Chemical analysis Flashcards
Pure substance
A single element/ compound
Not mixed with anything else
Pure vs impure substances
Pure = it will melt at a specific point/ boil at a specific point
Whereas impure = melts/boils at a range of temps
How to find if a substance is pure?
Increase the temperature
Record at what point it melts
Record what point it boils
If the temperatures are fixed = pure substance
What do heating graphs for impure substances look like?
When it’s changing state, it’s not at a fixed point.
It may slope upwards showing it’s not at a fixed point
Formulation
Complex mixture that has been designed as a useful product
How do we design formulations and why?
Quantity of each component has been carefully measured
So it has the properties we need
Examples of formulations
Fuels
Cleaning products
Paints
Medicine
Alloy
Fertiliser
Food
Physical separation techniques list
Filtration
Crystallisation
Distillation
Chromatography
What is the use of chromatography?
Allows us to separate substances based on their different solubilities
Ie ink into their different colours
How does chromatography work?
When the solvent travels up the paper
It dissolves the inks, carrying them up the paper
So it separates the colours
Stationary phase of chromatography
The paper because it doesn’t move
What is the mobile phase in chromatography?
The solvent (water) because it moves up the paper
Using chromatography, how do we determine if it’s a pure or impure colour?
A pure compound produces a single spot in all solvents
In an impure compound the compounds separate into different spots depending on the solvent
Do different solvents effect chromatography?
Yes
The positions could be further up or down the paper
Why use pencil in chromatography?
Using pen causes pen ink to move up the paper as well
Why exactly does paper chromatography work?
Because substances with a higher solubility can move further up the paper
And those with lower solubility will not go as high
How can we find out what an unknown chemical is in chromatography?
Do the chromatography of the unknown chemical
Draw the solvent line (where the water got to)
Measure distance from pencil line to the chemical
The measure how far the solvent moved from pencil line
Calculate Rf value and find out what that substance is
How to find Rf value
Distance moved by substance
———————————————
Distance moved by solvent
Problem with the method of finding Rf value
Several substances have same Rf value
Repeat using different solvent to narrow it down further
Newly found substance shave no Rf value to find
Test for hydrogen
Insert a burning splint into bung with hydrogen
Hydrogen will burn and make a pop sound
Test for oxygen
Use a glowing splint
Place it in oxygen and the splint will relight (burst into flames)
What is limewater?
Calcium hydroxide dissolved in water
Testing for carbon dioxide
Put some limewater into pipettes bubble the gas through limewater
Repeat several times = cloudy
Testing for chlorine
Insert damp litmus paper
It will be bleached (white)
What is the point of flame tests?
To find the metal ion in an unknown ionic compound
How to do a flame test?
Put a small amount of ionic compound onto wire
Place this in a blue Bunsen burner flame
This changes the colour which we can use to identify the metal
Colour lithium produces in flame test
Li = CRIMSON
Colour sodium produces in flame test
Na = YELLOW
Colour potassium produces in flame test
K = LILAC
Colour calcium produces in flame test
Ca = ORANGE/RED
Colour copper produces in flame test
Cu = GREEN
Problems with flame test
-Colours hard to distinguish esp if there is low concentration of the metal compound
-Sample may contain mixture of metal ions which masks flame colour
What do scientists do instead of flame tests?
Flame emission spectroscopy
How does flame emission spectroscopy work.
Compound is put in flame
Light given out by flame passes through spectroscope
Converts it into line spectrum
Position of these lines in a spectrum is specific to different metals
What else can flame emission spectroscopy tell us?
Concentration of metal ion
Because lines become more intense at higher concentration
what is flame emission spectroscopy an example of?
Instrumental method
Aka carries out by a machine
Advantages of instrumental methods
Quicker than by hand if we did flame tests
Sensitive so will work even if the compound is small
More accurate and correct than by hand
Metal hydroxide tests
Used for metals that don’t work for flame tests
How do metal hydroxide tests work?
Add sodium hydroxide to the substance
They produce a white precipitate which we then differentiate
How to find aluminium ions
Add sodium hydroxide to make white precipitate
Add more sodium hydroxide and it will re dissolve thus become colourless again
How to find magnesium ions
Add sodium hydroxide
Makes white precipitate
Adding more won’t redissolve it
Testing for copper ii
Add sodium hydroxide
Forms blue precipitate of copper II hydroxide
Testing for Iron II ions
Add sodium hydroxide
Forms green precipitate iron II hydroxide
Testing for Iron III ions
Forms brown precipitate of iron III hydroxide
Identifying non metal ions
Carbonate
Halide
Sulfate
how to identify carbonate ion?
Add dilute acid
The carbonate will react to form carbon dioxide gas, by effervescence
So bubble this gas through limewater to see if it turns colours
How to identify halide ions?
Add dilute nitric acid
Add dilute silver nitrate solution
Each halide produces different coloured precipitate
What colours do chloride ions produce in silver chloride?
White precipitate
What colours do bromide ions in silver bromide produce ?
Cream precipitate
What colours do iodide ions produce in silver iodide
Yellow
How to identify sulfate ion
Add dilute HCl
Add barium chloride solution
Forms white precipitate
