2.2 Separating and purifying substances Flashcards
Pure substance
In chemistry a pure substance may consist of a single element or compound which contains no other substances.
Mixture
A mixture consists of two or more elements or compounds that are physically mixed together, they are not chemically combined.
The chemical properties of the substances in a mixture remain unchanged.
Substances in mixtures can be separated by physical means.
Distinguishing purity
Pure substances melt and boil at specific and sharp temperatures.
Mixtures have a range of melting and boiling points as they consist of different substances that tend to lower the melting point and broaden the melting point range.
Melting and boiling points data can therefore be used to distinguish pure substances from mixtures.
Simple distillation
This is used to separate a liquid and soluble solid from a solution.
The solution is heated, and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask.
The vapour passes through the condenser, where it cools and condenses, turning into the pure liquid that is collected in a beaker.
After all the liquid is evaporated from the solution, only the solid solute will be left behind.
Fractional distillation
This is used to separate two or more liquids that are miscible with one another.
The solution is heated to the temperature of the substance with the lowest boiling point.
This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker.
All of the substance is evaporated and collected, leaving behind the other components of the mixture.
Filtration
Used to separate an undissolved solid from a mixture of the solid and a liquid / solution.
A piece of filter paper is placed in a filter funnel above a beaker.
A mixture of insoluble solid and liquid is poured into the filter funnel.
The filter paper will only allow small liquid particles to pass through as filtrate.
Solid particles are too large to pass through the filter paper so will stay behind as a residue.
Crystallisation
Used to separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in a cold solvent.
The solution is heated, allowing the solvent to evaporate, leaving a saturated solution behind.
Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution.
If the solution is saturated, crystals will form on the glass rod.
The saturated solution is allowed to cool slowly.
Crystals begin to grow as solids will come out of solution due to decreasing solubility.
The crystals are collected by filtering the solution, they are washed with cold distilled water to remove impurities and are then allowed to dry.
Paper Chromatography
This technique is used to separate substances that have different solubilities in a given solvent.
A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along with the samples.
The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent, so the samples don’t wash into the solvent container.
The solvent travels up the paper by capillary action, taking some of the coloured substances with it.
Different substances have different solubilities so will travel at different rates, causing the substances to spread apart.
Those substances with higher solubility will travel further than the others.
Identifying mixtures
Pure substances will produce only one spot on the chromatogram.
If two or more substances are the same, they will produce identical chromatograms.
If the substance is a mixture, it will separate on the paper to show all the different components as separate spots.
An impure substance therefore will produce a chromatogram with more than one spot.
Rf values
Rf = distance travelled by substance ÷ distance travelled by solvent
The Rf value will always lie between 0 and 1; the closer it is to 1, the more soluble is that component in the solvent
Potable water
Water that is safe for humans to drink is called potable water. Potable water is not pure water because it almost always contains dissolved impurities.
Purifying water
Filtration - This process removes large insoluble particles by passing the water through layers of sand and gravel filters that trap larger particles.
Sedimentation - Large insoluble particles sink to the bottom of a tank of water that has been left still for some time.
Iron sulphate or aluminium sulphate is sometimes added to help the fine particles clump together.
Chlorination - This process is used to kill bacteria and microorganisms which are too small to be trapped by the filters.
Cholera and typhoid are examples of bacterial diseases which can arise by the consumption of untreated water.
Making sea water potable
This process is done in some areas of the world where very hot and dry climates prevail and where a lack of water.
Sea water contains mainly salts and can therefore be distilled to separate the water and the salts.
The salt remains in the liquid while the steam is cooled and condensed to make potable water.
The process is extremely expensive as a lot of energy is required to heat the large volumes of water to 100 ºC.
The wastewater produced is also extremely toxic due to the very high concentration of salts and must be disposed of correctly.
Water in chemical analysis
Most chemical investigations involve the use of water at some stage of the process.
Normally deionised water is used, which is water that has had metallic ions such as calcium or copper removed.
Deionisation uses specifically designed ion-exchange resins that remove ions by exchange with hydrogen and hydroxide ions in water, which then recombine to form water molecules.
Deionised water is used as the ions could react with the substances under analysis and would give the experiment a false result.
Practical 1 (simple distillation)
Procedure - Ink consists of a solvent which has different dyes dissolved in it.
Solvents tend to have low boiling points than the dyes which tend to be more viscous so it will evaporate first.
Add anti-bumping granules and heat gently so as not to go past the boiling point.
Record the temperature of boiling point.
Practical 1 (chromatography)
Procedure - Use a ruler to draw a horizontal pencil line 2 cm from the end of the chromatography paper
Use a different capillary tube to put a tiny spot of each colouring A, B, C and D on the line.
Pour water into the beaker to a depth of no more than 1 cm and clip the top of the chromatography paper to the wooden spill. The top end is the furthest from the spots.
Carefully rest the wooden spill on the top edge of the beaker. The bottom edge of the paper should dip into the solvent.
Allow the solvent to travel undisturbed at least three quarters of the way up the paper.
Remove the paper and draw another pencil line on the dry part of the paper as close to the wet edge as possible. This is called the solvent front line.
Measure the distance in mm between the two pencil lines. This is the distance travelled by the water solvent.
For each of food colour A, B, C and D measure the distance in mm from the start line to the middle of the spot.
