C8 Chemical Analysis

Purity and Formulations

a pure substance contains only one type of compound or element

only pure substances melt and boil at specific temperatures

  • water boils at 100 degrees Celsius under standard atmospheric conditions, making it a pure substance that is essential for various chemical processes

  • sodium chloride boils at 801 degrees Celsius

helps to test for unknown substances

physical test-testing the physical properties of that substance

chemical test-reacting with another chemical

impure substances don’t have specific melting and boiling points and instead melt and boil over a range of temperatures

impurities in a substance lowers the melting point but increases the boiling point

a formulation is a mixture that has been prepared using a specific formula

  • precise amounts of different components

  • have a particular function

    examples include: fuels, cleaning agents, paint, medicines, alloys, fertilisers, food and drink, etc

different components are always present in the same proportions

Paper Chromatography

  1. take a piece of filter paper and draw a line with a pencil near the bottom of the sheet (baseline)

  2. add sample of ink to baseline

  3. find a beaker and fill it with a shallow amount of water or ethanol

  4. place the filter paper into the solvent and do not submerge the baseline

  5. place a lid to stop the solvent evaporating

  6. wait for solvent to seep up the paper, the dyes dissolve in the solvent and move up with it

  7. different dyes travel up the paper at different rates, so each one much be a different substance

  8. non-soluble chemicals cannot dissolve so stay on the baseline

  9. once solvent reaches near the top of the paper, leave it out to dry

this is called a chromatogram

the mobile phase is the substance the molecules can move in (solvent)

the stationary phase is the substance the molecules can’t move in (paper)

in chromatography, the chemicals constantly change phase, the time they spend in the two phases determines how fast they move up the paper

more soluble chemicals spend more time in the mobile phase and move faster

less soluble chemicals spend more time in the stationary phase and move slower

how far the chemicals travel is dependent on the properties of that substance

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

Tests for Gases

chlorine

test tube full of sample gas

piece of blue litmus paper and dampen it, place it in test tube

if chlorine is present, the paper will turn red then bleaches white

chlorine is poisonous, so wear a mask or do it in a fume cupboard

oxygen

glowing splint glowing red at the end

get gas sample and place the splint into the tube

if the gas is oxygen, the splint will relight

hydrogen

burning splint, test tube contains hydrogen

produces a squeaky pop as the heat energy provided by the flame causes the hydrogen to burn with the oxygen in the air to form water

carbon dioxide

gas solution in one tube

calcium hydroxide (limewater) in the other

bubble the limewater and if the gas is CO2, the gas will go cloudy

Test for Anions

anions = negatively charged ions

test for carbonates

carbonate + acid → salt + carbon dioxide + water

add dilute HCl to test sample, add any CO2 gas and run through limewater

if any carbonate is produced, the sample will go cloudy

test for sulfates

barium ions + sulfate ions → barium sulfate (white precipitate)

white precipitate= positive result (sulfates are present)

  1. add dilute HCl to remove carbonate and sulfite ions (impurities)

  2. add barium chloride solution

  3. if any sulfate ions are present, white precipitate will form and the test is positive

test for halide ions

tests for chloride ions, bromide ions and iodide ions

  1. add dilute nitric acid to remove carbonate and sulfite

  2. add silver nitrate

    the silver ions from silver nitrate react with halide ions to form a silver precipitate

reacts with chloride

  • forms silver chloride, white precipitate

reacts with bromide

  • forms silver bromide, cream precipitate

reacts with iodide

  • forms silver iodide, yellow precipitate

can see which halide ions were present

Test for Cations

cations = a positively charged ion

nearly all cations are metal ions, as metals form positive ions

the only exception is ammonium (NH4+) which is a non-metal cation

flame tests

one way to test is to see what color flames they produce when they burn

different metal ions produce different colors when burning, and so we can use the color of the flame to help identify the type of metal

  1. take a platinum (or nichrome) wire loop, and clean it by dipping it in some dilute hydrochloric acid, rinsing it in distilled water, and then heating it over a Bunsen burner flame

  2. dip the wire loop into the compound you want to test

  3. hold the wire loop in the clear blue part of the Bunsen burner flame (the hottest part)

  4. see what color the flame turns as the compound burns

  • lithium ions (Li+) burn with a crimson flame

  • sodium ions (Na+) burn with a yellow flame

  • potassium ions (K+) burn with a lilac flame

  • calcium ions (Ca2+) burn with an orange-red flame

  • copper ions (Cu2+) burn with a green flame

metal hydroxide tests

a reaction with a solution of sodium hydroxide (NaOH) and seeing what color the solution turns

this is possible because some metal ions form colored precipitates when they react with hydroxide ions, and these colored precipitates then determine the color of the solution

copper(II) (formula Cu2+) forms a blue precipitate

  • Cu2+(aq) + 2OH-(aq) ➔ Cu(OH)2 (s)

calcium (formula Ca2+) forms a white precipitate

  • Ca2+(aq) + 2OH-(aq) ➔ Ca(OH)2 (s)

iron(II) (formula Fe2+) forms a green precipitate

  • Fe2+(aq) + 2OH-(aq) ➔ Fe(OH)2 (s)

iron(III) (formula Fe3+) forms a brown precipitate

  • Fe3+(aq) + 3OH-(aq) ➔ Fe(OH)3 (s)

magnesium (formula Mg2+) forms a white precipitate

  • Mg2+(aq) + 2OH-(aq) ➔ Mg(OH)2(s)

aluminum (formula Al3+) forms a precipitate that is white at first, but if there is excess NaOH, it then redissolves to form a colourless solution

  • Al3+(aq) + 3OH-(aq) ➔ Al(OH)3 (s)

Flame Emission Spectroscopy

when metal ions are heated. they emit light

the wavelengths of that light are specific to that particular metal ion

a spectroscope detects the individual wavelengths that are emitted

  1. metal ions are heated until they emit light

  2. the light is detected by a spectroscope which can distinguish between individual wavelengths of light emitted

  3. as each metal ion emits a unique combination of wavelengths, it will produce a unique line spectrum

  4. identify an unknown metal cation by comparing its line spectrum to those of known metal cations in a data bank

benefits

every different metal ion produces a different pattern of wavelengths and has a unique line spectrum

the intensity of the lines shows the concentration of the ion so it can be used to find the concentration of a particular ion in a solution

if a sample contains multiple different cations ions, the spectrum will show the lines for all of them

benefits of instrumental methods

very accurate

very sensitive (detect even tiny amounts of a substance)

very fast (can usually be automated)