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Made by Esta Finesilver
Chemistry Revision…read more

Slide 2

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Unit C3 ­ Topic 1 ­ Chemical Detection…read more

Slide 3

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Made by Esta Finesilver
Specification Point: how water is used in everyday life and why it is important not
to waste it.
We all require a safe supply of water in order to survive. Over 70% of the human body is
water and it is recommended to drink 2 litres a day. Water seems to be everywhere (rivers,
oceans) millions of people die because they do not have a safe supply of water. It is
therefore important that we look after our water sources and do not waste water as the
consequences can be severe.
We need water for many reasons. Water is treated by screening and filtering to remove
solid particles, chemical treatments to eliminate impurities and chlorination to kill bacteria.
Analytical chemists check the purity of the water throughout its treatment.
The purity of water is checked against government standards, which set maximum limits
for chemicals and micro-organisms. Standards are high to ensure that water is not only
safe to drink but looks and tastes good as well. Water comes from natural sources, so
water pollution is a major concern. Pollution comes from Industry, Fertilisers used by
farmers and even using detergents and other household chemicals which are then washed
down the drain.…read more

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Made by Esta Finesilver
Specification Point: analysis may be qualitative or quantitative
There are two main types of analysis. Qualitative analysis which investigates the
kind of chemical present in each sample and Quantitative analysis, which
measures the amount of each chemical present.
Specification Point: why substances need to be identified and their purity
The purity of water is checked against government standards, which set maximum limits
for chemicals and micro-organisms. Our standards are high and ensure that water is not
only safe to drink but looks and tastes good as well.…read more

Slide 5

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Made by Esta Finesilver
Testing for Ions
Specification Point: ionic substances are identified by identifying each type of ion they contain.
Metals usually form positive ions called cations, by losing electrons. E.g. Sodium
Na Na+ + e-
Non-Metals usually form negative ions called anions, by gaining electrons. E.g. Chlorine
Cl2 + 2e- 2Cl-
Specification Point: why the test for each ion must be unique.
Ionic compounds are mostly colourless crystalline solids although some are coloured. To analyse
ionic substances, a series of tests is needed. The tests are unique for each ion to ensure the
correct conclusions on the analysis.
Flame tests: If a clean piece of wire is dipped in a solution of a compound and then held in the
hot part of a Bunsen flame, the colour produced indicates the metal ion present.
Element Flame Colour
Calcium (Ca2+) Brick Red
Sodium (Na) Yellow However, the colours of lithium and strontium
Potassium (K+) Lilac ion flame tests are far too similar to be used as a
Copper (Cu2+) Green test for the ions.
Lithium (Li) Crimson
Strontium (Sr2+) Crimson
Specification Point: The tests for Na+, K+, Ca2+, Cu2+ using flame tests.…read more

Slide 6

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Made by Esta Finesilver
Looking for Cations
Specification Point: precipitation reactions form the basis of some tests for ions
Flame tests are a good starting point for any analysis but other tests are needed. A series of
tests is required to find a unique set of results for each ion.
The next test usually involves adding a few drops of sodium hydroxide solution to the unknown
sample. Many metal hydroxides are insoluble so a precipitation reaction can occur and the
precipitate formed can identify the ion present.
A solution can be obtained by simply dissolving the solid in pure water. If insoluble in water then
hydrochloric avid
Specification and nitric
Point: acid for
The tests canAlbe
3+ tried as
, Ca2+, Cu 2+possible solvents.
, Fe2+, Fe3+ , NH + using sodium hydroxide solution.
Cation Symbol Precipitate The precipitate will be a solid hydroxide of the cation.
Ammonium NH4+ (aq) None E.g. If the compound was copper (II) sulphate, the
Aluminium Al3+ (aq) White precipitate would be copper (II) hydroxide.
Calcium Ca2+ (aq) White CuSo4 (aq) + 2NaOH (aq) Cu(OH)2 (s) + Na2SO4 (aq)
Copper (II) Cu2+ (aq) Blue Further tests are needed for NH4, Al, Ca ions. Al and
Iron (II) Fe2+(aq) Green Ca can be distinguished by adding excess sodium
Iron (III) Fe3+(aq) Brown (Rust)
hydroxide. Ca is unchanged but Al precipitate starts to
NH4 forms no precipitate. To test for the ion heat the unknown sample with concentrated
sodium hydroxide. If the ions are present the smelly alkaline gas called ammonia will be given
off. This can be detected as it turns universal indicator paper blue.…read more

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