Chapter -2.pdf (Mixture, element and compound)

notes for edexcel topic-2

1. Chapter -2
Elements, Compounds And Mixtures
Elements
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 known elements.
Examples: hydrogen, oxygen , carbon
Compounds
A pure substance made up of two or more elements chemically combined together. There are
unlimited types of compounds. Cannot be separatedby physical methods of separation.
Examples: copper (II) sulphate, calcium carbonate
Mixtures
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

5. Simple differences between Mixtures and Compounds
•In water ( a compound ) , every single water molecule has two hydrogen atoms combined with one
oxygenatom.(i.e. in fixed proportion)
•In a mixture of hydrogen and oxygen gases, the two could be mixed together in any proportion.
•In a mixture of elements, each element keeps its own properties, but the properties of compound
are quite different.
•A mixture of hydrogen and oxygen is a colourlessgas which explodes when you put a flame to it.
•But, the compound water is a colourlessliquid which just puts out a flame.

6. Iron and Sulphur
•In a mixture of iron and Sulphur, the iron is grey and the sulphuris yellow.
•The iron reacts with dilute acids (hydrochloric acid) to produce hydrogen ;the
sulphurdoesn’t react with the acid .
•But the compound iron sulphide(FeS) reacts differently with acids to produce
poisonous hydrogen sulphidegas which smell of bad eggs.
•In an iron sulphide(FeS) the proportion of iron to Sulphur is always exactly the
same.

7. Ease of Separation
•Mixturescan be separated by physical means(methods that don’t involve any chemical reactions).
•Physical means are things like changing the temperature or dissolvingpart of mixture in a solvent
such as water.
•A mixture of iron and suphurus quite easy to separate into two elements using a magnet.
•The iron sticks to the magnet and the Sulphur doesn’t.

8. •The elements in a compound cannot be separated by physical means .
•To convert iron sulfide into separate samples of iron and Sulphur requires chemical reactions.
•For example, a mixture of hydrogen and oxygen gases can be separated physically based on their
respective temperaturebut to separate the water compound into hydrogen and oxygen , you have
to change it chemically using electrolysis.

9. Melting point and Boiling point
•The melting point of water is 0˚C and boiling point is 100˚C .
•But dissolving 100g of common salt in 1litre of water lowers the melting point to about -0.6˚C and
raises the boiling point to about 100.2˚C.
•Pure substances (elements and pure compounds) melt and boil at fixed temperature.
•Mixture(not pure substances) usually melt or boil over a range of temperature.
•The presence of impurities( unwanted substances) lowers the melting point of a substance and
raises the boiling point.
•The melting point is very useful in determiningwhether or not a substance is pure.
•Aspirin melts at 138˚C .If the melting of the sample you made is 128--134˚C, it is quite impure
because it melts bellows the melting point of pure aspirin.

10. Separation of Mixtures
•Examples of mixtures are processing of crude oil , producing fresh water from salt water and
enrichment of uranium.
•In forensic science, the components of a mixture usually have to be separated before they can be
analysed.
Filtration
•Filtration can be used to separate a solid from a liquid .
•Sand can be filtered from water by filtration .
•The substance leftin the filter paper is called the residue and the liquidthat comes through is called
the filtrate.
•Filtration can also be used to separate two solids from each other if only one of them is soluble in
water.

11. Filtration

12. Crystallisation
•Crystallisationcan be used to separate a solute from a solution.
•The sodium chloride solution is heated in a evaporating basin to boil
off some of the water until an almost saturated solution is formed.
•This can be tested by dipping a glass rod into the solution and seeing
if the crystals form quickly on its surface when it is removed.
•The Bunsen burner is then turned off and the crystals allowed to form
as more water evaporates and the solution cools.
•The crystals can now be removed from the mixture by filtration.

13. Crystallisation

14. Simple Distillation
•Simple distillation can be used to separate the components of a solution.
•The water boils and is condensed back to a liquid by the condenser .The salt remains in the flask.
•The water is always fed into the condenser at the lower end.
•That way it fill the condenser jacket better and if the flow of water stop for any reason the
condenser jacket remains full of water.

15. Fractional Distillation
•Fractional distillation is used to separate a mixture of liquids such as ethanol (alcohol) and water.
•Ethanol and water are completely misciblewith each other (mix them together in any proportion
and they will form a single liquid layer).
•They can be separated using their different boiling points :water boils at 100˚C and ethanol at
78˚C.
•Both liquids boil, but by careful heating ,temperature of the column can be controlled so that all the
water condenses in the column and trickles back into the flask.
•Only the ethanol remains as a vapourall the way to the top of the fractionating column and out into
the condenser.
•Note that ;The fractionating column is often packed with glass beads.

16. Fractional Distillation

17. Paper Chromatography
•Paper chromatography can be used to separate a variety of mixtures.
•Paper chromatography can also be used to separate a mixture of colourlesssubstances such as
sugar ,but some method must be used to make spots visible on the paper.
•It can be used to separate mixtures of colouredinks or food colourings.
•Most inks and food colouringsare not just made up of one colourbut contain a mixture of dyes.
•In picture , the mixture have contained a minimum of three different dyes or could be more.
•It is possible that one of the spots is made up of two coloureddyes that by coincidence moved the
same distance.

18. Paper Chromatography
•Really, it is used to separate substances that have different solubilitiesin a given solvent (e.g.,
different colouredinks that have been mixed to make black)
•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 solvent, so the samples don’t wash into the solvent container.
•The solvent travels up the paper by capillary action , taking some of the colouredwith it.
•Different substances have different solubilitiesso 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 inks or dyes.
•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 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.

20. Interpreting chromatograms
??????
??????values
•These values are used to identify the components of mixtures and to describe how far the spots
move.
•The ??????
??????value of a particular compound is always the same but it is a different, however, on the
solvent used.
•If the solvent is changed then the value changes
•Calculating the ??????
??????value allows chemists to identify unknown substances because it can be
compared with ??????
??????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
??????
??????= distance travelled by substance ÷distance travelled by solvent
•The ??????
??????value will always lie between 0 and 1; the closer it is to 1, the more soluble is that
compound in the solvent
•The ??????
??????value is a ratio and therefore has no units.

21. Paper Chromatography Analysis

22. Chapter 2
Answers
Element Compound Mixture
Hydrogen Magnesium oxide Seawater
Calcium Copper (II)sulfate Honey
Blood
Mud
Potassium iodide solution
1.
2.(a) mixture (b) mixture (c) element (d) element (e) compound (f)
compound
3. Substance X is the pure substance –it melts at a fixed temperature. Substance Y is impure –it
melts over a range of temperatures.
4. (a) crystallization (b) simple distillation (c) fractional distillation (d) chromatography
(e) filtration

23. 5. For example; Stir with a large enough volume of cold water to dissolve all the sugar. Filter to leave
the diamonds on the filter paper. Wash on the filter paper with more water to remove any last traces of
sugar solution. Allow to dry.
6.(a) M (b) R
(c) 0.45±0.01 ( measure to the centreof the spot and remember to measure from the base line and
not from the bottom of the paper)
(d) G and T (e) P