Qualitative Analysis

Analytical chemistry deals or works with qualitative and quantitative analysis of substances. In inorganic qualitative analysis, the given compound is analysed or diagnosed for the radicals, i.e., cation (+ve) and the anion (-ve), that it contains. Physical procedures like nothing (taking) the colour, smell, or taste of the substance have a minimal scope because of the corrosive, poisonous nature of the chemical compounds. Therefore, what one has to resort (rectify) to is the chemical analysis of the substance that has to be carried out along with the physical examination of the compound under consideration. Examples are- Titration (volumetric analysis), Gravimetric analysis, Various wet chemistry tests, Combustion analysis, and Inert gas fusion.

Table of content

1. Aim

2. Physical Examination of the Salt

3. Dry Heating Test

4. Charcoal Cavity Test

5. Result

Aim

To deal with the concept of Qualitative analysis using the Qualitative analysis experiment.

Materials Required

1. Test Tubes

2. Test Tube Stands

3. Boiling Tube

4. Test Tube Holder

5. Measuring Cylinder

6. Delivery Tube

7. Filter Paper

8. Salts

9. Water

10. Blowpipe

Theory

1. Physical Examination of the Salt:

The examination of the unknown salt involves studying colour, smell and density. The test is not very reliable but is certainly helpful in identifying some coloured cations. Characteristic smell helps identify ions such as ammonium, acetate and sulphide.

Procedure

Observe the colour of the salt, which may provide useful information about the cations. The table below gives the characteristic colours of the salts of some cations.

Observations

2. Dry Heating Test

This test is performed by heating a small amount of salt in a dry test tube. Quite valuable information can be gathered by carefully performing and noting the observations. On heating, some salts undergo decomposition, thus evolving the gases or may undergo characteristic changes in the colour of the residue.

Procedure

• Take about 0.1 g of the dry salt in a clean and dry test tube.

• Heat the above test tube for about one minute and observe the colour of the residue when it is hot and when it becomes cold. Observing changes indicates the presence of cations, which may not be taken as conclusive evidence.

Observations

3. Charcoal Cavity Test

This test is based on the fact that metallic carbonates, when heated in a charcoal cavity decompose to give corresponding oxides. The oxides appear as coloured incrustation or residue in the cavity. In certain cases, the oxides formed partially undergo reduction to the metallic state producing metallic beads or scales.

Procedure

• Make a small cavity in a charcoal block with the help of a charcoal borer. Do not apply pressure; otherwise, it will crack. 

• Fill the cavity with about 0.2 g of the salt and about 0.5 g of anhydrous sodium carbonate. 

• Moisten the salt in the cavity with one or two drops of water. Otherwise, the salt/mixture will blow away.

• Use a blowpipe to heat the salt in a luminous (reducing) flame and observe the colour of the oxide/ metallic bead formed in the cavity when hot and cold. Obtain oxidising and reducing flame. 

• Always bore a fresh cavity for testing the new salt.

Observations

Result

• The physical examination of the salt (their cations) can be found out with the study of their characteristic colours.

• On heating, some salts undergo decomposition, thus evolving the gases or may undergo characteristic changes in the colour of residue.

• The nature of oxides can also be found using the charcoal cavity test.

Precautions

1. If you have touched any salt, wash your hands at once. It may be corrosive to the skin.

2. Never taste any salt; it may be poisonous. Salts of arsenic and mercury are highly poisonous.

3. Use a perfectly dry test tube for performing the test. While drying a test tube, keep it in a slanting position with its mouth slightly downwards so that the drops of water which condense on the upper cooler parts do not fall back on the hot bottom, as this may break the tube. 

4. For testing a gas, a filter paper strip dipped in the appropriate reagent is brought near the mouth of the test tube, or the reagent is taken in a gas detector, and the gas is passed through it.

5. Do not heat the tube strongly at one point, as it may break.

Lab Manual Questions

1. What is qualitative data analysis?

Ans. Qualitative data analysis involves organising, analysing, and interpreting qualitative data.

2. What is the significance of qualitative analysis in organic compounds?

Ans. Qualitative analysis of the organic compound helps determine the presence or absence of a particular compound. But it does not help in determining the presence of mass or concentration of the compound.

3. What is inorganic qualitative analysis?

Ans. Inorganic qualitative analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds.

4. What is a qualitative analysis of carbohydrates?

Ans. The qualitative analysis of carbohydrates is used to characterise carbohydrates present in an unknown solution based on various chemical assays.

Viva Questions

1. What is qualitative analysis?

Ans. The type of analysis deals with the methods used to determine a compound's constituents.

2. Name any iron salt which is light green.

Ans. Ferrous sulphate.

3. What is the colour of nickel salts?

Ans. Bluish green or green.

4. What is a radical?

Ans. A radical may be defined as an atom or group of atoms that carries a charge and behaves as a single unit in chemical reactions.

5. What are acidic and basic radicals?

Ans. Radicals carrying a positive charge are called basic radicals, and those carrying a negative charge are called acidic radicals.

6. Why do inorganic salts ionise when dissolved in water?

Ans. Due to the high dielectric constant of water, the force of attraction holding the two ions in a salt decreases. Thus, the two ions separate. The ions are further stabilised by solvation.

7. What type of bond is present in an inorganic salt?

Ans. Electrovalent bond.

8. What is the colour of iron salts?

Ans. Ferrous salts are usually light green, while ferric salts are generally brown.

9. Give examples of some coloured basic radicals.

Ans. Cu2+,Fe2+,  Fe3+, Cr3+, Ni2+, Co2+ and Mn2+.

Practical Questions

1. Identify the test which is not a part of the qualitative analysis.

1. Iodine test

2. Flame test

3. Litmus test

4. Kastle-Meyer test

Ans. The litmus test is not a part of qualitative analysis.

1. Which of the following is not a physical method?

1. Atomic emission spectroscopy

2. Inert gas fusion

3. Trace element analysis

4. none

Ans. Inert gas fusion is not a physical method.

2. Qualitative analysis is  used for separating the specific constituents from a mixture.

1. True

2. False

3. Not known

4. None

Ans. False the given statement is not true.

3. Identify the one which does not come under the chemical methods of qualitative analysis?

1. Volumetric

2. Gravimetric

3. Titrimetric

4. Magnetic Susceptibility

Ans. Magnetic Susceptibility does not come under the chemical methods of qualitative analysis.

4. Covalent molecules can be identified using quantitative methods.

1. True

2. False

3. Can’t predicted

4. None

Ans. False covalent molecules can’t be identified using quantitative methods.

5. Identify the element that Lassaigne’s test cannot detect.

1. Sulfur

2. Fluorine

3. Nitrogen

4. Phosphorus

Ans. Lassaigne’s test cannot detect fluorine.

6. What is the colour of nickel salts?

1. White

2. Bluish green

3. Brown

4. Silver

Ans. Nickel salts are Bluish green in colour.

7. Which compound gets precipitated in the detection of carbon and hydrogen?

1. Calcium carbonate

2. Copper

3. Copper Sulphate

4. Carbon dioxide

Ans. Calcium carbonate gets precipitated in the detection of carbon and hydrogen.

8. What is the precipitate's colour obtained in the Sulphur test?

1. Blue

2. White

3. Green

4. Black

Ans. Black colour will be obtained.

Conclusion

From the above experiment, we can conclude that the physical examination of the salt (their cations) can be found by studying their characteristic colours. Also, on heating, some salts undergo decomposition, thus evolving the gases or may undergo characteristic changes in the colour of the residue. The nature of oxides can also be found using the charcoal cavity test.