C3.1 Qualitative analysis Flashcards
What are two reasons chemical analysis is used on food and other products?
Purity and safety
What are the two main types of chemical analysis?
Qualitative and quantitative analysis
What is qualitative analysis used for?
Investigating what kind of substances are present in a sample
What is quantitative analysis used for?
Measuring the amount of each substance present in a sample
What two things must an analyst identify to identify the ionic compound?
The anion and cation
How can you be sure that the answer is correct when using an ion test?
The ion test must give a positive result to only one type of ion
What can flame tests be used for?
Detecting some metal cations
What colour flame does the flame test for calcium produce?
Brick red
What colour flame does the flame test for sodium produce?
Yellow
What colour flame does the flame test for potassium produce?
Lilac
What colour flame does the flame test for copper produce?
Green/blue
How are cations that can’t be detected with a flame test identified?
By adding a few drops of sodium hydroxide solution
Why does adding a few drops of sodium hydroxide solution to an unknown substance help identify a cation?
Because many metal hydroxides are insoluble, so a precipitation reaction can occur. The precipitate formed can then be used to identify the ion present
What colour precipitate does a cation of aluminium form when sodium hydroxide solution is added to it?
White
What colour precipitate does a cation of calcium form when sodium hydroxide solution is added to it?
White
What colour precipitate does a cation of copper(||) form when sodium hydroxide solution is added to it?
Pale blue
What colour precipitate does a cation of iron(||) form when sodium hydroxide is added to it?
Green
What colour precipitate does a cation of iron(|||) form when sodium hydroxide solution is added to it?
Brown (rust)
How can you distinguish calcium and aluminium ions when they both form a white precipitate when a few drops of sodium hydroxide solution is added to them?
You add sodium hydroxide in excess
What happens to the white calcium ion precipitate when sodium hydroxide solution is added in excess?
It remains unchanged
What happens to the white aluminium ion precipitate when sodium hydroxide solution is added in excess?
The precipitate redissolves to form a colourless solution
What is the precipitate when adding sodium hydroxide solution to an unknown substance?
A solid hydroxide of the cation
What would the precipitate formed from the reaction of copper(||) sulphate and sodium hydroxide be?
Copper(||) hydroxide
Why must an ion test only give a positive result for one type of ion?
If a positive result was produced for two or more different ions you wouldn’t be sure which of these ions was present in the compound
What are the three ways the water supply in Britain is cleaned?
- filtered to remove solid particles
- chlorinated to kill bacteria
- chemically treated to remove other impurities
What group is chlorine found in?
Group 7 (halogens)
How can halide ions be identified?
By acidifying the solution with dilute nitric acid, then adding silver nitrate
What precipitate is produced when dilute nitric acid and silver nitrate are added to a solution containing halides?
A silver halide precipitate
What colour precipitate is formed when silver nitrate is added to an acidified solution containing chloride ions?
White
What colour precipitate is formed when silver nitrate is added to an acidified solution containing bromide ions?
Cream
What colour precipitate is formed when silver nitrate is added to an acidified solution containing iodide ions?
Yellow
Why can ions in solution move about independently?
Because they are separated
What ions do many types of fertiliser contain?
Ammonium ions
Why can fertilisers be dangerous?
They can get into rivers and lakes and harm water life
How do you test for ammonium ions?
The substance is warmed with sodium hydroxide solution. If a smelly alkaline gas is given off that turns damp red litmus paper blue it is ammonia gas
How is a carbonate identified?
You add a dilute acid to the substance. If the gas given off turns limewater milky it is carbon dioxide gas and so the substance was a carbonate
How does water get dissolved substances inside it?
- from the rocks over which it flows
- from the chemicals used in water treatment
Why are maximum limits for the amounts of chemicals and microorganism found in water set?
- so that it looks and tastes good
- so that it’s safe to drink
What are three chemicals used in water treatment?
- chlorine
- aluminium salts
- sodium fluoride
Why is sodium fluoride added during water treatment?
It has been shown to reduce tooth decay
Why are aluminium salts added during water treatment?
To remove small particles of solids
Why are some scientists concerned about the use aluminium fluoride within water treatment?
High levels of aluminium have been linked to Alzheimer’s disease and damage to the digestive system
How do aluminium salts remove small particles of solids from water?
They stick to small dirt particles to form clumps so that they can be removed more easily
What is a pure liquid?
A liquid that only contains one substance
What does an acid + a metal produce?
A salt + hydrogen
What does an acid + a base produce?
A salt + water
What does an acid + a metal oxide produce?
A salt + water
What does an acid + a metal carbonate produce?
A salt + water + carbon dioxide
