This chapter covers methods to separate mixtures and identify their components.
1. Purpose of Separation:
To remove impurities or to isolate pure, useful substances from a mixture.
2. Key Physical Separation Techniques:
Chromatography: Separates dissolved solutes (like ink dyes) based on their differing solubility and adsorption.
Distillation: Separates a volatile liquid from a non-volatile solute. Fractional distillation is for miscible liquids with different boiling points (e.g., alcohol from water).
Separating Funnel: Used to separate two immiscible liquids (like oil and water) by draining the denser bottom layer.
Sublimation: Isolates a volatile solid that turns directly into vapour (e.g., ammonium chloride) from a non-sublimable solid.
Differential Extraction: Isolates an organic compound from water using a solvent in which the compound is more soluble.
3. Qualitative Analysis (Chemical Tests):
This process identifies anions (negative ions) and cations (positive ions) in a salt by observing gas evolution, precipitate formation, or colour changes.
Preliminary Tests: Include checking appearance, flame tests, and reaction with dilute acid.
Anion Tests: Specific chemical tests to identify ions like carbonate, chloride, sulphate, and nitrate.
Cation Tests: Specific chemical tests to identify ions like lead, copper, iron, zinc, and ammonium.
In essence, this chapter provides the foundation for separating mixtures and identifying unknown substances through physical and chemical methods.
EXERCISE- 1 (A)
1) Write the probable colour of the following salts.
(a) Ferrous salts
(b) Ammonium salts
(c) Cupric salts
(d) Calcium salts
(e) Aluminium Salts
Ans: (a) Ferrous salts : Light green
(b) Ammonium salts : Colourless
(c) Cupric salts : Blue
(d) Calcium salts : Colourless
(e) Aluminium salts : Colourless
2) Name:
(a) a metallic hydroxide soluble in excess of NH4OH.
(b) a metallic oxide soluble in excess of caustic soda solution.
(c) a strong alkali
(d) a weak alkali
(e) two coloured metal ions
(f) two coloured metal ions
(g) a metal that evolves a gas which burns with a pop sound when boiled with alkali solutions.
(h) two bases which are not alkalis but dissolve in alkalis to yield colourless solutions.
(j) a coloured cation not a representative element.
Ans: (a) Cu(OH)2
(b) ZnO
(c) NaOH
(d) NH4OH
(e) Na+, Ca2+
f) Fe2+, Mn2+
(g) Aluminium
(h) Zn(OH)2 and Al(OH)3
(i) PbO
(j) Ammonium ion
3) Write balanced equations for Q.2 (g) and (i)
Ans:
2Al + 2NaOH + 2H2O ⟶ 2NaAlO2 + 3H
(Hot and conc.) Sodium meta aluminate
(colourless)
PbO + 2NaOH ⟶ Na2PbO2 + H2O
(Yellow) sodium plumbate
(colourless, soluble)
4) What happens when ammonia solution is added first dropwise and then in excess to the following solution:
(i) CuSO4 (ii) ZnSO4 (iii) FeCI3
Write balanced equations for these reactions
Ans: (i) With Copper Sulfate (CuSO₄)
First dropwise: A pale blue precipitate of copper(II) hydroxide is formed.
In excess: The precipitate does not dissolve in excess ammonia. Instead, it dissolves to form a deep ink-blue solution due to the formation of a soluble complex ion.
Balanced Equations:
CuSO₄ + 2NH₄OH → Cu(OH)₂↓ + (NH₄)₂SO₄
(Pale blue precipitate)
Cu(OH)₂ + 4NH₃ → [Cu(NH₃)₄]²⁺ + 2OH⁻ + SO₄²⁻
(Deep blue complex, tetramminecopper(II) ion)
(ii) With Zinc Sulfate (ZnSO₄)
First dropwise: A white gelatinous precipitate of zinc hydroxide is formed.
In excess: The precipitate dissolves to form a colourless solution due to the formation of a soluble complex ion.
Balanced Equations:
ZnSO₄ + 2NH₄OH → Zn(OH)₂↓ + (NH₄)₂SO₄
(White precipitate)
Zn(OH)₂ + 4NH₃ → [Zn(NH₃)₄]²⁺ + 2OH⁻ + SO₄²⁻
(Colourless complex, tetramminezinc(II) ion)
(iii) With Iron(III) Chloride (FeCl₃)
First dropwise: A reddish-brown precipitate of iron(III) hydroxide is formed.
