Practical Skills for the BTEC applied and vocational scientist
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Weighing
•
Clean the balance of any debris.
•
If you have to record several weights during an investigation, you should always reweigh
using the same balance.
Top pan balance
Analytical balance
Usually shows values to two decimal places,
for example 0.01 g. This is less accurate than
an analytical balance.
Usually shows values to five decimal places, for
example 0.00001 g. This is more accurate than
a top pan balance.
The door must always
be closed when
readings are taken.
Questions
Complete the following passage:
The ……………............. of a substance can be determined using either a top pan
balance or an analytical balance. A ……………............. is the less accurate of the
two pieces of apparatus, usually measuring mass to the nearest ……………..............
An ……………............. is more accurate as it measures mass usually to the nearest
…………….............. An analytical balance has casing around it to protect the sample
being weighed from ……………............. in the laboratory. All balances should be
……………............. with a ……………............. to remove debris before being used. Displays
must be allowed to settle to ……………............. before anything is weighed and the door
of analytical balances must always be ……………............. when readings are taken. The
……………............. function allows the display to be set to zero when substances are being
weighed out into vessels, such as weighing boats, for which the mass does not need to be
recorded.
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Filtering
A separation technique called filtering is used to separate solids from liquids. During your post-16
studies you will probably use filtration extensively, including a technique called vacuum filtration.
Filtering works by placing filter paper in a funnel and passing liquid through it. The filter paper
contains microscopic pores that allow the liquid to pass through but which are too small to allow
the solid to pass through. The solid collects on the filter paper and the mixture is separated.
How do we filter?
The apparatus is set up as follows:
The filter paper needs to be folded in half, and then in half again; one
edge is then separated from the others to create a paper ‘funnel’ (it
looks like a cone). Ask your teacher or technician to demonstrate this
if you are unsure.
The filter paper is placed in a funnel, which can either be clamped
above or rest in the neck of a conical flask. Any solids will remain on
the filter paper after the liquid has passed through. This is called the
residue.
The liquid that passes through the filter paper will
collect in the flask. This is called the filtrate.
How is vacuum filtration set up?
Vacuum filtration works by exactly the same principles as basic filtration shown above; however,
it is faster because it happens under reduced pressure (the solution gets sucked through by a
vacuum). Vacuum filtration is generally used when it is the solid that we need to keep.
The apparatus is set up as follows:
This is called a Hirsch funnel. Flat filterpaper discs fit inside this funnel.
Practical Skills for the BTEC applied and vocational scientist
Sometimes solid particles will be present in a liquid and need to be removed. Solids present could
be formed as a product of a chemical reaction, or they may be solid reactants that have not been
used up.
This is called a Büchner flask. It is attached, via the side arm
and rubber tubing, to a vacuum. Your laboratory may have
vacuum taps or your technician or teacher may show you how to
set this up using ordinary water taps.
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Practical Skills for the BTEC applied and vocational scientist
20
Distillation
Different chemical substances have different boiling points. If a mixture contains compounds or
elements that all have different boiling points then each time one of the substances’ boiling-point
temperature is reached it will boil and can be separated off from the mixture and collected. This
process is called distillation and it is used widely throughout chemistry, including the industrial
process of fractional distillation, which separates the components or fractions of crude oil.
How do we carry out a distillation?
The apparatus is set up as follows:
The thermometer allows the temperature to be monitored. It is
often important to note the temperature at which substances
within the mixture boil off as it can give clues to their identity.
thermometer
The thermometer is held in place by a bung that prevents any
gases escaping and being lost.
The mixture to be separated
is placed in a roundbottomed or pear-shaped
flask. All the components of
the mixture will have different
boiling points.
Hot gases pass through the
condensing column and are cooled
down by the cold water flowing
through the outer jacket. They
condense (return to liquid form) and
run down the column.
water out
mixture
condenser
distilling
flask
cold
water in
distillate
Heat is applied to the mixture.
