Qualitative Analysis Notes Form Four Chemistry
Is the analysis concerned with the investigation and identification of a chemical species which make substances.
Qualitative analysis involves a set of analytical procedures that provide non numerical data about an analyte.
Qualitative analysis can indicate whether an atom, ion, functional group or compound is present or absent in a sample.
Categories of qualitative analysis.
Qualitative analysis is classified into two categories which are;
1. Qualitative organic analysis
Is the qualitative analysis which deals with identification of functional groups, chemical bonds or molecules in the samples.
2. Qualitative inorganic analysis
This is the analysis which deals with the determination of ionic or elemental composition of samples.
IMPORTANCE OF QUALITATIVE ANALYSIS
Qualitative analysis has several applications in real life as described in the following areas
1. Qualitative analysis helps in determination of ions present in a chemical substance.
The ions present in a chemical substance are determined through carrying out analytical tests involving identifying the physical and chemical properties of the unknown substance.
2. Qualitative analysis helps in determination of gases produced in a chemical reaction.
Qualitative analysis involves procedures which are useful in identifying the gases involved in a chemical reaction.
Example; the reddish brown fumes which indicates the presence of nitrogen dioxide gas in the sample.
3. Qualitative analysis helps in identification of chemical compounds.
During qualitative analysis experiment, chemical compounds display different properties including solubility, characteristic smell, flame colour, and reaction products which helps to determine the chemical substance under investigation.
4. Qualitative analysis helps in determination of the pH and types of nutrients in a soil sample.
The determination of soil pH involves observation of the colour changes of the indicator when subjected into a solution of a soil sample through qualitative analysis procedures.
The tests for cations and anions helps to identify the ions such as (K+, Ca2+, SO42-, NO3- and PO43-) which aid in deciding the type of fertilizer to be applied to a particular soil.
5. Qualitative analysis helps in identification of environmental pollutants.
Environmental scientists utilize qualitative analytical tests to determine the chemical composition of a specific contaminant.
Example the determination of pollutants in water.
6. Qualitative analysis is applied in medical and criminological analysis.
Qualitative analysis tests are used to identify a wide range of diseases, medical conditions and criminal evidences such as determination of sugar content in blood, pregnancy and blood grouping.
Examples
When a reagent is dipped in urine, it reacts with the substances present, causing a precise colour or physical change that indicate a particular medical conditions such as pregnancy.
When antibodies are introduced in blood corpuscles, they clump and aid in the detection of the blood group. Forensic/criminal scientists use qualitative analysis to identify the nature of the compounds at crime area for declaring a suspected criminal.
7. Qualitative analysis is applied in chemical industries.
Products in chemical industries are normally contaminated with chemical pollutants such as toxic metals which can be detected through qualitative analysis.
QUALITATIVE ANALYSIS TECHNIQUES
Qualitative analysis needs to be carried out carefully because even small errors may cause misinterpretation of the contents of the tested sample.
The following are some important aspects that should be observed when performing qualitative analysis experiments:
1. Adding the test reagent carefully.
The reagents should be added carefully because some of them may cause vigorous reactions. Initially the reagent should be added in small amount, and then added in excess.
2. Avoiding contamination.
Contamination of apparatus, samples and chemicals, lead to inaccurate results. For example, the presence of a contaminant that contains a cation with similar characteristics with the test analyte, may lead to false positive results.
The following are some precautions that should be taken to avoid contamination of chemicals, apparatus and samples:
i. All glassware should be thoroughly cleaned, rinsed with distilled water and dried before use.
ii. Do not lay a glass rod on a laboratory bench as this can lead to contamination; instead, put it in a clear beaker or any other clean container.
iii. Do not touch the sides of the test tube with the tip of a dropper as this can cause contaminations to be picked up and transferred to another solution.
iv. Distilled water should be the one used for qualitative analysis procedure.
v. Use tap water for normal cleaning and not for rinsing apparatus or dissolving chemicals
vi. Use separate glassware or apparatus for preparation of standard solutions and samples.
3. Heating a solution below its boiling point.
Direct heating of solution should be avoided. Use a water bath when heating the solution
4. Evaporating a solution.
Evaporation should always be done in an evaporating dish. The dish can be held using a pair of tongs and waved through a flame or placed on a wire gauze and heated using a burner over a tripod stand. Alternatively, the sample can be placed in an evaporating dish or a filter paper and exposed to sunlight to allow evaporation.