In excess: The precipitate does not dissolve in excess ammonia solution. It remains as an insoluble hydroxide.
Balanced Equation:
FeCl₃ + 3NH₄OH → Fe(OH)₃↓ + 3NH₄Cl
(Reddish-brown precipitate)
5) What do you observe when caustic soda solution is added to the following solution, first a little and then in excess:
(a) FeCI3
(b) ZnSO4
(c) Pb(NO3)2
(d) CuSO4 Write balanced equations for these reactions
Ans: (a) FeCl₃ (Ferric Chloride)
A reddish-brown precipitate of ferric hydroxide (Fe(OH)₃) forms. It is insoluble in excess sodium hydroxide.
(b) ZnSO₄ (Zinc Sulphate)
A white precipitate of zinc hydroxide (Zn(OH)₂) forms. It dissolves in excess NaOH, forming a colorless solution of sodium zincate (Na₂ZnO₂).
(c) Pb(NO₃)₂ (Lead Nitrate)
A white precipitate of lead hydroxide (Pb(OH)₂) forms. It dissolves in excess NaOH, forming a clear solution of sodium plumbate (Na₂PbO₂).
(d) CuSO₄ (Copper Sulphate)
It is insoluble in excess NaOH and decomposes upon heating to black copper oxide (CuO).
6) Name the chloride of a metal which is soluble in excess of ammonium hydroxide. Write an equation for the same.
Ans: The chloride that dissolves in excess ammonium hydroxide is Zinc Chloride (ZnCl₂).
It first forms a white precipitate of zinc hydroxide:
ZnCl₂ + 2NH₄OH → Zn(OH)₂↓ + 2NH₄Cl
This white precipitate dissolves in excess ammonium hydroxide to form a soluble, colorless complex ion:
Zn(OH)₂ + 4NH₄OH → [Zn(NH₃)₄]²⁺ + 2OH⁻ + 4H₂O
This complex is called tetrammine zinc(II) ion.
7) On adding dilute ammonia solution to a colourless solution of a salt, a white gelatinous precipitate appears. This precipitate however dissolves on addition of excess of ammonia solution identify (choose from Na, Al, Zn, Pb, Fe)
(a) Which metal salt solution was used?
(b) what is the formula of the white gelatinous precipitate obtained?
Ans: (a) The metal salt solution used was zinc (Zn) salt solution.
This conclusion is drawn from the nature of the precipitate formed. When a common base like sodium hydroxide (NaOH) is added to a solution containing Zn²⁺ ions, it reacts to form a characteristic precipitate. Zinc ions are known for producing a white, gelatinous solid upon reaction with hydroxide ions, which aligns perfectly with the observation described in the question. Other metal ions might form coloured precipitates or dissolve in excess NaOH, but zinc’s behaviour is distinct and matches the given evidence.
(b) The formula of the white gelatinous precipitate is Zn(OH)₂.
The precipitate results from a double displacement reaction between the zinc salt (for example, ZnCl₂ or ZnSO₄) and the sodium hydroxide (NaOH). The zinc ion (Zn²⁺) combines with two hydroxide ions (OH⁻) from the sodium hydroxide to form an insoluble compound. This compound is zinc hydroxide, which has the chemical formula Zn(OH)₂. Its white colour and distinctly gelatinous, jelly-like texture are its key identifying features.
8) Name:
(a) a yellow monoxide that dissolves in hot and concentrated caustic alkali
(b) a white, insoluble oxide that dissolves when fused with caustic soda or caustic potash
(c) a compound containing zinc in the anion
Ans: (a) A yellow monoxide that dissolves in hot and concentrated caustic alkali
Compound: Lead(II) Oxide (Chemical formula: PbO)
Explanation: Lead(II) oxide is a distinctive yellow solid. It is classified as an amphoteric substance, which means it can react with both strong acids and strong bases. While it is largely unreactive with water, it undergoes a specific reaction when treated with a hot, concentrated solution of sodium hydroxide (caustic soda) or potassium hydroxide (caustic potash). In this reaction, it dissolves to form a soluble compound called sodium plumbite.
The chemical reaction is represented as:
PbO + 2NaOH → Na₂PbO₂ + H₂O
(b) A white, insoluble oxide that dissolves when fused with caustic soda or caustic potash
Compound: Aluminium Oxide (Chemical formula: Al₂O₃)
Explanation: Aluminium oxide, a common constituent of clays and minerals like corundum, is a very hard, white, and insoluble powder. Its amphoteric nature is not readily apparent at room temperature. To observe its reaction with a base, it must be fused—meaning it is heated to a high temperature until it melts together—with a solid base like sodium hydroxide. This process overcomes the compound’s stability and allows it to react to form soluble sodium aluminate.
