Standard Grade Chemistry
Summary Notes
Topic 7 : Properties Of Substances
Learning Outcomes
General


















Credit




Metal elements and carbon (graphite) are conductors of electricity and most
non-metal elements are non-conductors of electricity.
Covalent compounds (solid, liquid, solution) do not conduct electricity.
Compounds made from metal elements do not conduct electricity in the solid
state, and that these compounds do conduct electricity when dissolved in
water or when molten.
Electric current is a flow of charged particles.
Electrons flow through metals and ions flow through solutions.
Electrical energy chemically changes a solution which conducts.
Positive ions form when atoms lose electrons.
Negative ions form when atoms gain electrons.
Solid compounds can be ionic or covalent and that compounds that exist as
liquids or gases at room temperature are covalent.
Ionic solids exist as networks / lattices of oppositely charged ions.
Give examples of covalent substances which are insoluble in water but which
dissolve in other solvents.
When ionic compounds dissolve in water, the lattice breaks up completely.
During electrolysis positive metal ions are attracted to the negative
electrode and negative, non-metal ions are attracted to the positive
electrode.
Electrical energy chemically changes a molten ionic compound.
The melt conducts because the ions are free to move. The ions are not free
to move in the solid.
An electrolyte contains ions that can move, e.g. a solution or molten ionic
compound.
During electrolysis the electrolyte is broken down to its elements.
Some ions are coloured e.g. Cu2+ is blue and Cr2O72+ is orange. These can be
seen moving during electrolysis.
In electrolysis, a D.C. supply must be used if the products are to be
identified.
Covalent and ionic network substances have high melting and boiling points as
they have a lattice structure. This makes them solid at room temperature.
Covalent molecules have low melting and boiling points as there is little
attraction between molecules. This means that covalent substances can be
solid, liquid or gas.
In electrolysis, negative ions attracted to the positive electrode, lose
electrons to become atoms and positive ions attracted to the negative
electrode, gain electrons to become atoms.
Only ionic compounds have ions free to move in solution and when molten so
only ionic compounds can take part in electrolysis.
Classification Of Substances
In Chemistry we often place substances which have similar properties into
groups. We say we are classifying them. Elements and compounds are common
classifications you have already come across in chemistry. Elements can also be
classified as metals and non-metals.
The terms covalent and ionic are also used to classify some substances. To use
the terms ionic and covalent correctly, we need to know the types of element
joined in a substance.
A substance is covalent if it
contains only non-metal atoms
joined together
A substance is ionic if it contains
metal and non-metal atoms joined
together
Examples : H2O, CO2, NH3
Examples : NaCl, MgF2, Fe2O3
Conductors And Insulators
Some substances allow electricity to pass through them. These substances are
called conductors. Substances which do not conduct electricity are called
insulators.
The results of experiments involving the conductivity testing of solids and
liquids allow the following conclusions to be drawn about solids and liquids which
conduct electricity.

Only metals and graphite conduct in the solid state.

