AQA Chemistry 3 GCSE

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The periodic table
Newlands and Mendeleev attempted to classify the elements by arranging them in order of their
atomic weights. Called the periodic table because similar properties occur at regular intervals.
The early periodic table was incomplete and some elements were placed In inappropriate groups if
the strict order of atomic weights was followed.
Mendeleev left gaps for elements that he thought had not been discovered
When electrons, protons and neutrons were discovered in the early 20th century, the periodic table
was arranged in order of atomic number. When this was done, the elements were placed in the
appropriate groups.
The modern periodic table can be seen as an arrangement of the elements in terms of their
electronic structures.
Elements in the same group have the same number of electrons in their outer shell.
The trends in reactivity within groups in the periodic table can be explained because the further
away the outer electron:
The more easily electrons are lost
The less easily electrons are gained
Group 1 ­ the alkali metals:
Metals with low density
React with non- metals to form ionic compounds in which the metal ion carries a charge of
+1. The compounds are white solids which dissolve in water to form colourless solutions
React with water releasing hydrogen
Form hydroxides which dissolve in water to give alkaline solutions
The further down the group an element is:
The more reactive the element
The lower its melting point and boiling point
The elements in group 7- halogens:
Have coloured vapours
Consist of molecules which are made up of pairs of atoms
Form ionic salts with metals in which the chloride, bromide or iodide ion/halide ion carries a
charge of -1
Form molecular compounds with other non- metallic elements
The further down the group an element is:

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The less reactive the element
The higher its melting point and boiling point…read more

Page 3

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A more reactive
halogen can displace
a less reactive
halogen from
aqueous solution of
its salt.
The transition
elements have
similar properties
and some special
properties because
the inner shell
is being filled in the
atoms of the
elements between
groups 2 and 3. This
is because the third
can hold up to 18
electrons, once two
electrons have
occupied the fourth
shell.…read more

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Many transition
elements have ions
with different
charges, form
coloured compounds
and are useful
as catalysts.
Acids, alkalis and
Arrhenius- acids H+
Alkalis OH-
Lowry and Bronsted-
base= proton
Acid= proton donor
(moles of acid x C of
alkali x V of alkali) =
(moles of alkali x C of
acid x V of acid)
Grams = moles x
An acid can be
defined as a proton
donor. A base can be
defined as a proton
acceptor.…read more

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Water must normally
be present for a
substance to act as
an acid or as a base.
Acids produce
hydrogen ions in
aqueous solution.
The H+ ion is a
proton. In water this
proton is
hydrated and is
represented as
Alkalis produce
hydroxide ions,
OH-(aq), in aqueous
solutions. Acids and
alkalis are classified
by the
extent of their
ionisation in water.
A strong acid
or alkali is
one that is
ionised in
Examples of
strong acids
sulphuric and nitric
acids.…read more

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The volumes of acid and alkali solutions that react with each other can be measured by titration
using a suitable indicator:
strong acid + strong alkali ­ any acid-base indicator
strong acid + weak alkali ­ methyl orange indicator
weak acid + strong alkali ­ phenolphthalein indicator.
If the concentration of one of the reactants is known, the results of a titration can be used to find
the concentration of the other reactant.…read more

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Page 9

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The solubility of soluble
gases in water increases as
the temperature
decreases and as the
Dissolving CO2 in
water under high
pressure makes
carbonated water.
When the pressure
released, the gas bubbles
out of the solution.
Carbonated water is used
to make fizzy drinks
Dissolved oxygen
is essential for
aquatic life.…read more

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Can increase costs
because more
soap is needed
When heated it can
produce scale that
reduces the
efficiency of
heating systems
and kettles
Can be good-
calcium compounds
are good for
Hard water can be made
soft by adding sodium
carbonate which reacts
with calcium and
ions forming a precipitate
of calcium carbonate and
magnesium carbonate. or
using an ion exchange
column containing
hydrogen ions or sodium
ions which replace the
calcium and magnesium
ions.…read more


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