7.3 Elements, Mixtures and Compounds
7.3.1 Properties of substances
Different substances have different properties and so have different uses. Most metals
for example are hard, strong, dense, conduct electricity, conduct heat well, are malleable
(can be pressed into thin sheets) and ductile (can be drawn into wires). Some are,
however, harder or stronger or better conductors than others and so we choose the best
material for a particular job.
7.3.2 Elements, mixtures and compounds
A substance which contains just one type of atom is known as an element.
There are well over 100 types of atom. For example, the element carbon contains only
carbon atoms. An atom is the smallest part of an element you can get - elements consist
of just one type of atom (but there are countless numbers of atoms, not just one)! Each
element has its own symbol. For example the symbol for carbon is C and the symbol for
iron is Fe.
The atoms of different elements can join together in chemical reactions, forming
compounds.
A COMPOUND is two or more different elements which are chemically bound together.
The properties of compounds are usually very different from the properties of the
elements they contain. For example sodium is a very reactive shiny metal and chlorine is a
very poisonous green gas. Sodium chloride, which is made when these two elements react,
is commonly known as salt and is safe to eat.
A MIXTURE is when two more elements (or compounds) are mixed together but not
chemically joined.
Element
Element
Compound
TOPIC 7.3 ELEMENTS, MIXTURES & COMPOUNDS
Mixture
Mixture
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7.3.3 Elements and the Periodic Table
Every element has its own chemical symbol. It is usually one or two letters long (but can
sometimes be three). Every symbol begins with a capital. The second and third letters are
lower case. Sometimes the symbols are really obvious: O = oxygen; Li = lithium; Mg =
magnesium. However, sometimes it is not easy to tell what the symbol stands for (because
the symbol come from a name that is not an English word) e.g. W stands for tungsten
(from the word wolfram); Na for sodium (from natrium).
The chemical symbols are used all over the world, not matter what language people speak.
All the different elements are arranged in a chart - the periodic table. The elements are
arranged so similar elements are found in the vertical columns, called groups. The
horizontal rows are called periods. The metals are on the left, the non-metals on the
right.
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7.3.4 Elements Combining
Compounds
Most of the substances around you are compounds which have been made by combining
some of the 100 or so elements mentioned earlier. Compounds are two or more elements
chemically bonded together. The name of a chemical compound comes from simply
combining the names of the elements you reacted together to make your compound.
E.g.
sodium + chlorine  sodium chloride
iron + oxygen  iron oxide
The -ide is added to the end to tell us the elements have joined together, forming a
compound. The metal always appears first in the name (if there is one). When a compound
contains oxygen and 2 other elements, the compound was not made by reacting all three
elements together directly but by another route. The name of compounds of this type (i.e
with 2 elements and also oxygen) contain the names of the first two elements and end in ate to indicate the presence of oxygen.
E.g.
A compound containing copper, sulphur and oxygen is called copper sulphate.
Formulae
There is always a fixed number of atoms of each type in a compound. For example water
is made up of two hydrogen atoms and one oxygen atom and has the formula H2O. This is
always the case for water and if something has a different formula then it isn’t water.
A chemical formula is made up of the symbols for the elements it contains.
E.g. water is H2O, carbon monoxide is CO, Carbon dioxide is CO2. Look out for prefixes
like mono-, di, tri- which tell you how many of something there are and help to tell
compounds like CO and CO2 apart.
Compounds can be broken back down into the elements from which they are made using
techniques such as heating (thermal decomposition) or by passing an electric current
through it (electrolysis).
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Solutions
A mixture contains two or more different substances which are not chemically joined
together. Because they are not chemically bonded together, the components of a liquid
can be separated fairly easily. You meet a wide variety of mixtures in everyday life, for
example:
• vinegar is a mixture of two liquids (water and ethanoic acid)
• hair gel is a mixture of a solid and a liquid
• shaving foam is a mixture of a liquid and a gas
One particular type of mixture is called a SOLUTION. A solution is obtained when a solid
is dissolved in a liquid, which is most commonly water. Solutions are clear and look the
same all the way through because the dissolved solid is completely mixed with the water.
