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Chemistry revision
Topic 1:
Water testing
Safe water
Safe limits
Water solutions
Hard and soft water
Preparing soluble salts…read more

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Water testing
Must give a positive result with only one type of ion.
Qualitative ­ investigate the types of substance present is a sample
Quantitative ­ measure the amount of each substance present in a sample
Qualitative tests are carried out before quantitative tests, because you need
to know what's present in the compound first. Analytical chemists use
qualitative and quantitative tests to test the purity of water, because they
need to identify the substance in the water, and see the value there is.
Flame tests: (to detect certain metal cations)
Sodium (yellow)
Potassium (lilac)
Calcium (red)
Copper (green/blue)…read more

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Water testing:
Hydroxide precipitates
The test for metal cations using sodium hydroxide solution using sodium hydroxide
-Dissolve sample in water
-Add few drops of sodium hydroxide solution
Metal cations in solution form insoluble precipitates when this is done.
Sample Result with excess
Cation Symbol Effect of adding sodium hydroxide solution containing sodium hydroxide
Aluminium Al White precipitate Aluminium Precipitate dissolves to
form a colourless
Calcium Ca White precipitate solution
Copper (II) Cu Pale blue precipitate
Calcium White precipitate
Iron (II) Fe Green precipitate
Iron (III) Fe Brown (rust) precipitate
Sodium ions and potassium ions are detected using flame tests. Calcium ions and
copper (II) can be detected using flame tests or hydroxide precipitate tests.…read more

Slide 4

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Safe water
Chemists working in the water industry need to test the purity of drinking
water to make sure the water doesn't contain harmful dissolved
Halide ions:
Add a few drops of acidified nitric acid to the sample (The nitric acid reacts
with, and removes, other ions that might also give a confusing precipitate
with silver nitrate).
Then add a few drops of silver nitrate. A precipitate forms, chloride white,
bromide cream and iodide yellow. (Milk, cream, butter!)
Ammonium ions:
Add sodium hydroxide solution. Warm it using a Bunsen burner flame. Smell
the gas given off, ammonia has a distinct smell, and it turns damp red
litmus paper blue.…read more

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Safe limits
The government set standards for water purity to make sure water looks good, tastes good, and is safe
to drink.
Dissolved substances in water can come from rocks, and from chemicals used in water treatments.
Analytical chemists are employed by water authorities to test the purity of water, and that it follows
government standards.
Why are chemicals added to water?
-Chlorine is added to kill harmful microorganisms
-Aluminium sulphate is added to remove small solid particles
Testing blood: (moderns tests can detect small amounts of substances in the blood)
-High levels of aluminium in the blood are linked with Alzheimer's disease.
-Symptoms such as high blood pressure and kidney disease would alert doctors to order blood tests for
sodium ions.
Test for carbonate ions:
-add dilute hydrochloric acid to a sample containing carbonate ions, you will briefly see bubbles. The
gas is carbon dioxide. It turns limewater milky.…read more

Slide 6

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Water solutes:
Hard water
Hard water:
Water in rivers, lakes and reservoirs may flow over rocks
containing calcium or magnesium ions. Calcium
carbonate is the main compound in the limestone and
chalk found in some parts of the country. Some of the
calcium or magnesium ions dissolve in the water,
forming hard water.
- Does not lather easily with soap.
- Calcium and magnesium ions react with soap, forming
a precipitate (scum, causing soap to be wasted)…read more

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