GCSE AQA Chemistry 'Unit 3a' Revision Guide

Revision guide for the first half of the GCSE AQA Chemistry 'Unit 3a' / 'C3' exam paper.

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GCSE Chemistry
`Unit 3a' written by Applequestria
Table of Contents
Table of Contents..................................................................................................................................... 1
The early periodic table ........................................................................................................................... 2
Newlands's Law of Octaves: ................................................................................................................ 2
Mendeleev's Periodic Table of Elements: ........................................................................................... 2
The modern periodic table ....................................................................................................................... 3
Trends within the periodic table .............................................................................................................. 4
The alkali metals (group one): ............................................................................................................. 4
The halogens (group seven): ............................................................................................................... 4
Trends within the periodic table (continued) ........................................................................................... 5
The transition elements (between group two and group three):........................................................ 5
Hard and soft water ................................................................................................................................. 6
The two types of hard water: .............................................................................................................. 6
Use of titration, to compare the hardness of water samples method: ............................................... 6
Use of titration, to compare the hardness of water samples results: ................................................. 6
Hard and soft water (continued) .............................................................................................................. 7
Purifying water......................................................................................................................................... 8
Making ammonia ..................................................................................................................................... 9
Making ammonia (continued) ................................................................................................................ 10
Alcohols ................................................................................................................................................. 11
Methanol, ethanol and propanol: ..................................................................................................... 11
Uses of alcohols:................................................................................................................................ 11
Carboxylic acids...................................................................................................................................... 12
Carboxylic acids: ................................................................................................................................ 12
Uses of carboxylic acids: .................................................................................................................... 12
Esters ..................................................................................................................................................... 13
Esters: ................................................................................................................................................ 13
Uses of esters: ................................................................................................................................... 13
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The early periodic table
In the nineteenth century, scientists attempted to classify the elements by arranging them in order of
their relative atomic masses (mass number, because atomic number wasn't discovered until the
twentieth century when electrons, protons and neutrons were discovered). When the elements were
listed by arranging them in order of their relative atomic masses, scientists noticed that similar
properties occurred at regular intervals. The list can be arranged in a table so that elements with
similar properties are in columns (groups).…read more

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The modern periodic table
When electrons, protons and neutrons were discovered in the early twentieth century, the periodic
table was arranged in order of atomic number instead of relative atomic mass (mass number), which
placed the elements in 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 vertical column (group) have the same number of electrons in their
highest occupied energy level (outer shell).…read more

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Trends within the periodic table
The alkali metals (group one):
These include lithium, sodium and potassium.
They have low density (the first three elements are less dense than water).
The further down the group an element is, the lower the melting and boiling points.…read more

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Trends within the periodic table (continued)
The transition elements (between group two and group three):
Compared with the alkali metals (group one), transition metals have high density.
They have higher melting and boiling points (except for mercury).
They are stronger and harder (except for mercury).
They are much less reactive (so they don't react vigorously with water and oxygen).
Transition metals often have ions with different charges:
Copper can form Cu+ and Cu2+ ions.
Chromium can form Cr2+ and Cr3+ ions.…read more

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Hard and soft water
Soft water readily forms lather with soap. Hard water readily forms scum with soap because it reacts
with the soap (meaning more soap is needed to form a later). However, soapless detergents do not
form scum because it does not have soap.
Hard water is caused by calcium ions and magnesium ions in dissolved compounds (such as
magnesium sulphate and calcium sulphate). These compounds are dissolved when water
comes in contact with rocks (such as limestone, chalk and gypsum).…read more

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Hard and soft water (continued)
Boiling hard water:
Temporary hard water becomes soft when boiled. When heated, the hydrogencarbonate ions (HCO3-)
decomposes to form carbonate ions (CO32-) which react with calcium and magnesium ions to form
insoluble precipitates of calcium carbonate and magnesium carbonate. Permanent hard water
remains hard when boiled. When heated, the sulphate ions (SO42-) do not decompose.
Ca(HCO3)2 (aq) CaCO3 (s) + H2O (l) + CO2 (g) Calcium hydrogencarbonate thermally
decomposes to form calcium carbonate, water
and carbon dioxide.…read more

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Purifying water
Water of correct quality is essential for life. For humans, drinking water should have sufficiently low
levels of dissolved salts (such as poisonous phosphate salts and nitrate salts) and microbes (which can
cause diseases such as cholera and dysentery). Water of correct quality is produced by:
Choosing an appropriate source (most drinking water comes from clean reservoirs).
The water is filtered by using mesh screens (to remove large solids).
Chemicals are added to make solids and microbes stick together (to be removed).…read more

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Making ammonia
A reversible reaction is a reaction where the products of the reaction themselves react to produce the
original reactants. When a reversible reaction occurs in a closed system, equilibrium is reached when
the reactions occur at exactly the same rate in each direction. A closed system is where reactants and
products cannot escape.…read more

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Making ammonia (continued)
The Haber process is an important industrial process as it produces ammonia (NH3) which is used to
make fertilisers. The raw materials for the Haber process are nitrogen and hydrogen. Nitrogen is
obtained from the air and hydrogen is obtained from natural gasses and other sources. Some of the
nitrogen and hydrogen react to form ammonia. The reaction is reversible so ammonia breaks down
again into nitrogen and hydrogen.…read more

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