The only liquids that conduct electricity are molten ionic substances
and molten metals (including mercury)
Electrical Conductivity
Conductors
Metals conduct electricity in both solid and liquid states
Graphite conducts electricity
Ionic compounds conduct electricity when molten
Ionic compounds conduct electricity when in aqueous solution
Insulators
Non-metals elements (except graphite) do not conduct electricity
Covalent compounds do not conduct electricity
Ionic compounds do not conduct electricity when solid
Ionic Bonding
Ions are charged particles which are formed by atoms gaining or losing
electrons. Ionic compounds are made of ions. This is why we classify these
compounds as ionic.
Forming ions is one of the ways atoms can join to each other. Atoms can join to
each other by transferring electrons from one atom to another.
The atom which loses electrons changes into a positive ion. Metal atoms usually
lose electrons to form positively charged ions.
The atom which gains electrons changes into a negative ion. Non-metal atoms
usually gain electrons to form negatively charged ions.
Opposite electrical charges attract. The attraction of opposite electrical
charges holds the ions together. This attraction between oppositely charged
ions is what we call the ionic bond.
The size of the electric charge on an ion depends on the number of electrons
lost or gained by the atom. The number of electrons lost or gained is normally
given by the valency of the atom.
Magnesium is a metal. It is therefore a positive ion. Magnesium has a valency of
2. Magnesium atoms therefore lose two electrons to form Mg2+ ions.
Chlorine is a non-metal. It therefore forms a negative ion. Chlorine has a
valency of 1. Chlorine atoms therefore gain 1 electron to form Cl- ions.
Ion-Electron Half Equations
The loss or gain of electrons can be shown in a type of equation called an ionelectron half equation. An ion-electron half equation is a type of equation which
shows an ion and electrons. It only shows either the loss or gain of electrons
in a reaction where electrons are transferred. The complete reaction involves
both the loss and gain of electrons.
The magnesium atom loses 2 electrons to become a magnesium ion.
This could be written as :
magnesium atom  magnesium ion + 2 electrons
Mg  Mg2+ + 2e
The chlorine atom gains one electron to become a chloride ion.
Note that the ions of non-metal atoms usually have names ending in –ide.
chlorine atom + 1 electron  chloride ion
Cl + e  ClNote that in reality a diatomic chlorine molecule (Cl2) would first split into 2
chlorine atoms. Each of these atoms would gain an electron to form two chloride
ions.
Ions Containing More Than One Kind Of Atom (Group Ions)
Some ions are made from more than one atom. These ions are often called group
ions.
Science data books often list the formulae of the common group ions
Examples
Sulphate ion
Nitrate ion
Carbonate ion
Ammonium ion
22SO4
NO3
CO3
NH4+
The size of the charge on the ion tells us the valency of the ion.
The chemical formulae of compounds containing these groups can be worked out
in the same way your learned in TOPIC 4.
Ionic Bonding Explained
Atoms can bond (join) by sharing electrons. When an atom shares electrons with
another it is attempting to achieve a stable electron arrangement. i.e. an
electron arrangement which is the same as a Noble Gas. If a pair of atoms
share electrons to achieve Noble Gas electron arrangements, the shared pair of
electrons is called a covalent bond. This type of bonding is called covalent
bonding.
In ionic bonding the metal atoms loses electrons and gives these electrons to
the non-metal atom. This transferring of electrons from the metal atom to the
non-metal atom results in both atoms getting an electron arrangement which is
the same as a Noble Gas. Millions of atoms are involved in these electron
transfers, and the oppositely charged ions which are formed attract to form a
regular arrangement of ions called a crystal lattice.
Sodium chloride is an example of a substance formed by ionic bonding.
Atom
Electron
Arrangement
sodium
(Na)
2,8,1
Chlorine
(Cl)
2,8,7
Ion Formed
transfers an
electron to the
chlorine atom
gains an electron
from the sodium
atom
Na+
Electron
Arrangement
Of Ion
2,8
Cl-
2,8,8
Ionic And Covalent Bonding – Summary
Atoms bond to achieve more stable Noble Gas electron arrangements.
Atoms can join in two ways :
Ionic bonding which involves the transfer of electrons from one atom to
another.
[Remember – metal atoms form positive ions by losing the electrons in their
outermost electron level and non-metal atoms form negative ions by gaining
electrons into their outermost electron level.]
Covalent bonding which involves the sharing of electrons between two non-metal
atoms.
Properties And Structures Of Ionic And Covalent Compounds
Ionic compounds conduct electricity when molten or when dissolved in water
(i.e. in solution). They do not conduct when solid.
Covalent substances, whether elements or compounds, never conduct electricity.
Conduction of electricity is one property where ionic and covalent substances
differ. We can look at other properties :

