chapter 4 elements, compounds and mixtures © 2013 marshall cavendish international (singapore)...
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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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