Topic 8 chemical analysis

Chemistry

define chemical analysis

about the instruments and methods we use to separate, identify and quantify different substances

Give some examples of ways to separate mixtures

chromatography, crystallisation, filtration, distillation

Define a pure substance

contains only one type of compound or element

is water a pure or impure substance? Why?

water is a pure substance as it only contains water (H2O) molecules

How do you know if a substance is pure

Because they melt and boil at specific temperatures

water temperature does water boil and freeze at

boils at 100C and melts at 0C

How do you test a unknown liquid to see if its a pure substance

by boiling or melting it and compare it to other pure substances - this is known as a physics test. Or add the substance to another chemical - this is known as a chemical test.

What don’t impure substances have

don’t have a specific melting or boiling point

give me facts about impure substances

-melt/boil over a range of temperatures depending on how much each substance there is in a mixture

-lowers the melting point and increases the boiling point

define formulations

mixtures that have been prepared using a specific formula —> meaning there’s a precise amount of different elements in there as it has a particular function/purpose

give one example where formulations are used in modern world

for pharmaceutical industry- medicines

what is paper chromatography used for

used to separate different dyes in an ink

what are the steps to paper chromatography

1:take piece of filter paper and use a pencil to draw a line at the bottom of the sheet which is called the baseline 2: add sample of ink to the pencil line 3:fill a beaker with a shallow amount of solvent e.g. water 4:place filter paper in solvent but make sure the pencil line and ink isn’t submerged and place a lid to stop the solvent from evaporating. 5:wait for solvent to seep up paper as it does the different dyes that make the ink should move up as well. 6:each off the different dyes will travel up filter paper at different rates and will sperate out showing that each must be a different substance. 7:once solvent has almost reach the top of the paper take it out and leave to dry which creates patterned spots called a chromatogram

what is the mobile phase in chromatography

substance the molecules can move in (Liquid or Gas) - in this cane ours would be the solvent/water we used

what’s the stationary phase in chromatography

substance or material that molecules cant move in (solid or really thick liquid) this can would be the filter paper we used

if chemicals are more soluble in solvent what phase will they be in more- and what does it mean?

more time in the mobile phases therefor they are moving faster

if chemicals are less soluble or more attracted to the paper what phase will it spend more time in and what does it mean?

more time in the stationary phase and so moving slower

how far each of the chemicals travel is dependent of what

-on properties of that substance, how long they are left in experiment

how do you work out the Rf value

Rf=distance travelled by the substance/distance travelled by the solvent

what does the Rf value mean

is the ratio between the distance travelled by substance and distance travelled by solvent

when chromatography is done with a pure substance what will happen

nothing will be separated and you will have a single spot which shows a specific Rf value

Why may Rf value change for a pure substance

-depending on type of solvent used or type of paper

what gases can you test for

oxygen, chlorine, hydrogen and carbon dioxide

what is the test for chlorine

1: test tube full of sample gas that might contain chlorine, take a piece of blue litmus paper and dampen it, 2:put litmus paper in test tube- if chlorine is present the paper will turn from blue to white 3:chlorine is a poisonous gas so do this experiment with a mask or in a fume cupboard

how do you test for oxygen

1:glowing splint- flame is blown out but still glowing at the end, 2:get sample of oxygen in tube and place splint in tube, 3:if gas is oxygen then the splint will relight

how do you test for hydrogen

1:get a test tube containing hydrogen and having a burning splint, 2:place splint close to test tube and the top and you should hear a squeaky pop if hydrogen is present. Heat energy provided by flame causes hydrogen to burn with the oxygen in the air to form water

how do you test for Carbon dioxide

1:get gas sample in a tube, and calcium hydroxide in a separate tube (limewater), 2:bubble gas sample through calcium hydroxide and as long as gas is co2 the solution will turn cloudy

in the test for carbon dioxide why does the limewater turn cloudy

CO2 and calcium hydroxide react to form calcium carbonate and water