5. Testing the pH of a solution.
When testing the pH of a solution using a clean glass rod, a drop of the test solution is placed on the litmus paper. The colour change of the litmus paper is noted while it is still wet.
Qualitative Analysis Notes Form Four Chemistry
FORMAT OF THE QUALITATIVE ANALYSIS REPORT
The report of qualitative analysis experiments is usually organized in tabular form with three columns showing the brief explanation of the experimental procedure or test performed, observations and inferences.
1. Experiment column
Tests carried out on an unknown sample should be written in the “Experiments” column and reported in simple past tense.
Example, “A small amount of a sample was picked using a clean nichrome wire and heated on a flame”
2. Observation column
The appearance of the sample and changes which have been observed during experiments are written in the “Observations” column.
The observations include colour, texture, formation of precipitates, evolution of gas, flame colour and production of special sound during experiment.
3. Inference column.
What can be inferred from the observations are written in the “Inferences” column which leads to the systematic identification of the unknown salt after investigation.
Report format for qualitative analysis experiments
| Experiments | Observations | Inferences |
QUALITATIVE ANALYSIS PROCEDURE FOR SOME SELECTED IONS
The qualitative analysis procedure involves three sets of experiments as follows;
- Preliminary tests
- Tests in aqueous solutions
- Confirmatory tests
1. Preliminary tests
The preliminary tests are generally for solid samples. This involves the following:
- Appearance of the sample: colour, texture, deliquescence and colour.
- Action of heat.
- Flame test.
- Solubility in water.
- Action of dilute hydrochloric acid or dilute sulphuric acid.
- Action of concentrated sulphuric acid.
a. Appearance of a compound.
The appearance of a compound can help in its identification. This includes the colour, texture and deliquescence of the compound in solid form.
Colours of some common anhydrous salts.
| TYPES OF SUBSTANCES | NAMES OF SALTS AND THEIR FORMULAE | COLOUR |
| Carbonates | Silver carbonate , Ag2CO3 | Yellow |
| Copper(II) carbonate, CuCO3 | Green | |
| Barium carbonate, BuCO3 | White | |
| Calcium carbonate, CaCO3 | White | |
| Chlorides | Silver chloride, AgCl | White |
| Lead(II) chloride PbCl2 | White | |
| Copper(II) chloride CuCl2 | Yellowish brown | |
| Barium chloride BaCl2 | White | |
| Hydroxides | Silver hydroxide, AgOH | Greyish-brown |
| Copper(II) hydroxide Cu(OH)2 | Pale blue | |
| Iron(II) hydroxide Fe(OH)2 | Dirty green | |
| Iron(III) hydroxide Fe(OH)3 | Reddish-brown | |
| Lead(II) hydroxide Pb(OH)2 | White | |
| Zinc hydroxide Zn(OH)2 | White | |
| Sulphates | Barium sulphate BaSO4 | White |
| Zinc sulphate ZnSO4 | White | |
| Copper(II) sulphate CuSO4 | White | |
| Lead(II) sulphate PbSO4 | Bright white | |
| Iron(II) sulphate FeSO4 | Green |
Colour and texture are not accurate determinants of the cations and anions present in salts but give prior information to describe a chemical substance.
b. Flame test
A flame test involves dipping a clean nichrome wire ( or glass rod or a base of a test tube) in concentrated hydrochloric acid then in the unknown substance, followed by heating the mixture over a non-luminous flame. The flame changes to a characteristic colour of the element.
The flame colours of some metal ions.
| FLAME COLOURS | METAL IONS |
| Golden yellow colour flame | Na+ may be present. |
| Brick red colour flame | Ca2+ may be present. |
| Bluish-green colour flame | Cu2+ may be present. |
| Blue-white (pale-blue) colour flame | Pb2+ may be present. |
| Yellow (orange) sparks | Fe2+, Fe3+ may be present. |
| No definite colour flame | Zn2+, NH4+ may be present. |
c. Effects of heat on solid samples.
Different substances decompose on heating to give different products.
When the hydrated compound is heated, the colourless liquid (water) condenses on the inner part of the test tube. A test for the presence of water can be performed by using cobalt chloride paper on anhydrous copper (II) sulphate.
When heating if the gas is given off, note its colour and odour. Also after heating look for the colour of the residue formed when hot and when cold.
