Summary of AQA GCSE Chemistry

Edexel IGCSE Chemistry Revision Notes

States of matter
SOLIDS: (particles in contact; attractions hold them in fixed positions)
 have definite shape and volume


LIQUIDS: (particles in contact, and attract each other, but are free to move around)
 have a definite volume
 they take the shape of a container



GASES: (particles widely separated; little or no attraction; move freely until they collide)
 spread out (diffuse) and into all the space that is available and will mix completely
with any other gas(es) in the same space
 are compressible (i.e. can be squashed up into a smaller volume)




The following diagram shows the names given to the various changes of state.
Sublimation

Evaporation (at
any temperature)
Melting
(at fixed temp = m.p.t)
Boiling
(at fixed temp = b.p.t)

SOLID LIQUID GAS/VAPOUR

Freezing Condensation
(at fixed temp = m.p.t) (at fixed temp = b.p.t)
Sublimation

When substances change state, energy is involved.
Change of State Energy Change Reason for energy change

Solid  Liquid Heat taken in Energy needed to break bonds between the
particles in the solid
Liquid  Gas Heat taken in Energy needed to break the forces between
the particles in the liquid
Liquid  Solid Heat given out Energy given out as particles bond together
in the solid
Gas  Liquid Heat given out Energy given out as particles bond together
in the liquid




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,Edexel IGCSE Chemistry Revision Notes


Kinetic Theory
The properties of solids, liquids and gases can be explained by kinetic theory.
Kinetic theory states that matter is made of tiny particles that move all the time.
The main points of the theory are;
 All matter is made of tiny, invisible, moving particles.
 The particles move all the time. The higher the temperature the faster they move.
 Heavier particles move more slowly than light ones at a given temperature.

DIFFUSION: this is the mixing of atoms or molecules due to their continuous and random
motion. e.g. Mixing of bromine vapour and air.

BROWNIAN MOTION: the constant random movement of tiny particles (e.g. smoke
particles, or pollen on a drop of water) is caused by collision with (invisible) air or water
molecules, which are themselves in continuous and random motion. e.g.



Experiments using gases diffusing in a tube are used to examine the motion of the particles.




Ammonia and hydrochloric acid particles under Brownian motion as they hit air particles in
the tube.
When they meet white smoke (ammonium chloride) forms.
As the ammonia travels farthest along the tube we know that;
 The ammonia particles move faster than hydrochloric acid particles.
 The ammonia particles are lighter than the hydrochloric acid particles.




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,Edexel IGCSE Chemistry Revision Notes


Heating and Cooling Curves
We can use kinetic theory to explain changes in state when substances are heated or cooled.


D
Temp



B
C


A
time (steady heating)

 In a solid the particles are held in position by bonding to their neighbours.
(A on the graph).
 As the solid is warmed the particles vibrate but cannot move.
(Between A and B on the graph).
 When heated enough the particles vibrate so much that they can tear themselves free
from their positions, and the substance melts to a liquid.
 At the melting point heat needs to be added to melt the solid and break the bonds
between the particles, so the temperature doesn’t rise until it is all liquid.
(Between B and C on the graph).
 As the liquid is heated up the particles gain energy.
(Between C and D on the graph).
 At the boiling point heat needs to be added to change break the forces between the
particles in the liquid turning the substance into a gas.




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, Edexel IGCSE Chemistry Revision Notes


Elements, Mixtures and Compounds
THE ATOMIC THEORY
This theory assumes that all elements are made up of "atoms". If you were to divide a lump of
an element into smaller and smaller pieces you would eventually come to a piece that could not
be divided any further - a single ATOM of the element. Atoms are therefore very small. We can
see this if we dilute a solution of potassium manganate(VI) many times. It is still coloured even
when it is very dilute.

Definition: An atom is the smallest particle of an element that can exist or take part in a
chemical change.

MOLECULES
All elements are made up of atoms. In some gaseous elements (e.g. argon) single atoms move
around freely. But in other gaseous elements, single atoms cannot exist on their own at ordinary
temperatures: in these elements the free-moving particles consist of pairs of atoms.
The two atoms forming a pair (a MOLECULE) are joined together by a chemical "bond". This
is the case with hydrogen (H2), oxygen (O2) and nitrogen (N2). Such substances are said to be
diatomic.




An ELEMENT is a pure substance made up of only one type of ATOM.

A COMPOUND is a pure substance which contains two or more elements, chemically bonded
together in a fixed proportion.

A MIXTURE is a group of substances that are not chemically bonded together.

Compounds Mixtures
Proportions of elements are fixed. Proportions may vary.

Properties different from those Properties are simply those of
of the elements. the separate elements.

Cannot be separated into elements Can be separated by a physical
without chemical reaction. change (e.g. dissolving one
of the elements)

There is usually an energy change No energy change when the
when a compound is made from elements are mixed.
its elements.




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