Practical activity
Identification of species
Collecting samples
qualitativeResults that are not expressed as numbers or numerical data, eg they can be expressed as present or absent, or as colours. analysis can identify the different substances in a sample. It is used to:
- test chemicals during manufacturing processes
- test mineral samples
- check toxins in waste
- undertake environmental tests of water
- test soils
Representative sampling
It is important that the sample tested is similar or representationalTo represent reality or an aspect of real life. of the substance as a whole.
This is difficult if the material being tested is an uneven mixture such as soil. For this reason several samples need to be collected from different locations.
Identifying ions in unknown salts in the laboratory
Identifying ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. in unknown salts in the laboratory requires some logical thinking and detective work.
A series of tests, including flame testA qualitative identification technique where samples are heated and the metal ions present are identified by characteristic flame colours. and chemical tests, gradually reveal evidence of the identity of the unknown compoundA substance formed by the chemical union of two or more elements..
The observation from each test results in an inference or deduction about what the evidence means. This will either be that an ion is present in the unknown compound or that it is not present.
It is important that all safety precautions are followed that are appropriate to the substances being used to carry out the tests. It is particularly important to take care when working with an unknown compound. It cannot be assumed that this is harmless.
Identification of the positive ion
For a positive ion, a flame test is used on a solid sample:
- if a flame colour is seen, the inference is that Li+, Na+, K+, Ca2+ or Cu2+ ions are in the salt
Hydroxide precipitation reactionA reaction in which an insoluble solid is formed when certain solutions are mixed. require the solid sample to be dissolved before sodium hydroxide solution is added:
- if a coloured precipitateA suspension of particles in a liquid formed when a dissolved substance reacts to form an insoluble substance, eg in a precipitation reaction. is observed, the inference is that Fe2+, Fe3+ Cu2+ ions are present
A further test is needed to distinguish between Zn2+ ions and Ca2+ ions. When excess sodium hydroxide solution is added:
- the zinc hydroxide precipitate dissolves to form a colourless solution
- the calcium hydroxide precipitate does not dissolve
Identification of the negative ion
The table below shows what inference can be made if a change is observed. If no change is observed, it rules out the ion being tested for.
| Test | Observation | Inference |
| Add dilute hydrochloric acid and test gas with lime water | Bubbles that turn limewater cloudy | CO32- |
| Add a few drops of hydrochloric acid and barium chloride (or barium nitrate) solution | White precipitate | SO42- |
| Add a few drops of nitric acid and silver nitrate solution | White precipitate, Cream precipitate, Yellow precipitate | Cl- Br- I- |
| Test | Add dilute hydrochloric acid and test gas with lime water |
|---|---|
| Observation | Bubbles that turn limewater cloudy |
| Inference | CO32- |
| Test | Add a few drops of hydrochloric acid and barium chloride (or barium nitrate) solution |
|---|---|
| Observation | White precipitate |
| Inference | SO42- |
| Test | Add a few drops of nitric acid and silver nitrate solution |
|---|---|
| Observation | White precipitate, Cream precipitate, Yellow precipitate |
| Inference | Cl- Br- I- |
Question
What is the inference for each observation?
Use the inferences to work out the identify of salts A, B and C.
| Salt | Test | Observation | Inference |
| A | Flame test | Bright yellow flame colour | |
| A | Dilute nitric acid and silver nitrate solution added | White precipitate forms | |
| B | Dilute sodium hydroxide added | Green precipitate forms | |
| B | Dilute hydrochloric acid and barium chloride solution added | White precipitate forms | |
| C | Dilute sodium hydroxide added | Blue precipitate forms | |
| C | Dilute nitric acid added | Brief bubbling, limewater turns milky |
| Salt | A |
|---|---|
| Test | Flame test |
| Observation | Bright yellow flame colour |
| Inference |
| Salt | A |
|---|---|
| Test | Dilute nitric acid and silver nitrate solution added |
| Observation | White precipitate forms |
| Inference |
| Salt | B |
|---|---|
| Test | Dilute sodium hydroxide added |
| Observation | Green precipitate forms |
| Inference |
| Salt | B |
|---|---|
| Test | Dilute hydrochloric acid and barium chloride solution added |
| Observation | White precipitate forms |
| Inference |
| Salt | C |
|---|---|
| Test | Dilute sodium hydroxide added |
| Observation | Blue precipitate forms |
| Inference |
| Salt | C |
|---|---|
| Test | Dilute nitric acid added |
| Observation | Brief bubbling, limewater turns milky |
| Inference |
Salt A: Sodium chloride, NaCl.
Salt B: Iron sulfate, FeSO4.
Salt C: Copper carbonate, CuCO3.
| Salt | Test | Observation | Inference |
| A | Flame test | Bright yellow flame colour | Shows that Na+ ions are present |
| A | Dilute nitric acid and silver nitrate solution added | White precipitate forms | Shows that Cl- ions are present |
| B | Dilute sodium hydroxide added | Green precipitate forms | Shows that Fe2+ ions are present |
| B | Dilute hydrochloric acid and barium chloride solution added | White precipitate forms | Shows that SO42- ions are present |
| C | Dilute sodium hydroxide added | Blue precipitate forms | Shows that Cu2+ ions are present |
| C | Dilute nitric acid added | Brief bubbling, limewater turns milky | Shows that CO32- ions are present |
| Salt | A |
|---|---|
| Test | Flame test |
| Observation | Bright yellow flame colour |
| Inference | Shows that Na+ ions are present |
| Salt | A |
|---|---|
| Test | Dilute nitric acid and silver nitrate solution added |
| Observation | White precipitate forms |
| Inference | Shows that Cl- ions are present |
| Salt | B |
|---|---|
| Test | Dilute sodium hydroxide added |
| Observation | Green precipitate forms |
| Inference | Shows that Fe2+ ions are present |
| Salt | B |
|---|---|
| Test | Dilute hydrochloric acid and barium chloride solution added |
| Observation | White precipitate forms |
| Inference | Shows that SO42- ions are present |
| Salt | C |
|---|---|
| Test | Dilute sodium hydroxide added |
| Observation | Blue precipitate forms |
| Inference | Shows that Cu2+ ions are present |
| Salt | C |
|---|---|
| Test | Dilute nitric acid added |
| Observation | Brief bubbling, limewater turns milky |
| Inference | Shows that CO32- ions are present |
Hazards, risks and precautions
The hazards and the precautions needed to reduce the risk of harm include:
| Hazard | Possible harm | Possible precaution |
| Barium chloride solid | Harmful if inhaled and toxic if swallowed | Only use dilute solutions supplied by your teacher |
| Silver nitrate solution | Causes serious eye irritation Causes skin irritation | Wear eye protection, avoid skin contact by using dropper bottles or by wearing gloves |
| Hazard | Barium chloride solid |
|---|---|
| Possible harm | Harmful if inhaled and toxic if swallowed |
| Possible precaution | Only use dilute solutions supplied by your teacher |
| Hazard | Silver nitrate solution |
|---|---|
| Possible harm | Causes serious eye irritation Causes skin irritation |
| Possible precaution | Wear eye protection, avoid skin contact by using dropper bottles or by wearing gloves |
Fran Scott demonstrates how to test for various ions and interpret the results