The liquid part of a solution is called the SOLVENT. The solid part of a solution is called
the SOLUTE. (Solutions can also be obtained by dissolving some gases and some liquids in
water). Not all solids dissolve in a particular liquid. The ones that do dissolve are said to
be SOLUBLE; the ones that do not dissolve are said to be INSOLUBLE. The more soluble
a substance is, the more of it dissolves in water. Water, ethanol, propanone, petrol, white
spirit and turpentine are all examples of solvents.
To re-cap:
SOLUTE: the solid that is dissolved
SOLVENT: the liquid that the solute is dissolved into
SOLUTION: the mixture formed by dissolving the solute into the solvent.
Dissolving
When a substance dissolves, the solute and the solvent simply mix. Dissolving is therefore
known as a physical process as no new substances are formed.
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The diagram shows a model of sugar particles dissolving in water particles. As the
sugar particles separate, the smaller water particles fit into the spaces between the
larger sugar particles. The water and sugar particles are attracted to each other, so
they move closer together when they are mixed. This is why the total volume is often
slightly less than the volumes of the two separate components.
Dissolving can be speeded up by heating the solvent up (sugar dissolves much faster in
a hot cup of tea than in cold water). The hotter the solvent, the faster the molecules
are moving, the more energy they have and the more collisions there between solvent
and solute. The solute molecules will also have more energy. Stirring the mixture also
speeds up dissolving as does grinding the solid into smaller pieces.
Separating mixtures
Since a mixture is simply two different substances in the same container and which are
not chemically joined together, they can easily be separated. To separate mixtures we
can use a variety of techniques - the method used depends upon the mixture.
Filtration
Separates an insoluble solid (one which does not dissolve)
from a liquid, e.g. sand and water can be separated using
filtration.
Evaporation
Separates a soluble solid (one which dissolved) from a liquid,
e.g. water can be evaporated from salty water, leaving the salt
behind.
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Distillation
Separates a solvent (liquid) from a
solution (solid dissolved in a liquid).
The liquid evaporates from the
solution, is cooled and condenses
(which you then collect). Different
liquids boil at different
temperatures, so it is possible to
distill a solution containing many
liquids, e.g. water can be separated
from salty water.
Fractional distillation
This technique is used to separate two miscible (mixable)
liquids like water and ethanol. These liquids have different
boiling points and so as the mixture is heated the substance
with the lower boiling point boils off and is cooled and
collected, leaving the other liquid behind. The large
fractionating column helps to prevent the second liquid
from boiling over before the first liquid has all gone.
Chromatography
This is a technique used to separate dissolved substances
which have different colours. It works because some of the
coloured substances dissolve in liquid better than others, so
traveling further up the paper, e.g. an ink or dye can be split
into the different colours which make it up.
Separating funnel
This is used to separate two immiscible (do not mix) liquids like oil
and water. The less dense oil floats on top of the water and the tap
at the bottom can be used to remove the water, leaving the oil in
the flask.
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TOPIC SUMMARY
An element is a substance that contains only one kind of _________. For example a piece
of iron only contains __________ atoms. Elements cannot be broken down into
____________ substances in the laboratory.
A compound is 2 or more different______________ chemically combined or bonded.
The properties of a compound can be very different to the elements it contains. For
example, hydrogen is a very flammable gas and oxygen is also a reactive gas but water
(H2O) is a colourless ____________ that does not react with much. Although there are
only over 100 elements, there are ______________ of compounds. If only two elements
combine to make a compound, the name of the non-metal part changes to an –ide. An
example would be:
sodium + chlorine  sodium ___________
A mixture contains 2 or more substances that are not chemically combined or bonded.
They are usually _________ to separate and this can often be done in the laboratory. A
common type of mixture is a solution. A typical solution contains a solid that is called the
__________ , and the liquid, which is called the ______________. An easy way to
separate a solution is to ______________ the liquid. Some solids do not dissolve and are
said to be ______________. An example would be sand and water. Another way to
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separate these two substances would be to ____________ the sand from the water.
Other common methods of separation include: ________________ and
__________________.
solute
solvent
simpler
chromatography
evaporate
distillation
elements
insoluble
TOPIC 7.3 ELEMENTS, MIXTURES & COMPOUNDS
atom
chloride
liquid
iron
easy
millions
filter
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