4.8.3 -IDENTIFICATION OF IONS BY CHEMICAL AND SPECTROSCOPIC MEANS

what type of ions do flame tests identify

cations

which cations do flame tests identify

lithium, sodium, potassium, calcium and copper

how to carry out flame tests

• dip the loop of an unreactive metal wire such as nichrome or platinum in dilute acid

• hold it in the blue flame of a bunsen burner until there is no colour change

• dip the loop into the solid sample / solution and place it in the edge of the blue bunsen flame

what is a limitation of flame tests

• if a sample containing a mixture of ions is used some flame colours can be masked

why should the nichrome/platinum wire be dipped in acid

to prevent contamination

lithium flame test colour

crimson

sodium flame test colour

yellow

potassium flame test colour

lilac

calcium flame test colour

orange red

copper flame test colour

green

what are all the flame test colour results

• lithium compounds result in a crimson flame

• sodium compounds result in a yellow flame

• potassium compounds result in a lilac flame

• calcium compounds result in an orange-red flame

• copper compounds result in a green flame

what type of ions does adding sodium hydroxide solution identify

cations

describe how to carry out a sodium hydroxide solution test

• add just a few drops of NaOH at first and very slowly

• If it is added too quickly and the precipitate is soluble in excess, you might miss the formation of the precipitate dissolves as quickly as it forms if excess solution is added

• a small amount is thus added, very gradually and any colour changes or precipitates formed are noted

• then the NaOH is added in excess and the reaction is observed again

positive result of the sodium hydroxide solution test with aluminium

• white precipitate (aluminium hydroxide)

• this dissolves in excess sodium hydroxide solution to form a colourless solution

positive result of the sodium hydroxide solution test with calcium

• white precipitate (calcium hydroxide)

• insoluble so remains in excess NaOH

positive result of the sodium hydroxide solution test with magnesium

• white precipitate (magnesium hydroxide)

• insoluble so remains in excess NaOH

positive result of the sodium hydroxide solution test with copper(II)

• blue precipitate (copper hydroxide)

• insoluble so remains in excess NaOH

positive result of the sodium hydroxide solution test with iron(II)

• green precipitate (iron hydroxide)

• insoluble so remains in excess NaOH

positive result of the sodium hydroxide solution test with iron(III)

• brown precipitate (iron hydroxide)

• insoluble so remains in excess NaOH

how to form a balanced equation

Cu²⁺ + 2OH^- → Cu(OH)₂

you need as many OH- ions as the charge on the metal ions

the Na from the NaOH and whatever the metal ion was bonded with will react to form a compound together

Cu(OH)2 = precipitate

CuCl₂ + NaOH → Cu(OH)₂ + 2NaCl

describe a test for carbonates ions

• adding dilute acid and testing the gas released

• carbonates react with dilute acid to form carbon dioxide gas

• if carbonates are present, effervescence should be seen and the gas produced is CO2

• it should be bubbled through limewater, this forms a white precipitate of calcium carbonate

• this white precipitate turns limewater cloudy/milky

describe a positive test result for the test for carbonates

• after effervescence, the gas bubbled through limewater should cause it to become cloudy/milky

describe a test for halide ions

• add dilute nitric acid

• then, add silver nitrate solution

• If a halide is present, it forms a silver halide precipitate

describe a positive test result for the test for (all) halides

silver chloride → white

silver bromide → cream

silver iodide → yellow

describe a positive test result for silver chloride - CHLORIDE IONS

silver chloride → white

describe a positive test result for silver bromide - BROMIDE IONS

silver bromide → cream

describe a positive test result for silver iodide - IODIDE IONS

silver iodide → yellow

describe a test for sulfate ions

• add dilute hydrochloric acid (to remove any carbonates)

