Topic 5- Qualitative Analysis – Chemistry Form Four
The Concept of Qualitative Analysis
The Meaning of Qualitative Analysis
- acidic or non-metallic radicals or negative ions;
- basic or metallic radicals, including ammonium ion.
The Importance of Qualitative Analysis in Real Life
Detection of poisonous substances in the environment
This is achieved through dissolution of a substance in distilled water. Then the characteristics of the solution formed serve as a clue to establish the type of elements present in a tested sample. The mixture formed following dissolution of a solid sample may be a clear solution, an emulsion or a precipitate. The solution or emulsion is further analysed to detect the ions present in it. The precipitate is then separated from the filtrate and both are subjected to further tests to identify the kind of elements present.
Many chemical reactions are accompanied with evolution of gases, as one of the products of the reaction. In some cases, the smell of the gas may not suffice to detect the gas, especially if the gas is colourless and odourless. In such cases, the gas is subjected to various qualitative analysis tests in order to establish its identify.
Finding the nature and identity of chemical substances
Soils of given pH give specific colours when their solutions are added to certain types of indicators. This procedure is purely qualitative because it involves observation of the change in colour of indicators to determine the pH of the soil. For further details on the measurement of soil pH, read a topic on Soil Chemistry (Chapter Three) in this book.
Qualitative tests are performed to determine the type of minerals contained in a particular soil. Such tests include test for nitrate, sulphate, chloride and phosphate ions. Determination of soil composition gives soil scientists information necessary for conservation.
Qualitative analysis techniques are applied in medical field, for example in carrying out various tests such as testing blood and urine samples, determining the level of blood sugar, pregnancy diagnose and blood grouping. Most of these analytical tests are done to diagnose a wide range of diseases and medical conditions.
- in detecting the causative agents for typhoid (salmonella typhi), the blood is left to clot, or it is centrifuged in order to separate blood corpuscles from plasma. The plasma is then subjected to various qualitative tests to detect the presence of salmonella typhi.
- in pregnancy diagnosis, a certain chemical is added to urine, where a specific change in colour of the urine confirms whether one is pregnant or not.
Many industries use qualitative analysis in their efforts to perform better and make quality products.In chemical industries, the products often get contaminated by unknown contaminants. Qualitative tests are done to detect the contaminants. This may be followed by quantitative analysis aiming at finding out the quantity (amount) of the contaminant present.
- All the apparatus should be cleaned and dried thoroughly and must remain clean throughout the experiment.
- Do not lay a glass rod on a dirt laboratory bench as it can get contaminated easily.
- Avoid touching the side of a test-tube with the tip of a dropper. The contaminant can be picked up and transferred to another solution, a fact that would contaminate the solution, thus producing false results.
- Only distilled water from the wash bottle should be used to dissolve the solids. Spring, rain or tap water contains chemicals that can lead to wrong results and conclusions.
- Colour and smell
- Flame test
- Solubility in water
- Dry heating
- Action on litmus (for gases evolved)
- Dilute acid test (dilute H2SO4)
- Concentrated acid test (concentrated H2SO4)
- Wet test for acid radicals
Carry out preliminary test on an unknown sample
Preliminary tests include noting the appearance (colour, texture, feel, etc) of the sample, detecting the smell of the gas liberated, observing the action of a substance on litmus, and even the taste of the substance under investigation. The preliminary tests give a clue about the type of the elements present in a sample.)