When the boiling point for
individual substances is
reached they will begin to boil
off, that is, become gaseous.
Water flows in from the bottom
and out from the top. This
ensures that the outer jacket of
the condenser fills completely
and enables the most efficient
cooling.
Liquid droplets of each
substance collect
here. This is called the
distillate. The test tube
or conical flask would
need to be changed for
each component that is
separated.
Because the various boiling points of a mixture’s components can be close, this can make
collecting pure distillates difficult. Often the substance you wish to collect is the least volatile (the
one with the highest boiling point). This means you can use distillation to boil off impurities and
leave behind a more and more concentrated substance in the flask.
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Distillation
Put the following sentences in the order that describes the process of distillation:
1
The liquid droplets will run down the inside of the condensing column to be collected
in a test tube or conical flask via a glass tube connected to the condenser.
2
The condensing column is connected to the cold water supply with water entering
at the bottom of the column and exiting at the top. This ensures the outer jacket
completely fills with water and enables the most efficient cooling.
3
When the substance is no longer condensing and has boiled off, the test tube or
conical flask should be changed so that any other substances collected will be kept
separate, if the procedure is being used to collect different components.
4
As substances reach their specific boiling points they become gaseous and boil off,
the temperature at which this happens should be noted down. Substances with the
lowest boiling points will boil off first.
5
Gases pass up through the flask and into the condensing column. Due to the cold
water passing through the outer jacket hot gases now condense back to liquids.
6
A mixture containing substances with different boiling points is placed into a roundbottomed flask, connected to a condensing column, and sealed with a bung and
thermometer.
7
If the substance being isolated has the highest boiling point (is the least volatile) this
substance will remain in the flask, becoming more highly concentrated each time an
impurity is boiled off. When all impurities are thought to have been boiled off the heat
should be removed.
8
The mixture is heated carefully.
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Practical Skills for the BTEC applied and vocational scientist
Questions
47
How Bonding Influences Structures and Properties
Post-16 Courses
You will be familiar with
covalent, ionic and metallic
bonding, and be used to drawing
dot-and-cross diagrams to
represent each type of bonding.
You will need to recognise which type of bonding a
substance contains and be able to discuss how this
influences properties such as boiling point and electrical
conductivity.
You will need to be confident at drawing dot-and-cross
diagrams, including double or triple bonds.
Is there a way of knowing which type of bonding is present?
Topic Builders
GCSE
37
Yes. If the substance is purely metal elements, the bonding present is metallic bonding.
positive nuclei
All the positive nuclei act together as one large nucleus, and all the
electrons are free to move around, or are delocalised, and are attracted to
the nuclei jointly, meaning there is a massive force of attraction.
delocalised electrons
If non-metal elements are bonded with metal elements, then the bonding present is ionic
bonding.
1⫺
2⫹
X
Magnesium has formed a 2+ ion and both chlorines have formed
1– ions. Ions must be shown in brackets with their charge at the
top right-hand corner.
Cl
Mg
1⫺
X
Cl
Magnesium is a group 2 element, so has two outer electrons. Two
chlorine atoms can each accept one of these electrons as they are
both in group 7.
Because they have opposite charges the ions are attracted and
held together by electrostatic attraction – an ionic bond.
Note that we just show Mg with an empty outer shell; it isn’t
usually necessary to show the shells underneath.
Dots and crosses are used to represent the electrons; all electrons
are the same – it just helps us remember which element owned
them at the start.
If non-metal elements bond with other non-metal elements (this includes hydrogen), then
the bonding present is covalent bonding (this can include non-metal elements bonded to
themselves, for example Cl2).
H
H
C
H
H
Carbon is a group 4 element, so has four electrons in its outer shell
(shown as dots here); hydrogen only has one electron (shown as crosses).
If they overlap or merge their orbitals, each feels as though it has a full
shell of electrons (remember hydrogen only needs one more electron
to fill the first energy level). Each shared pair of electrons is a single
covalent bond.
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