melting and boiling points

solubility in water and in other solvents
Ionic substances have high melting points and covalent substances usually have
low melting points.
Ionic substances are soluble in water whereas covalent substances are usually
insoluble in water. However, covalent substances are soluble in covalent solvents,
e.g. paint in turpentine or nail varnish in acetone. Ionic substances are insoluble
in covalent solvents.
These patterns in properties of ionic and covalent substances are connected to
their structures.
The ions in a crystal of an ionic compound are closely packed together.
All ionic compounds have their ions arranged so that ions with the same charge
do not touch each other.
This regular arrangement of ions is called a crystal lattice.
The crystal lattice is a giant structure containing millions of ions. The formula of
an ionic compound is the simplest ratio of the number of positive ions to the
number of negative ions.
In sodium chloride there are equal numbers of sodium ions and chloride ions in
the crystal lattice so the formula is Na+Cl-.
In an ionic crystal lattice the ions are held together by strong forces of
attraction between the positive and negative charges. It takes a lot of energy
to separate the ions from one another so ionic compounds have high melting and
boiling points.
When an ionic substances dissolves in water the crystal lattice breaks up
completely and the ions are dispersed throughout the water.
Covalent Structures
Covalent substances usually have low melting and boiling points. Some covalent
substances do however have high melting points e.g. silicon dioxide (sand).
Covalent bonding, due to the sharing of outer electrons between atoms can
result in two different structures.
Most covalent substances, e.g. carbon dioxide, have very low melting and boiling
points and are therefore liquids or gases at normal room temperature. They
exist as small, separate molecules, and although the molecule is held together
by the strong covalent bonds, there are only very weak forces between the
molecules. This type of covalent substance is said to have a covalent molecular
structure.
Other covalent substances, e.g. silicon dioxide, have very high melting and boiling
points and are therefore solids at normal room temperature. They exist as giant
networks of atoms where all the atoms are joined by the strong covalent
bonds. This type of covalent substance is said to have a covalent network
structure.
Conduction Explained
The only types of substance that conduct electricity are metals, graphite, ionic
solutions and molten ionic substances. The passage of electricity through a
substance is called an electric current. An electric current is a flow of charged
particles.
In metals and graphite the charged particles which pass through the substance
are electrons; in ionic solutions and charged particles are ions.
We know metals conduct electricity when solid and liquid. To understand
conduction of electricity in metals you need to know how the atoms in a metal
are arranged and held together. The bonding which holds the atoms in a metal
together is called metallic bonding.
Metallic bonding is the name given to the type of bonds which hold the atoms in
a metal together.
When metals conduct electricity, electrons leaving the metal to move towards
the positive terminal of a battery are replaced by electrons coming from the
negative terminal. In this way the metal conducts electricity without changing in
any way.
Conduction Of Ionic Solutions
The carbon rods make the electrical connection between the wires and solution.
They are called the electrodes.
The passing of the electric current into a copper (II) chloride solution causes a
chemical reaction to occur at the electrodes. The copper (II) chloride is broken
down by the electric current into copper metal and chlorine gas.
The breaking down of ionic compounds using electricity is called electrolysis.
The electrical energy chemically changes a solution which conducts.
The Cu2+ ions move towards the negative electrode and form copper metal and
the Cl- ions move towards the positive electrode and form chlorine gas. It is the
movement of ions in the solution (or in the molten state) which allows ionic
compounds to conduct electricity.
For electrolysis to take place the ions in an ionic compound have to be free to
move. When an ionic compound is molten or in solution its ions become free to
move. Compounds which conduct electricity when molten or in aqueous solution
are called electrolytes. All ionic solutions and melts are electrolytes.
Electrolysis will take place in all substances which contain free ions when a
direct current (d.c.) is passed into the substance. A d.c. supply must be used if
the products are to be identified.
Positive ions (metal ions) are attracted to the negative electrode.
Negative ions (non-metal ions) are attracted to the positive electrode.
i.e. ions are attracted to the oppositely charged electrode where they lose their
charge to become neutral atoms.
Example : Electrolysis of copper (II) chloride solution
Each chloride ion loses one electron to the positive electrode and changes into a
neutral chlorine atom. The electrons flow to the positive terminal of the power
pack. Chlorine atoms then join to form Cl2 molecules which bubble off.
The negative electrode releases two electrons to each copper ion which causes
copper atoms to form. A layer of copper forms on the surface of the electrode.
Ion-electron half equations can be written for the reactions which takes place
at the electrodes during electrolysis. Writing these equations is made easier by
using information in your Data Book page 7 – Electrochemical Series –
Reduction Reactions.
In the electrolysis of copper (II) chloride solution, Cu2+ ions are changed into
copper atoms at the negative electrode.
Cu2+(aq) + 2e  Cu(s)
At the positive electrode, chloride ions are changed into chlorine molecules. The
equation in the Data book needs to be reversed to read :
2Cl-(aq)  Cl2 (g) + 2e
Coloured Ions
Many ionic compounds are coloured. The colour of an ionic compound comes
from the ions from which it is made.
Though many ionic compounds are coloured, there are large numbers of
colourless ionic compounds. This means that many ions are colourless.
The only way to find out which ions are coloured and which are colourless is to
look at a large number of ionic compounds. From their formulae you can work out
which ions are responsible for the colour in the coloured compounds.
The ions in an electrolyte are attracted to the electrodes when the current is
switched on.
Remember negative ions (non-metal ions) are attracted to the positive electrode
and positive ions (metal ions) are attracted to the negative electrode.
We can use this knowledge to prove that these coloured ions really exist by
watching how the colours move when a current is passed through a solution of a
coloured compound.
Chemical Formulae Using Group Ions
Step 1
Step 2
Step 3
Step 4
Write down the symbol of any
element or the formula of any
group ion (no charge) which is
involved
Work out the valencies for each
element or group ion and write
them below the symbol for each
element or group ion.
SWAP the valencies around
DIVIDE (cancel) if possible to
obtain the chemical formula
K
CO3
K
CO3
1
K
2
CO3
2
1
K2CO3
When writing chemical formulae of ionic compounds we can show the
charges on the ions. This gives a type of formula called the ionic formula.
The ionic formula of potassium carbonate is written in ionic form as
(K+)2CO32ALTERNATIVE METHOD
Step 1
Step 2
Write down the symbol of any
element or the formula of any
group ion (no charge) which is
involved
Find charges of the ions.
Step 3
Work out how many of each ion you
require to make the charges match
Step 4
Write formula
Cu
NO3
Cu2+ NO3Cu2+ NO31
2
Cu2+ (NO3-)2