This document contains everything from the AQA GCSE triple higher chemistry specification and notes are made by spec point. This document combines a variety of resources to make what covers everything at GCSE needed for a top grade. Just using my documents uploaded and past papers resulted in grad...
Chemistry – Key Topic 1
4.1 Atomic structure and the periodic table
4.1.1 A simple model of the atom, symbols, relative atomic mass,
electronic charge and isotopes
4.1.1.1 Atoms, elements and compounds
Atom = the smallest part of an element that can exist. They are the chemical building
blocks of the world
Element = the simplest substances that are made up of only one type of atom and
cannot be broken down into anything smaller
Compound = made up of two or more different elements chemically combined in
fixed proportions. They can only be separated into elements by chemical reactions
Molecule = a group of atoms chemically bonded together
Law of conservation of mass = the total mass of the products formed in a reaction is
equal to the total mass of the reactants. Mass is not lost during a reaction
All substances are made of atoms. An atom is the smallest part of an element that
can exist
There are about 100 different elements. They are shown in the periodic table
Compounds are formed from elements by chemical reactions. Chemical reactions
always involve the formation of one or more new substances, and often involve a
detectable energy change. Compounds contain two or more elements chemically
combined in fixed proportions and can be represented by formulae using the symbols
of the atoms from which they were formed. Compounds can only be separated into
elements by chemical reactions
Chemical reactions can be represented by word equations or equations using symbols
and formulae
4.1.1.2 Mixtures
A mixture consists of two or more elements or compounds not chemically combined
together. The chemical properties of each substance in the mixture are unchanged
- The properties of a mixture are just a mixture of the properties of the separate
parts – the chemical properties of a substance aren’t affected by it being a prat of
a mixture
, Compounds Mixtures
Compounds have a fixed composition Mixtures have no fixed composition
Chemical reactions must be used to The different elements or compounds
separate elements in a compound in a mixture can be separated again
more easily
There are chemical bonds between There are no chemical bonds between
atoms of the different elements in the atoms of the different substances in a
compound mixture
Mixtures can be separated by physical processes such as filtration, crystallisation,
simple distillation, fractional distillation and chromatography. These physical
processes do not involve chemical reactions and no new substances are made
- These separation techniques rely on differences in the physical properties of the
substances in the mixture, such as different solubilities in a solvent or different
boiling points
Filtration
This technique is used to separate an insoluble substance from a solvent for example
sand and water
The liquid is poured through the filter paper which removes larger particles but not
smaller particles, so allows solvent to pass through
Distillation
this technique is used to separate a) a soluble substance from a solvent, the solvent is
collected, for example salt and water b) Two or more miscible solvents for example ethanol and
water or crude oil. the solvents are separated because they have different boiling points.
the process involves heating to evaporate the liquid to a gas and then cooling the gas so
it is condensed to a liquid, it is then collected
, Fractional distillation
this method separates several miscible liquids using a fractionating column. the separa-
tion is possible because of the different boiling points of the liquids in the mixture .
- it is difficult to get pure liquids from mixtures of liquids with similar boiling points by simple distillation,
as vapour is given off from each liquid before they actually reach their boiling point. So to aid separation,
you can use a fractionating column
- the vapours must pass over and between the glass beads in the fractionating column before they reach
the condenser. The temperature in the fractionating column is highest at the bottom of the column,
getting lower as the vapour rises up.
- The substance with the higher boiling point will condense more readily on the cooler glass beads nearer
the bottom of the column and drip back down into the flask beneath. The substance with the lower boil-
ing point will continue rising and pass over into the condenser, where it is cool enough to turn back into
the liquid state and be collected.
Crystallisation
This technique is used to separate a soluble substance
from a solvent, only the solid is collected. The solution is heated to the solvent evaporates and
crystals appear. The heat is then stopped and the rest of the solvent is allowed to evaporate
slowly(get larger crystals)
Chromatography
Technique separates different dyes in an ink.
- Draw a line near the bottom of a sheet of filter paper
- Add a spot of the ink to the line and place the sheet in a beaker of solvent. The solvent de-
pends on what is being tested. Some compounds dissolve well in water but sometimes other
solvents such as ethanol are needed
- Make sure the ink isn’t touching the solvent – you don’t want to dissolve it
- Place a lid on top of the container to stop the solvent evaporating
- The solvent seeps up the paper, carrying the ink with it
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