties Metals Metalloids Non-metals

Ductility and Malleability

Ductile (can be drawn into wires)

Malleable (can be hammered into different shapes without breaking

Sonorous (makes a ringing sound when struck)

Brittle (easily broken when hammered)

Brittle if solid

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neon atom

Atoms and Molecules

• An element is made up of tiny particles called atoms.

• Atoms are the smallest particles of an element that have the chemical properties of that element.

• The atoms of an element are different from that of another element.

4.1 Elements

gold atom

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A molecule is a group of two or more atoms that are chemically combined (joined together).

4.1 Elements

Diatomic molecules are those that are formed by the combination of two atoms. E.g. hydrogen, oxygen, nitrogen.

Elements like hydrogen, oxygen, ozone and sulfur exist as molecules.

Atoms and Molecules

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Polyatomic molecules consists of three or more atoms that are chemically combined together.

4.1 Elements

Polyatomic Molecules

ozone (O3)

phosphorus (P4)

sulfur (S8)

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4.1 Elements

4.2 Compounds

4.3 Mixtures

Elements, Compounds and Mixtures

Chapter 4

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Learning Outcomes

• define a compound;

• describe the difference between elements and compounds;

• deduce the formula of a compound from the ratio of elements present.

At the end of this section, you should be able to:

4.2 Compounds

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What is a Compound?

A compound is a pure substance that contains two or more elements that are chemically combined in a fixed ratio.

4.2 Compounds

What are compounds made of?

• molecules • ions

water molecule

sodium ion

chloride ion

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Properties of Compounds

Example:When magnesium is burned, it combines with oxygen in the air to form a compound called magnesium oxide.

A compound has different properties from the elements that form it.

4.2 Compounds

magnesium + oxygen magnesium oxideshiny grey solid colourless gas white solid

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Names of Compounds

4.2 Compounds

Each compound has a chemical name indicating the elements it contains.

Compound Elements present

sodium chloride sodium, chlorine

carbon dioxide carbon, oxygen

zinc oxide zinc, oxygen

calcium carbonate (marble) calcium, carbon, oxygen

copper(II) sulfate copper, sulfur, oxygen

hydrogen chloride hydrogen, chlorine

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4.2 Compounds

Chemical Formula

A compound can be represented by a chemical formula.

The chemical formula shows:

•the types of elements present;

•the ratio of the different atoms present.

Chemical formula for lead(II) nitrate

Pb(NO3)2

Pb1 lead atom

(N)21 × 2 =

2 nitrogen atoms

(O3)23 × 2 =

6 oxygen atoms

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Composition of Compounds

Every compound has fixed compositions of the different elements it contains.

4.2 Compounds

Chemical formula can be deduced by the ratio of atoms present in a compound.

Compound Chemical formula Ratio of atoms

hydrogen chloride HCl H : Cl = 1 : 1

carbon dioxide CO2 C : O = 1 : 2

carbon monoxide CO C : O = 1 : 1

sulfuric acid H2SO4 H : S : O = 2 : 1 : 4

sulfurous acid H2SO3 H : S : O = 2 : 1 : 3

ethanol C2H5OH C : H : O = 2 : 6 : 1 20

Decomposition of Compounds

In thermal decomposition, heat is used to break down compounds into elements or simpler compounds.

A chemical reaction is necessary to separate the elements in a compound.

4.2 Compounds

Compounds can also be broken down by using electricity. This process is known as electrolysis.

mercury(II) oxide

mercury

oxygen gas

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4.1 Elements

4.2 Compounds

4.3 Mixtures

Elements, Compounds and Mixtures

Chapter 4

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Learning Outcomes

• define a mixture;

• differentiate between compounds and mixtures.

At the end of this section, you should be able to:

4.3 Mixtures

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What are Mixtures?

4.3 Mixtures

Examples of mixtures:

• petrol

• air

• muddy water

• alloys such as brass, bronze and steel

Mixtures are made up of two or more substances that are not chemically combined.

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4.3 Mixtures

Types of Mixtures

A mixture can exist as a combination of

• two elements;

• two compounds;

• one element and one compound.

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Example: Neon and Hydrogen

4.3 Mixtures

Mixture of Two Elements

Neon is monatomic.

Hydrogen is diatomic.

hydrogen(H2)

neon(Ne)

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Example:Water and Carbon dioxide

4.3 Mixtures

Mixture of Two Compounds

A water molecule is made up of two hydrogen atoms and one oxygen atom.

A carbon dioxide molecule is made up of two oxygen atoms and one carbon atom. water

(H2O)

carbon dioxide(CO2)

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Example:Hydrogen and Ammonia

4.3 Mixtures

Mixture of an Element and a Compound

Hydrogen is an element.

Ammonia is a compound.

Each ammonia molecule consists of three hydrogen atoms and one nitrogen atom.

hydrogen(H2) ammonia

(NH3)

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Differences between Mixture and Compound

4.3 Mixtures

Mixture Compound

Separation

The components of a mixture can be separated by physical processes such as magnetic separation, filtration or distillation.

A compound can only be broken down into its elements or into simpler compounds by chemical processes (e.g. thermal decomposition or electrolysis).

Properties

The chemical properties of a mixture are the same as those of its components.

The physical and chemical properties of a compound are different from those of its constituent elements.

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Differences between Mixture and Compound

4.3 Mixtures

Mixture Compound

Energy changes

No chemical reaction takes place when a mixture is formed — usually there is little or no energy change.

A chemical reaction takes place when a compound is formed — usually there is an energy change, e.g. the reactants get hot or cold.

Composition

The components of a mixture can be mixed in any proportion.

The elements in a compound are always combined in a fixed proportion.

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4.3 Mixtures

Separation

Example:Comparing a mixture of iron filings and sulfur with a compound iron(II) sulfide

Mixture Compound

Can be separated by physical methods

E.g. In a mixture of iron filings and sulfur, iron filings can be separated with the use of a magnet.

Can only be separated by chemical methods or by using electricity

E.g. Iron in iron(II) sulfide cannot be simply separated with a magnet.

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4.3 Mixtures

Physical and Chemical Properties

Example:Comparing a mixture of iron filings and sulfur with a compound iron(II) sulfide

Mixture Compound

Has the same chemical properties as its components

E.g. In a mixture of iron filings and sulfur, each component retains their colour, texture and magnetic property (iron).

Has different physical and chemical properties from those of its components

E.g. Iron(II) sulfide has a different appearance from iron filings or sulfur; has no magnetic property.

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4.3 Mixtures

Energy Change

Example:Comparing a mixture of iron filings and sulfur with a compound iron(II) sulfide

Mixture Compound

No chemical reaction takes place. Little or no energy change.

E.g. Mixture of iron filings and sulfur – no chemical change takes place without heating.

Chemical reaction takes place. Heat and light usually given off.

E.g. When a mixture of iron filings and sulfur is heated, it glows red hot to form iron(II) sulfide. 33

4.3 Mixtures

Example:Comparing a mixture of iron filings and sulfur with a compound iron(II) sulfide

Composition

Mixture Compound

• No fixed composition.

E.g. A mixture of iron filings and sulfur can have more or less of each component.

• Fixed composition.

E.g. The compound iron(II) sulfide is formed in the ratio of iron to sulfur (by mass) of 7:4.

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Concept Map

Elements, Compounds and Mixtures

Chapter 4

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Elements, Compounds and Mixtures

Chapter 4

Acknowledgements

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