Table 5.2 Identification of gases
Gas | Colour | Smell | Action on litmus | Test |
CH3COOH | Colourless | Vinegar-like | Acidic | Liberated as dense white fumes |
N2 | Colourless | Odourless | Neutral | No chemical test |
Water vapour | Colourless | Odourless | Neutral | Turns white CuSO4 blue |
NO2 | Reddish-brown | Pungent | Acidic | Not as red as Br2 vapour and does not condense on the sides of the test tube |
NH3 | Colourless | Pungent | Alkaline | Forms thick white fumes when in contact with HCl gas |
HCl | White fumes | Irritating | Acidic | Forms thick white fumes when in contact with NH3 gas |
HBr & Br2 | White fumes & reddish- brown gas | Choking | Acidic & bleaches | HBr resembles HCl, & Br2, condenses to a red liquid on the sides of the test tube |
Cl2 | Pale green | Bleaches | Choking | Gives white fumes with NH4OH |
I2 | Violet | Choking | Bleaches | Turns starch iodide paper blue-black |
CO2 | Colourless | Odourless | Slightly acidic | Turns lime water milky |
CO | Colourless | Odourless | Neutral | Burns with pale blue flame |
H2 | Colourless | Odourless | Neutral | Burns with a „pop‟ sound |
H2S | Colourless | Rotten eggs | Acidic | Burns with blue flame to SO2, blackens lead acetate paper. |
O2 | Colourless | Odourless | Neutral | Re-ignites a glowing splint |
SO2 | Colourless | Irritating smell of burning sulphur | Acidic | Decolourizes KMnO4 solution, turns K2Cr2O7 from orange to green |
SO3 | Colourless | Pungent | Acidic | Fumes in moist air forming dense white fumes |
Colours of some ions in solution
- If a compound and its solution in water are colourless, it is probable that a transition metal is absent.
- If its colour is black, it is probably an oxide or a sulphide.
- If the solid and its solution in water are coloured, probably a transition metal is present.
The following are the colours of some transition element ions


Prepare stock solutions from soluble and insoluble salts
- All common nitrates of metals are soluble.
- All common sodium, potassium and ammonium salts are soluble.
- All common chlorides are soluble except those of silver, mercury (I) and lead (II).
- All common sulphates are soluble except those of lead and barium. The sulphates of calcium and silver are sparingly soluble.
- All carbonates, sulphites and phosphates of sodium, potassium, and ammonium are soluble but other common carbonates are insoluble.
- Sodium, potassium and ammonium hydroxides [(ammonia solution, NH3(aq)]* are soluble but other common hydroxides are insoluble.
- All sulphides are insoluble except those of the alkali metals, the alkaline earth metals and ammonium.
Different substances decompose on heating to give different products. Many compounds decompose on heating and leave solid residues. In performing this test, the compound is heated in an ignition tube or dry test tube. Heating is continued until no further change occurs. The gas evolved, residue left or sublimate formed on heating may help to identify the acid radical present in a compound.
Table 5.5 Action of heat on the test substance


Treat the solid with cold dilute sulphuric or hydrochloric acid. Identify the gas evolved. If there is no reaction with the cold acid, heat the mixture gently. Heat carefully and ensure the mixture does not boil. The gas evolved can be identified as follows:
Table 5.6 Detection of acidic radicals

As in the above test, the acid is added to the solid substance and if no reaction occurs, the mixture is warmed gently, but the mixture should not be boiled. Then, the gas given off is identified. In addition, observe any product, other than the gas, which results from the reaction.
Table 5.7 Action of conc. H2SO4


- Oxalates give CO2 with concentrated H2SO4 but not with. HCl.
- Nitrates do not give NO2 when treated with HCl but nitrites give NO2 gas.
- If no result is obtained in the above tests, the salt is probably a sulphate, chromate or phosphate.
Cations and Anions Identifies
Then, the resulting solutions are tested for radicals. Table 5.8 shows a list of test/experiments, each of which confirms the presence of a given ion. Depending on the availability of reagents, students can do any of the listed tests to confirm the ions present in test solutions.
Table 5.8 confirmatory tests for anions





TESTS FOR METALLIC RADICALS
Some metal ions can be identified by the colour of their flame during a flame test. Flame test involves mixing solutions of ions with conc. HCl and then heating them on a platinum or nichrome wire over a non luminous flame. Alternatively, a dry solid can be used instead of the solution.To perform this experiment, dip a platinum or nichrome wire into concentrated hydrochloric acid and hold it just above the blue part of the flame. Repeat the process until the wire is clean. After that dip the clean wire into the acid and the dip it into the solution of the test substance (or its solid particles). Heat the substance strongly and observe the colour of the flame formed.

Action of hydrochloric acid

Action of sulphuric acid

Action of sodium hydroxide
Table 5.9 Reaction of cations with dilute NaOH


Reaction equations:



Action of aqueous ammonia
Table 5.10 Reaction of cations with aqueous ammonia



Confirmatory tests for cations
Table 5.11 Confirmatory tests for cations





Confirmatory tests for iron salts
- Potassium hexacyanoferrate (II) solution
- Potassium hexacyanoferrate (III) solution
- Potassium thiocyanate solution
Table 5.12 Tests for iron salts

Table 5.13 Tests for bromides and iodides


Write-up of analytical data
Table 5.14 Table of results
