Chemical analysis Flashcards
(45 cards)
What is a pure substance ?
A pure substance is something that only contains one compound or element throughout, not mixed with anything else
How to measure the purity of a substance
You can test the purity of a substance by its melting or boiling point and comparing it with the melting point or boiling point of a pure substance, which you can find from a databook. The closer your measured value is to the actual melting or boiling, the purer the sample is.
Boiling and melting point in pure substances
A chemically pure substance will boil and melt at a specific temperature.
Boiling and melting point in impure substances
Impure substances boil and melt at a range of substances. Impurities will lower the melting point and increase the boiling point
What is the formulation?
A formulation is a useful mixture that has been designed as a useful product
In a formulation, the quantity of each component is carefully measured so that the product hs the properties we need.
Examples of formulation
Fuels, cleaning products, paints, medicines, alloys, fertilisers and food.
why are formulations really important
Formulations are really important in the pharmaceutical industry. For example, by altering the formulation of a pill, chemists can make sure it delivers to the correct part of the body, has the right concentration, is consumable and has a long enough self life.
Is paper chromatography a physical process
It is a physical process, they do not involve chemical reactions and no new substances are made
What does paper chromatography do
It allows us to separate substances depending on their solubility
How does chromatography work
During a chromatography experiment, the substances in the sample constantly move between the mobile and the stationary phases, an equilibrium is formed between the two phases.
When the mobile phase moves up through the stationary phase, anything dissolved in the mobile phase moves up with it. Chemicals that spend more time in the mobile phase than the stationary phase (chemicals that are weakly attracted to the paper) will move further up the stationary phase. But chemicals that are strongly attracted to the stationary phase (spend more time in the stationary phase than the mobile phase) won’t move very far
Stationary phase
where the molecules cannot move, lie the paper
Mobile phase
where the molecules can move like a solvent
Pure chemicals and paper chromatography
a pure chemical will produce a single spot in all solvents
Chemicals in a mixture and paper chromatography
the chemicals in a mixture may separate into different spots depending on the solvent
Chromatography required practical
1) use a ruler to draw a horizontal line using a pencil, about 2 cm from the bottom of the chromatography paper
2) Mark five pencil spots at equal spaces across the line. Leave at least 1 cm clear at each side.
3) Put a small spot of the ink onto the pencil spots. Make sure the spots are small so that they do not spread into each other.
4) place water into a beaker to a depth of 1 cm
5) Place the paper into the water, the bottom of the paper should dip into the water. The pencil line with the ink should be above the surface of the water, otherwise, the water will wash the ink off the line. the ides of the paper should not touch the sides of the beaker as this might interfere with the way the water moves.
6) place a lid on the beaker to reduce evaporation of the solvent
7) At this stage the water will move up the paper and the colours will be carried up
8) Remove the paper when the water has travelled across three quarters up the paper
9) use a pencil the mark the point where the water has reached
10) hang the paper to dry
What is chromatography
chromatography is an analytical method used to separate the substances in a mixture, you can then use it to identify the substances
The amount of time the molecules spend on each phase depends on two things
how soluble they are in the solvent
How attracted they are to the paper
molecules with a higher solubility in the solvent, and which are less attracted to the paper, will spend more time in the mobile phase, and they will be carried further up the paper
Rf value formula
distance travelled by substance / distance travelled by solvent
How can you identify a substance using the rf value
We can identify the chemical using rf values. First, measure the distance from the baseline to the centre of each spot, then measure the distance travelled by the solvent from the baseline. Then using the formula calculate the rf value and using a database to tell you the identity of the chemical
HOWEVER
several chemicals may have this rf value, we may need to repeat the experiment using a different solvent to narrow it down further. If the chemical has never been analysed before, there will not be a rf value on the database, we would need to carry further analysis to identify it
what does the rf value depend on?
the solvent
Test for hydrogen gas
we remove the bung of a test tube containing hydrogen and insert a burning splint, you should hear a squeaky pop sound because hydrogen burns rapidly with the o2 in the air to form H20
Test for oxygen gas
Place a glowing splint into a test tube of oxygen, the oxygen will relight the splint
Test for carbon dioxide gas
Use an aqueous solution of calcium hydroxide like lime water - calcium hydroxide dissolved in water. Draw some of the gas using a plastic pipette, bubble the gas through the lime water if we repeat this a few times the lime water should turn cloudy
Test for Chlorine
Insert damp litmus into the mouth of a test tube, chlorine bleaches litmus paper and turns it white, it may turn red for a moment at first because a solution of chloride is acidic