- If the brown gas is given off, the compound is most likely to be a nitrate and the brown gas is most likely to be nitrogen dioxide gas (NO2)
- Sulphites decomposes to give a colourless gas (sulphur dioxide) which has a chocking smell.
- Some carbonates decompose when heated to give a colourless and odourless gas (carbon dioxide) that forms white precipitate with lime water.
NOTE: the characteristics of the gases evolved infer anion(s) present in the sample(s), whereas the colours of residues infer the cation(s) present in the compound.
When white sublimate with the colourless gas which turns a wet red litmus paper blue, the compound is likely to be ammonium and the gas is ammonia gas.
CAUTION: When heating, hold the test tube in a slanting position and away from the observers and neighbours.
Example of cation residues after heating with their colours when hot and when cold
| CATION | COLOUR WHEN HOT | COLOUR WHEN COLD |
| Zn2+ | Yellow | White |
| Pb2+ | Reddish-brown | Yellow |
| Cu2+ | Black | Black |
| Ca2+, Na+ | White | White |
| Fe2+, Fe3+ | Reddish-brown | Reddish-brown |
Action of dilute hydrochloric acid or sulphuric acid on the solid samples.
Dilute hydrochloric acid or sulphuric acid is added to the test sample until its solution is acidic. A cation that generates an insoluble chloride will form a precipitate.
Action of concentrated sulphuric acid on solid sample.
Concentrated sulphuric acid is added dropwise to the solid sample until its solution become acidic.
A cation that generates an insoluble sulphate will form a precipitate.
CAUTION: concentrated sulphuric acid is corrosive, hence handle it with caution, and do not boil it. Keep the test tube slanted and away from observers and neighbours.
2. Solubility of solid samples.
To test for the solubility of a substance, one spatulafull of the test substance is put in 1cm3 of distilled water and stirred.
If the substance is soluble, this amount will dissolve, if it is sparingly soluble, some of the amount will dissolve, but if it is insoluble, none of the amount will dissolve.
If the substance does not dissolve or dissolve slowly, warm it gently to speed up its dissolution.
The following are some general trends on the solubility of different compounds in water:
- All nitrates are soluble in water.
- All salts of sodium, potassium and ammonium are soluble in water
- All chlorides, bromides and iodides are soluble in water, except those of silver, mercury(I) and lead(II).
- All sulphates are soluble in water, except those of barium and lead(II). The sulphates of calcium and silver are sparingly soluble in water.
- All carbonates, sulphites and phosphates are insoluble in water, except those of sodium, lithium, potassium and ammonium.
- All sulphides are insoluble in water, except those of alkali metals, the alkaline earth metals and ammonium.
- All hydroxides are insoluble in water, except those of the alkali metals. The hydroxides of calcium, magnesium and barium are sparingly soluble in water.
- Ammonium hydroxide exists as aqueous ammonia.
2. Tests in solution
Preparation of the stock solution of the sample.
Transfer about 1g of the solid sample into a test tube. Add about 15cm3 to 20cm3 of distilled water and shake thoroughly. If the sample is insoluble in cold water, warm it. If the sample is insoluble in hot water, transfer 1g of the original solid sample in a test tube and then dissolve it in dilute nitric acid or dilute hydrochloric acid to make about 15cm3 or 20cm3 of the final solution.
Tests on a sample solution include the following:
- Action of sodium hydroxide (NaOH) solution.
- Action of aqueous ammonia (ammonia NH3 ) solution.
Action of sodium hydroxide (NaOH) solution on sample solutions.
Dilute NaOH solution is added dropwise to the test solution. The mixture is stirred or shaken well. If a precipitate forms, more sodium hydroxide solution is added. If no precipitates formed, the mixture is warmed.
Action of aqueous ammonia (ammonia NH3 ) solution.
Add dilute aqueous ammonia dropwise to the test solution. Stir or shake the mixture and observe any changes. If a precipitate forms, continue adding the aqueous ammonia solution.
3. Confirmatory tests.
After performing preliminary tests in solution on the sample of unknown compound, it is important to make conclusions about the ions present in the sample by performing confirmatory tests for every deduced ion.
Confirmatory tests can be obtained by combining the inferences made in the successive tests.
Confirmatory tests are divided into the confirmatory tests for cations which are performed to confirm the presence of selected cations and the confirmatory test for anions which are performed to confirm the presence of selected anions.
Qualitative Analysis GUIDES Chemistry