The fusion reaction is:
Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
(c) A compound containing zinc in the anion
Compound: Sodium Zincate (Commonly represented in solution as Na₂[Zn(OH)₄])
Explanation: In the vast majority of its compounds, such as zinc sulfate (ZnSO₄), zinc exists as a positively charged ion (cation, Zn²⁺). However, zinc is also amphoteric. When zinc oxide or zinc metal reacts with a strong base like sodium hydroxide, it forms a compound where the zinc atom is part of a complex, negatively charged ion (anion). This compound is known as sodium zincate. In this anion, called the zincate ion, zinc has effectively given up its usual cationic role to become the central atom within a complex anion.
9) What do you observe when freshly precipitated aluminium hydroxide reacts with caustic soda solution? Give a balanced equation.
Ans: Observation: Freshly precipitated aluminium hydroxide dissolves in caustic soda solution, forming a clear, colourless solution.
Reason: Aluminium hydroxide is amphoteric. It reacts with bases like caustic soda (NaOH) to form a soluble complex salt, sodium aluminate.
Balanced Equation:
Al(OH)3 + NaOH -> NaAlO2 + 2H2O
10) What do you understand about amphoteric oxide? Give the balanced equations for the reaction with three different amphoteric oxides with a caustic alkali. Write your observations if any.
Ans: An amphoteric oxide is a metal oxide that shows dual behavior, reacting with both acids and bases to form a salt and water.
Examples with Balanced Equations:
Aluminium Oxide (Al₂O₃):
Equation: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O
Observation: A white solid dissolves in sodium hydroxide to form a colorless solution.
Zinc Oxide (ZnO):
Equation: ZnO + 2NaOH → Na₂ZnO₂ + H₂O
Observation: The solid (yellow when hot, white when cold) dissolves.
Lead(II) Oxide (PbO):
Equation: PbO + 2NaOH → Na₂PbO₂ + H₂O
Observation: A yellow solid dissolves to give a clear solution.
11) Distinguish by adding:
(a) sodium hydroxide solution and
(b) Ammonium hydroxide solution to
(i) Calcium salt solution and lead salt solution
(ii) Lead salt solution and ferrous salt solution
(iii) copper salt solution and ferrous salt solution
(iv) Fe (II) salt solution and Fe (III) Salt solution
(v) Ferrous nitrate and lead nitrate
Ans: (a) Using Sodium Hydroxide Solution (NaOH)
(i) Calcium salt vs. Lead salt
Calcium salt: Forms a white precipitate of Ca(OH)₂ which is sparingly soluble (does not dissolve in excess NaOH).
Lead salt: Forms a white precipitate of Pb(OH)₂ which is soluble in excess NaOH, forming a clear solution of [Pb(OH)₄]²⁻.
(ii) Lead salt vs. Ferrous salt
Lead salt: Forms a white precipitate (Pb(OH)₂), soluble in excess NaOH.
Ferrous salt: Forms a dirty green precipitate of Fe(OH)₂, which is insoluble in excess NaOH.
(iii) Copper salt vs. Ferrous salt
Copper salt: Forms a pale blue precipitate of Cu(OH)₂, which is insoluble in excess NaOH (but may turn black on heating).
Ferrous salt: Forms a dirty green precipitate of Fe(OH)₂, insoluble in excess NaOH.
(iv) Fe(II) salt vs. Fe(III) salt
Fe(II) salt: Forms a dirty green precipitate of Fe(OH)₂.
Fe(III) salt: Forms a reddish-brown precipitate of Fe(OH)₃.
(v) Ferrous nitrate vs. Lead nitrate
Ferrous nitrate: Forms a dirty green precipitate of Fe(OH)₂.
Lead nitrate: Forms a white precipitate of Pb(OH)₂, soluble in excess NaOH.
(b) Using Ammonium Hydroxide Solution (NH₄OH)
(i) Calcium salt vs. Lead salt
Calcium salt: No precipitate forms (Ca²⁺ ions do not precipitate with NH₄OH).
Lead salt: Forms a white precipitate of Pb(OH)₂, which is insoluble in excess NH₄OH.
(ii) Lead salt vs. Ferrous salt
Lead salt: Forms a white precipitate, insoluble in excess NH₄OH.