• then, add barium chloride solution

• If sulfates are present, it forms a white precipitate

describe a positive test result for sulfate ions

white precipitate formed

what is the objective of the chemical tests required practicals

identify the ions in unknown single ionic compounds covering the ions

chemical tests rp: method for flame tests

1. Pour 1 cm3 of each known chloride solution into 5 test tubes.

2. Clean the nichrome wire by dipping it in dilute hydrochloric acid.

3. Dip the nichrome wire into solution and hold the tip in a blue bunsen flame.

4. Record the colour of the flame.

5. Repeat for the following solutions and make sure to clean the wire after each test.

6. Pour 1 cm3 of the unknown salt solution into test tube.

7. Dip the nichrome wire into solution and hold the tip in a blue bunsen flame.

8. Record the colour of the flame, you should be able to compare results with the known chloride with the matching colour flame.

chemical tests rp: flame test colours and results

lithium (Li+) - crimson

sodium (Na+) - yellow

potassium (K+) - lilac

Calcium (Ca2+) - orange-red flame

Copper (II) (Cu2+) – green flame

chemical tests rp: describe how to test for carbonates

• Place a 2 cm3 of limewater in a clean test tube.

• Add a little dilute hydrochloric acid to the unknown solution.

• if you see bubbles, transfer the gas produced to the limewater using a delivery tube.

• Repeat this process for the known sodium solutions to identify carbonates ions.

• result - bubbles produced and limewater turns cloudy/milky

chemical tests rp: positive result for carbonates

bubbles are produced, which is passed through limewater, limewater will turn cloudy

chemical tests rp: describe how to test for sulfates

• Add a 10 drops of dilute hydrochloric acid to the unknown solution in a test tube.

• Add a 2 cm3 barium chloride solution.

• Pour 1 cm3 of the known sodium solutions into separate test tubes.

• Add 5 drops of dilute hydrochloric acid and then 2 cm3 of barium chloride.

chemical tests rp: positive result for sulfates

white precipitate formed if present

chemical tests rp: describe how to test for halides

• Add a 10 drops of dilute nitric acid to the unknown solution in a test tube.

• Add a 1 cm3 silver nitrate solution to the test tube with the unknown solution.

• Pour 1cm3 of the know sodium solutions into separate test tubes

• Repeat steps 1 and 2 for which of the solutions.

• Record colour of precipitate formed in each test tube

chemical tests rp: positive result for halides

• Chloride – white precipitate produced

• Bromide – cream precipitate produced

• Iodide – yellow precipitate produced

chemical tests rp: describe the test for metal hydroxides

Add a few drops of dilute sodium hydroxide solution. Observe and record the colour of any precipitate formed.

chemical tests rp: positive result for metal hydroxides

aluminium → white precipitate (aluminium hydroxide) - this dissolves in excess sodium hydroxide solution to form a colourless solution

calcium → white precipitate (calcium hydroxide)- Insoluble so remains in excess NaOH

magnesium → white precipitate (magnesium hydroxide) - insoluble so remains in excess NaOH

copper (II) → blue precipitate (copper hydroxide) - insoluble so remains in excess NaOH

iron (II) → green precipitate (iron hydroxide) - insoluble so remains in excess NaOH

iron (III) → brown precipitate (iron hydroxide) - insoluble so remains in excess NaOH

what are the hazards for the chemical tests practical

hazard - barium chloride solid

harm - harmful if inhaled and toxic if swallowed

precaution - only use dilute solutions

hazard - silver nitrate solution

harm - causes eye irritation and skin irritation

precaution - wear eye protection, avoid skin contact by using dropper bottles or by wearing gloves

what are instrumental methods

using machines to carry out chemical analysis of elements and compounds

what can be identified using instrumental methods

elements and compounds

what are characteristics of instrumental methods

they are: accurate, sensitive and rapid

what are advantages of instrumental methods

• They provide greater accuracy

• they are faster and easier to use

• they are more sensitive than non-instrumental methods

therefore, better than chemical tests

what is flame emission spectroscopy

an example of an instrumental method used to analyse metal ions in solutions.

what are two uses of flame emission spectroscopy

• Identify metal ions in a sample

• determine the concentration of metal ions in dilute solutions

how does flame emission spectroscopy work

• the sample is put into a flame

• the light given out is passed through a spectroscope

• the intensity and wavelength of the light emitted is measured

• the output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their concentrations.

what is a strength of flame emission spectroscopy

works with mixtures of ions

how can ions be identified using flame emission spectroscopy

comparing the sample spectrum to reference spectra