Class 12 Chemistry: Step-by-Step Salt Analysis for CO32- and Zn2+ Ions

Inorganic salts are formed during the process of complete or partial neutralisation of an acid and base. During this process, the acid donates its hydrogen ions and releases an anion and the base donates its hydroxyl ions and releases a cation. These anions and cations merge to form a neutral salt, whereas the H+ and OH- ions merge and form neutral water. Inorganic salts are analysed for the presence of cations and anions, and this analysis is known as qualitative analysis. Initially, a physical examination of the salts is done for their colour and odour, following which, respective chemical tests for specific anions and cations are conducted.

Table of Contents

• Aim

• Apparatus Required

• Theory

• Procedure: Charcoal Cavity Test for Zinc

• Procedure: Confirmatory Test for Carbonate

• Procedure: Confirmatory Test for Zinc

• Observation

• Result

Aim

To analyse the given salt for acidic and basic radicals CO32- and Zn2+

Apparatus required

1. Test tubes

2. Test tube holder

3. Dropper

4. Burner

5. Dil sulphuric acid

6. Na2CO3 

7. Charcoal 

8. Dil Hydrochloric acid

9. Sodium hydroxide

10. Potassium ferrocyanide solution

11. Hydrogen sulphide gas

12. Ammonium hydroxide

Theory

1. The analysis of various ions present in salt is based on the principle of solubility product and common ion effect. As the ionic product of salt increases beyond its solubility product, precipitation of that ion takes place. The common ion effect controls the ionic product of various salts.

2. Anions such as carbonates, sulphides, sulphites, nitrites, and acetate when mixed with dil sulphuric acid release various gases. The gases thus evolved to give information regarding the anions which might be present in the salt.

3. Metallic carbonates when heated in a charcoal cavity decompose and give the respective oxide, which appears as a coloured residue inside the cavity. At times, oxides are reduced to metal due to the presence of carbon in the charcoal cavity.

Procedure

1. Physical examinations for the colour, odour etc. of the salt are conducted.

2. Preparation of Original Solution (O.S.): Take some amount of salt and dissolve it in distilled water- O.S. is prepared.

3. Dry Test for Anions:

1. Take a pinch of salt in a test tube and add 1-2 ml of dil. sulphuric acid to it.

2. Observe changes at room temperature, if no gas evolved, heat the above contents and observe.

4. Confirmatory Test for Anions:

1. Take a pinch of salt in a test tube and dissolve the salt in water. Record the observations.

2. Add dil HCl to the salt and observe the changes.

5. Dry Test for Cations-Charcoal Cavity Test:

1. Take a charcoal block and make a small cavity in it gently.

2. Take a small quantity of salt and mix it with a double quantity of anhydrous sodium carbonate.

3. Moisten the salt in with 1-2 drops of water.

4. Heat the above contents in a charcoal cavity using a blowpipe over a reducing flame and observe the colour of metallic residue formed in the cavity, when hot and cold.

6. Confirmatory Test for Cations:

1. Take O.S in a test tube and pass H2S gas through it and observe.

2. Dissolve the precipitate obtained from the above step by boiling it in dil HCl. 

3. Divide the solution obtained into two parts- 

• In part A add NaOH to the solution and observe the changes.

• Neutralise part B of the solution with NH4OH and add Potassium ferrocyanide solution and observe.

Observations:

Result

1. While performing the above tests, one acidic radical Zn2+ and one acidic radical CO32- were identified.

2. From the above ions obtained, we can conclude that the given salt was Zinc carbonate ZnCO3.

Precautions

1. Be careful while using strong acids such as HCl and H2SO4.

2. Keep nose and mouth away from the fumes released during the tests.

3. Always bore a new cavity while performing charcoal cavity tests gently, do not apply too much pressure or else the charcoal slab might break.

4. Use safety gear such as lab coats and gloves.

Lab Manual Questions

1. Give the steps involved in the salt analysis.

Ans: Qualitative analysis of any salt is performed in the following steps-

• A preliminary examination of the salt is performed.

• A dry test for anions and cations is performed.

• A wet test and confirmatory test are performed for acidic and basic radicals obtained from the above steps.

2. Describe the cobalt nitrate test.

Ans: The cobalt nitrate test is used to test the presence of Zinc, Magnesium, and Aluminium cations in the given salt. In this test, a cobalt nitrate solution is added to the given salt and the colour of the residue is observed. The residue is green, pink, and blue indicating the presence of Zinc, Magnesium, and Aluminium ions respectively.

3. How is a borax bead made?

Ans: Borax bead test is a preliminary test which is used to identify the possible cations present in the salt. For this purpose, a borax bead is made-

• Make a small loop at the end of the platinum wire and make it red-hot.

• Dip this platinum wire in borax powder and again heat till a colourless transparent borax bead is not formed.

