Separating substances

● 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