Ferrous salt: Forms a dirty green precipitate of Fe(OH)₂, insoluble in excess NH₄OH.
(iii) Copper salt vs. Ferrous salt
Copper salt: Forms a pale blue precipitate of Cu(OH)₂, which dissolves in excess NH₄OH to form a deep royal blue solution of [Cu(NH₃)₄]²⁺.
Ferrous salt: Forms a dirty green precipitate of Fe(OH)₂, insoluble in excess NH₄OH.
(iv) Fe(II) salt vs. Fe(III) salt
Fe(II) salt: Forms a dirty green precipitate of Fe(OH)₂.
Fe(III) salt: Forms a reddish-brown precipitate of Fe(OH)₃.
(v) Ferrous nitrate vs. Lead nitrate
Ferrous nitrate: Forms a dirty green precipitate.
Lead nitrate: Forms a white precipitate, insoluble in excess NH₄OH.
12) You are provided with two reagent bottles marked A and B. One of which contains NH4OH solution and the other contains NaOH solution. How will you identify them by a chemical test?
Ans: Add a few drops of MgSO₄ or MgCl₂ solution to a sample from each bottle.
Observation:
A white precipitate forms in the test tube containing NaOH.No precipitate forms in the test tube containing NH₄OH.
Reason: NaOH reacts to form insoluble magnesium hydroxide, while NH₄OH forms a soluble complex with magnesium ions.
INTEXT QUESTIONS:
1)What do you understand by the following:
(i) Analysis
(ii) Qualitative analysis
(iii) Reagent
(iv) Precipitation
Ans: (i) Analysis
The process of deconstructing a complex topic or substance into its smaller, fundamental parts to study its structure and composition.
(ii) Qualitative Analysis
A chemical method used to determine what elements, ions, or compounds are present in a given sample.
(iii) Reagent
A substance or compound that is added to a system to trigger a chemical reaction or to test for the presence of a specific substance.
(iv) Precipitation
The process where a solid, called a precipitate, forms and separates from a liquid solution as the result of a chemical reaction.
2) Write the probable colour of the following salts:
(i) Iron (III) chloride
(ii) Potassium nitrate
(iii) Ferrous sulphate
(iv) Aluminium acetate
(v) Calcium carbonate
Ans: (i) Yellow
(ii) Colourless
(iii) PaleGreen
(iv) Colourless
(v) Colourless
3) Name the probable cation present in each of the following solution:
(i) Yellow coloured solution
(ii) blue coloured solution
(iii) Light blue coloured solution
(iv) Pink coloured solution
Ans: (i) Fe3+
(ii) Cu2+
(iii) Cu+2
(iv) Mn2+
4) Name the metal hydroxides which are:
(i) Sparingly soluble
(ii) Insoluble
(iii) Soluble In caustic soda solution
Ans: (i) Ca(OH)2
(ii) Fe(OH)2 and Cu(OH)2
(iii) Zn(OH)2 and Pb(OH)2
5) What do you observe when ammonium salt is heated with caustic soda solution? Write the balanced equation.
Ans: When ammonium salt is heated with caustic soda solution, ammonia gas is evolved. The balance equation is:
NH4Cl + NaOH —> NaCl + H2O + NH3
(NH4)2SO4 + 2NaOH —> Na2SO4 + 2H2O + 2NH3
6) How will you distinguish NH4OH solution from NaOH solution?
Ans: The most reliable distinction is to add a few drops of ferrous sulfate (FeSO₄) solution to each.
With NaOH solution: A dirty green precipitate of ferrous hydroxide [Fe(OH)₂] forms.
With NH₄OH solution: A dirty green precipitate forms initially, but it quickly turns to a dirty brown color due to oxidation by air. This is a key observable difference.
In short: Add FeSO₄. A stable dirty green precipitate indicates NaOH. A precipitate that turns brown indicates NH₄OH.
7) Name the metal hydroxides which are: (i) Insoluble (ii) Soluble. In ammonium hydroxide
Ans: (i) Insoluble in ammonium hydroxide solution:
Iron (II) hydroxide (Fe(OH)₂)
Iron (III) hydroxide (Fe(OH)₃)
Zinc hydroxide (Zn(OH)₂) (Note: It is insoluble but dissolves in excess NH₄OH to form a complex)
(ii) Soluble in ammonium hydroxide solution:
Copper (II) hydroxide (Cu(OH)₂) (forms a deep blue complex)
Silver hydroxide (AgOH) (which decomposes to Ag₂O but then dissolves to form a complex)
