Experimental techniques
Measurement
● Time
○ Digital stopwatch
○ Measures up to 0.01s
● Temperature
○ Mercury-in-glass thermometer
○ Measures to the nearest celsius
● Mass
○ Electronic top pan balance
○ Measures up to 0.01 g
● Volume
○ Beaker
■ Estimate liquid volume
○ Measuring cylinder
■ Measures up to 0.1 cm3
○ Pipette
■ Measures fixed volumes accurately (e.g 20cm3)
■ Measures up to 0.1 cm3
○ Burette
■ Used for measuring variable amounts of liquid accurately
■ Measures up to 0.1 cm3
● A mixture is a substance which contains more than one substance
● Solute + Solvent → Solution
● A soluble solid usually gets more soluble as the temperature rises.
● A solution is called saturated when it can dissolve no more solute at that temperature
● A solution in water is known as an aqueous solution
● A volatile liquid is one that evaporates easily, it is a sign that the forces between its particles
are weak
● A pure substance has no particles of any other substance mixed with it.
● An unwanted substance mixed with the solution is known as an impurity
● Purity
○ You can find purity by checking their melting and boiling point
○ It has a definite, sharp melting and boiling point.
● Impurity
○ When a substance contains an impurity
■ Its melting point falls and its boiling point rises
■ It melts and boils over a range of temperatures, not sharply
○ The more impurities there are
■ The bigger the change in melting and boiling points
■ The wider the temperature range over which melting and boiling points occur
Seperation
● Filtration
○ The process by which a liquid is separated from
insoluble substances
○ A solid from a liquid
○ The insoluble solid is collected as the residue
while the liquid is collected as the filtrate.
● Crystallization
○ It is the process by which a solid forms where the atoms and molecules are highly
organised into structure known as a crystal
○ It works because soluble solids tend to be less soluble at lower temperatures
○ Method
■ Heat the solution to evaporate some of the water, until it becomes concentrated
■ The solution will eventually become saturated, so cool it down for crystals to form
■ Check that it is ready after by touching it with a glass rod. Crystals should form on
the cool glass
■ Leave the solution to cool, crystals start to form as temperature falls.
■ Remove the crystals by filtering, then rinse them with distilled water and dry them
with filter paper
● Evaporation
○ It is the process by which liquid changes
its state to gas at various temperatures
○ To separate solute from its solution
○ Method (salt and sand)
■ Add water to the mixture and
stir, salt dissolves in the water
■ Filter the mixture. The sand is
trapped in the filter paper, but
the salt solution passes through
■ Rinse the sand with water and dry
it in an oven
■ Evaporate the water from the
salt solution to obtain dry salt
○ Salt and sugar → use ethanol(dissolves sugar), should be evaporated over water bath
●
Simple distillation
○ It is the way to obtain solvent from a solution
○ Both the solvent and the solute are obtained through this process
○ Method(water from salt solution)
■ Heat the solution in the flask. As it boils, water vapour rises into the condenser,
leaving salt behind
■ The condenser is cold, so the vapor condenses to water in it
■ The water drops into the beaker. It is called distilled water
● Fractional distillation
○ Process by which components in a chemical mixture are separated into different parts
(fractions)
○ Method(ethanol and water)
■ Heat the mixture in the flask at
78 degree Celsius, the ethanol
begins to boil and a bit of water
too, a mixture of ethanol and
water vapour rises up in the
fractionating column.
■ The vapour condenses on the
glass beads in the column,
making the beads hot
■ When the beads reach 78
degree celsius, ethanol no longer
condenses on them, only water
does, so water goes back into the flask while ethanol goes into the liebig condenser.
■ There it condenses and pure ethanol drops into the beaker.
■ Eventually the thermometer reading rises above 78 degree celsius, a sign ethanol
has gone so you can stop heating
● Paper chromatography
○ A method used to separate dyes based on their solubility
○ Used to separate multiple substances from a solution
○ Method
■ Place a drop of blank ink in the centre of some filter paper. Let it dry. Then add 3
or 4 more drops on the same spot
■ Now drop water onto the ink spot, one stop at a time. THe ink slowly spreads out
and separates into rings of different colours
■ Suppose there are 3 rings: yellow, red and blue. This shows that ink contains 3 dyes
coloured yellow, red and blue
○ The filter paper with coloured rings is known as a chromatogram
○ Chromatography can be used to
■ Identify a substance
■ Separate mixtures of substances
■ Purify a substance by separating it from its impurities
○ 2 substances travel over the paper at different speeds because of their different
solubilities in the solvent.
○ The more soluble a substance is in a solvent the further it will travel up the chromatogram
● Identifying substances in a colourless mixture(5 different colourless solutions of amino acids)
○ Place a spot of each solution along a line drawn in pencil on slotted chromatogram (purpose
of the slots are to keep samples separate)
○ Label each spot in pencil at the top of the paper
○ Place a suitable solvent in the bottom of the beaker (amino acids → water / ethanoic acid
→ butanol)
○ Roll the chromatography paper into a cylinder and place it in the beaker, cover the beaker
○ The solvent rises up the paper, when it has almost reached the top, remove the paper
○ Mark a line in pencil on it to show where the solvent reached
○ Put paper in an oven to dry out
○ Next spray it with a locating agent to make the amino acid show up (Ninhydrin). After
spraying, heat the paper in the oven for 10 mins. Spots turn purple
○ Mark a pencil dot at the centre of each spot. Measure from the baseline to each dot, and
line with final solvent level
○ Rf = Distance moved by amino acid/ Distance moved by solvent
○ The Rf value of a compound is always the same for a given solvent under the same
conditions