• This borax bead is then dipped in salt solution and heated to determine the cations present in the given salt

4. Enlist the 4 different types of salts.

Ans: The 4 types of the salt present are:

• Acidic salt-Salt from a strong acid and weak base.

• Basic salt-Salt from a strong base and weak acid.

• Neutral salt-Salt from a strong acid and strong base.

• Mixed salt-Salt containing more than one cation or anion, except the H+ and OH- ions.

Viva Questions

1. What is a macro analysis of salt?

Ans: Macro analysis is when around 0.1 to 0.5g of salt is taken and around 20 ml of solution is used for the qualitative analysis of a given salt.

2. State the difference between organic and inorganic salts.

Ans: Salts which do not contain C-H bonds are known as inorganic salts, and salts having C-H bonds are known as organic salts.

3. Give reasons why-Sulphuric acid produces two types of salts.

Ans: H2SO4 is a diprotic acid, i.e. it contains two ionizable hydrogens. Therefore, sulphuric acid produces two types of salt-Hydrogen sulphate HSO4- and sulphates SO4 2-. 

4. Why does table salt sometimes appear yellow?

Ans: The yellow colour of table salt is due to the presence of metal excess defect because of which unpaired electrons occupy the anionic sites. Such electrons absorb energy from the visible spectrum, and hence the colour of the salt appears yellow.

5. What is a strong base?

Ans: A strong base is a chemical that in an aqueous solution dissociates completely into its constituents ions. For example KOH, in an aqueous solution, KOH dissociates into K+ and OH- ions.

6. Enlist the members present in group 5 of basic radicals.

Ans: Group 5 of basic radicals contain Barium, Calcium, and Strontium ions.

7. Why is Na+ in NaOH known as a basic radical?

Ans: Basic radicals are formed after the removal of OH- ions during the neutralisation reaction. Hence, during neutralisation when NaOH dissociates, it forms Na+ and OH- ions. Na+ ion comes from a base NaOH and hence is known as a basic radical.

8. How to clean a platinum loop?

Ans: To clean the platinum loop: 

• Dip the loop in conc. HCl.

• Hold the loop in a non-luminous flame until the wire imparts no colour to the flame. 

9. Enlist a few types of salts.

Ans: Table salt, Sea salt, Flake salt, Himalayan pink salt, Kosher salt, Rock salt, Celtic sea salt etc. are a few types of salt.

10. What is a radical?

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

Practical Based Questions (MCQs) 

1. Na2CO3 is known as___.

1. Sodium bicarbonate

2. Baking soda

3.  Sodium tricarbonate

4. Washing soda

Ans: Washing soda

2. Which bond is present between ions of a salt?

1. Covalent bond

2. Ionic bond

3. Hydrogen bond

4. Van Der waals interactions 

Ans: Ionic bond

3. Inorganic salts upon dissociation give____and____.

1. Salt, water

2. Only acidic radicals

3. Cation, anion

4. Only basic radicals

Ans: Cation, anion

4. Find the odd one out.

1. Sodium

2. Potassium

3. Zinc

4. Nitrite

Ans: Nitrite

5. H2SO4+ NaCl gives _____and ____

1. NaHSO4, H2O

2. NaHSO4, HCl (gas)

3. NaSO4, HCl (gas)

4. H2O, NaOH

Ans: NaHSO4, HCl (gas)

6. Which one of the following is false?

1. Copper sulphate is a blue colour salt.

2. Acidic radicals are formed by the removal of hydrogen ions

3. Nickel sulphate is a white colour salt

4. The borax bead test is a preliminary test for cations

Ans: Nickel sulphate is a white colour salt

7. Basic radicals are also known as____

1. Positively charged radicals

2. Negatively charged radicals

3. Anions

4. Acceptors of hydroxyl ions

Ans: Positively charged radicals

8. ____, ____and____are members of the second group of anions.

1. Cl-, Br-, I-

2. Cl-, S2-, NO2-

3. Br-, I-, CO32-

4. All of the above

Ans: Cl-, Br-, I-

9. Neutralisation reaction is

1. CoO + ZnO→CoO.ZnO

2. PbCl2 + 2KI→ PbI2 + 2KCl

3. Fe3+ + SCN-→[Fe(SCN)]2+

4. 2HCl + Mg(OH)2→ 2H2O + MgCl2

Ans: 2HCl + Mg(OH)2→ 2H2O + MgCl2

10. Acid radical is formed due to the removal of

1. An acid

2. A Base

3. Hydrogen ion

4. Hydroxyl ion

Ans: Hydrogen ion

Conclusion

From the above experiment, we conclude that the above-given salt sample contains the acidic radical CO32- and the basic radical Zn2+. Acidic radicals are anions and are negatively charged because they are formed due to the loss of hydrogen ions. Basic radicals are cations with a positive charge because they are formed due to the loss of hydroxyl ions. The neutralisation reaction between an acid and a base leads to the formation of salt and water.