elements, mixtures & compounds syllabus objectives: (a) describe the differences between...
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Elements, Mixtures & Compounds
Syllabus objectives:
(a) Describe the differences between elements, compounds and mixtures
(b) State the symbols of the elements and formulae of the compounds mentioned in the syllabus.
(b) Deduce the formulae of simple compounds from the relative numbers of atoms present and vice versa.
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Types of particles
ParticlesParticles
Molecules
IonsAtoms
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Atoms and Molecules
• Matter is made up of tiny particles. These particles are called atoms.
• Atoms are the smallest particle of an element that can still have the chemical properties of that element.
• A molecule is made up of two or more atoms chemically joined together by covalent bond.(Can a molecule be joined together by ionic bonds?)
• Some substances exist as atoms while others exist as molecules.
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Atoms and Molecules
• Examples of substances that exist as atoms• All metals e.g. Gold (Au), Copper (Cu)• Noble gases e.g. helium (He), neon (Ne)
E.g. helium
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Atoms and Molecules
• Examples of substances that exist as molecules• Diatomic molecules eg.
• oxygen (O2)• hydrogen(H2)• hydrogen chloride (HCl)
• Tri-atomic molecules eg. • water (H2O),• Carbon dioxide (CO2)
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Atoms and Molecules Atoms of the same element that are chemically
joined together form molecules of elements. Atoms of different elements that are chemically
joined together form molecules of compounds.
element compound elementcompound
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Nitrogen gas (N2)Bromine liquid (Br2)
These areELEMENTS !
Examples of substances that exist as molecules :-
Hydrogen gas (H2) Oxygen gas (O2) Chlorine gas (Cl2)
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Examples of substances that exist as molecules :-
Carbon dioxide gas(CO2) Water(H2O) Ammonia gas (NH3)
Methane gas(CH4)
These are COMPOUNDS !
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Elements
• An element is a substance which which cannot be broken down into simpler substances by chemical means.
• Elements contain only one type of atom.• Egs. Aluminium, carbon, sulfur
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Elements• Elements are represented by symbolsExamples..
Al C Cu Hg S
Aluminium Carbon Copper Mercury Sulfur
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Metals & Non-metals• Elements can be classified as metals or non-metals
Metals Non-metals
• Shiny• Solids at r.t.p. (except Hg)• Malleable and ductile• High melting pt. & boiling pt.• Good conductors of heat• Good conductors of
electricity
• dull• Either gases, liquids or
solids at r.t.p. • brittle• low melting pt. & boiling pt.
(except C)• Poor conductors of heat• Poor conductors of
electricity (except graphite)
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Compounds
• A compound is a substance which is made up of 2 or more elements chemically combined together.
• Compounds contain only one type of molecule
• E.g.s. Water (H2O), common salt (NaCl), carbon dioxide (CO2)
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Compounds
• A chemical reaction takes place when a compound is formed.
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Compounds
oxygen (
(silvery solid)a bright white light;
ELEMENTS COMPOUND
magnesium oxide colourlessgas)
magnesium magnesium burns with
• A chemical reaction takes place when a compound is formed.
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Compounds
• A chemical reaction takes place when a compound is formed.
ELEMENTS COMPOUND
(colourlessgas)oxygen
gas)
lighted splint
heat, light water
hydrogencolourless
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Mixtures
• A mixture contains 2 or more substances which can be easily separated by physical means.
• Mixtures contain two or more types of atoms or molecules (physical means
include : filtration, evaporation, distillation, chromatography etc.)
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Representing atoms, molecules, elements, mixtures and compounds:
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Representing atoms, molecules, elements, mixtures and compounds:
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Representing atoms, molecules, elements, mixtures and compounds:
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same properties as its components
unique physical and chemical propertiesproperties
can be easily separated by physical means
cannot be easily separated by physical meansseparation
a mixture of two elementsa compound of two elements
arrangement of atoms
variable melting and boiling points
fixed melting and boiling pointsmelting and
boiling points
variable composition by massfixed composition by masscomposition
MixtureMixtureSingle CompoundSingle Compound
mixtures vs compoundsmixtures vs compounds
Comparison between a single compound and a mixture:Comparison between a single compound and a mixture:
A good way to distinguish between a mixture and a compound is through using their melting and boiling points
A good way to distinguish between a mixture and a compound is through using their melting and boiling points
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Molecular FormulaeMolecular Formulae
• With the presence of both metallic and non-metallic elements, write the metallic element first.
• E.g. CaO, NaCl, FeS.• Number of atoms is written as a subscript
except for “1”.• E.g. FeCl2 (not Fe1Cl2); K2Cr2O7 (not K2Cr2O7)• Write oxygen atom at the end of the
formula.E.g. H2O, CuO, MgCO3
How do we deduce the formulae?
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• -ide: The compound has only 2 elements.
Sodium chloride (NaCl)
Iron (II) Sulfide (FeS)• -Hydroxide: The compound has OH- ion.
Sodium hydroxide (NaOH)
Ammonium hydroxide (NH4OH)
• -ate or -ite: The compound has element oxygen. The former one has more oxygen atoms than the latter one.
Sodium nitrate (NaNO3) & sodium nitrite (NaNO2)
Naming of compounds
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Chemical Formula
provides information about• the type of element present in the
compound• the relative number of atoms
(ratio) present
• Eg. (NH4)2SO4 contains..
CuSO4.5H2O contains…