1b. Elements, compounds and mixtures Flashcards
Element
A substance made up of atoms that all contain the same number of protons (one type of atom) and cannot be split into anything simpler. There are 118 know elements.
Examples: hydrogen, oxygen, carbon
Compound
A pure substance made up of two or more elements chemically combined together. There are unlimited types of compounds Cannot be separated by physical methods of separation.
Examples: copper (ii) sulphate, calcium carbonate
Mixture
A combination of two or more substances (elements and/ or compounds) that are not chemically joined together. Can be separated by physical methods of separation.
Examples: salt and water, air
Melting & Boiling Points
A pure substance has a fixed melting and boiling point, but a mixture may melt or boil over a range of temperatures
Simple Distillation
This is used to separate a liquid and soluble solid from a solution (e.g., water from a solution of salt water) or a pure liquid from a mixture of liquids
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 water 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 (e.g., ethanol and water from a mixture of the two)
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(s) of the mixture
For water and ethanol
Ethanol has a boiling point of 78 ºC and water of 100 ºC
The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker
When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated
Filtration
Used to separate an undissolved solid from a mixture of the solid and a liquid / solution ( e.g., sand from a mixture of sand and water)
Centrifugation can also be used for this mixture
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 cold (e.g., copper sulphate from a solution of copper (II) sulphate in water)
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 (e.g., different coloured inks that have been mixed to make black ink)
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 paper is called the stationary phase
The solvent travels up the paper by capillary action, taking some of the coloured substances with it; it is called the mobile phase
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
This will show the different components of the ink / dye
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 will show up with more than one spot, a pure substance should only show up with one spot
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
These values are used to identify the components of mixtures
The Rf value of a particular compound is always the same but it is dependent, however, on the solvent used
If the solvent is changed then the value changes
Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions
These values are known as reference values
Calculation
The Retention factor is found using the following calculation:
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
The Rf value is a ratio and therefore has no units
Practical: Investigate Paper Chromatography Using Inks & Food Colourings
Objective:
Investigate how paper chromatography can be used to separate and identify a mixture of food colourings
Hypothesis:
Rf values can be used to identify the components of an unknown mixture by comparison with Rf values of known substancesMaterials:
A 250 cm3 beaker
A wooden spill
A rectangle of chromatography paper
Four known food colourings labelled A–D
An unknown mixture of food colourings labelled U
Five glass capillary tubes
Paper clip
Ruler & pencil
Chromatography - Food Colourings, IGCSE & GCSE Chemistry revision notes
Diagram of the apparatus needed for paper chromatography
Practical Tip:
The pencil line must never be below the level of the solvent as the samples will be washed away
Method:
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
Use the fifth tube to put a small spot of the unknown mixture U on the line
Make sure each spot is no more than 2-3 mm in diameter and label each spot in pencil
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
Results:
Record your results in a suitable table
Required Practical 6:Investigating Chromatography Results, downloadable IGCSE & GCSE Chemistry revision notes
Evaluation:
The Rf values of food colours A, B, C and D should be compared to that for the unknown sample as well as a visual comparison being made
Conclusion:
The use of chromatography and Rf values is a viable method of identifying unknown mixtures given reference material
