Practical Work

(a) Chemical test to identify the gases are :

1. Ammonia — When a glass rod dipped in conc. HCl is brought near the gas, dense white fumes of ammonium chloride are formed.
2. Sulphur dioxide — Sulphur dioxide gas turns acidified potassium permanganate from pink to clear colourless and acidified potassium dichromate from orange to clear green.
3. Hydrogen chloride — Forms a curdy white ppt. on passage through AgNO₃ solution. The precipitate dissolves in excess of NH₄OH.
4. Chlorine — Pass the gas through silver nitrate solution, a white ppt. of silver chloride is formed.
5. Carbon dioxide — When the gas is passed through lime water, it turns milky and white ppt. of calcium carbonate appears.
6. Oxygen — Oxygen gas rekindles a glowing wooden splinter.
7. Hydrogen — Hydrogen gas burns with a 'pop' sound in air.

(b) Ammonia is a basic gas. It turns red litmus blue.

(c) Sulphur dioxide, Hydrogen chloride, Chlorine, Carbon dioxide. It turns blue litmus red.

(d) Chlorine

(e) Water vapour

(a) H₂ and CO₂ — Hydrogen gas is neutral to litmus whereas carbon dioxide turns blue litmus red.

(b) H₂ and O₂ — Hydrogen gas burns with a pale blue flame when a burning splinter is brought near it whereas oxygen gas rekindles a glowing wooden splinter.

(c) CO₂ and SO₂ — Carbon dioxide gas has no effect on acidified potassium permanganate (KMnO₄) and acidified potassium dichromate (K₂Cr₂O₇) solution whereas sulphur dioxide gas turns acidified potassium permanganate from pink to clear colourless and acidified potassium dichromate from orange to clear green.
2KMnO₄ + 2H₂O + 5SO₂ ⟶ K₂SO₄ + 2MnSO₄ + 2H₂SO₄
K₂Cr₂O₇ + H₂SO₄ + 3SO₂ ⟶ K₂SO₄ + Cr₂(SO₄)₃ + H₂O

(d) HCl and H₂S — Rod dipped in ammonia solution is brought near the mouth of the two gases. HCl gives dense white fumes of ammonium chloride and hence can be distinguished from hydrogen sulphide.
NH₃(aq) + HCl ⟶ NH₄Cl

(e) HCl and Cl₂ — Chlorine (Cl₂) turns moist starch iodide paper blue black whereas HCl does not.
Cl₂ + 2KI ⟶ 2KCl + I₂
Starch + I₂ ⟶ Blue black colour

(f) NH₃ and HCl — Ammonia turns Nessler's reagent from colourless to pale brown.
Hydrogen chloride shows no action with Nessler's reagent. It forms a curdy white ppt. on passage through AgNO₃ solution.
AgNO₃ [ag.] + HCl ⟶ AgCl ↓ [curdy white ppt.] + HNO₃
The ppt. of AgCl is soluble in NH₄OH but insoluble in dil. HNO₃

(g) SO₂ and Cl₂ — On passing Sulphur dioxide gas through lime water, it turns lime water milky.
Ca(OH)₂ + SO₂ ⟶ CaSO₃ ↓ [white ppt.] + H₂O
Chlorine gas does not turn lime water milky. It turns moist starch iodide paper blue black.
Cl₂ + 2KI ⟶ 2KCl + I₂
Starch + I₂ ⟶ Blue black colour

(h) NH₃ and SO₂ — Ammonia turns red litmus blue whereas sulphur dioxide turns wet blue litmus red and finally bleaches it, though the bleaching is temporary.

(a) Chlorine

(b) Ammonia

(c) Carbon dioxide

(d) Sulphur dioxide

(a) When CO₂ is passed through lime water first in small amounts it turns lime water milky. This is due to the formation of insoluble calcium carbonate.

Ca(OH)₂ + CO₂ ⟶ CaCO₃ ↓ + H₂O

When excess of the gas is passed through the solution, milkiness disappears. This is due to the formation of a soluble bicarbonate.

CaCO₃ + CO₂ + H₂O ⟶ Ca(HCO₃)₂ [soluble]

(b) When HCl is passed through silver nitrate solution, a white ppt. of silver chloride is formed.

HCl + AgNO₃ ⟶ AgCl ↓ + HNO₃

(c) When H₂S is passed through lead nitrate solution, black ppt. of PbS is formed.

Pb(NO₃)₂ [colourless] + H₂S ⟶ PbS ↓ [black] + 2HNO₃

(d) Cl₂ turns potassium iodide (KI) solution blue black

Cl₂ + 2KI ⟶ 2KCl + I₂

(e) Water vapour turns blue cobalt chloride paper pink.

CoCl₂ + 2H₂O ⟶ CoCl₂.2H₂O

(a) Sodium carbonate [Na₂CO₃] and Potassium carbonate [K₂CO₃]

(b) Potassium nitrate [KNO₃] and Sodium nitrate [NaNO₃]

(c) Nitrogen dioxide [NO₂]

(d) Chlorine [Cl₂]

(e) Hydrogen sulphide [H₂S]

(a) zinc carbonate, copper carbonate and lead carbonate :

On heating zinc carbonate, light amorphous white solid changes to pale yellow.

On heating copper carbonate, light green amorphous powder changes to black.

On heating lead carbonate, heavy white crystalline solid crumbles with a crackling sound.

(b) zinc nitrate and copper nitrate :

On heating zinc nitrate, white solid changes to yellow when hot and white when cold.

On heating copper nitrate, bluish green crystalline solid melts to form a bluish green mass and gives off steamy vapours which condense on cooler parts of the test tube.
On further heating, the bluish green mass changes to a black residue.

(c) copper sulphate and copper carbonate :

On heating copper sulphate, blue crystalline solid crumbles to form a white amorphous powder and gives off steamy vapours which condense on cooler parts of the test tube to form a colourless liquid (water).
On strong heating, a black solid is formed and a gas is evolved that turns moist blue litmus red and changes the colour of acidified K₂Cr₂O₇ solution from orange to green suggesting that it is SO₂.

On strongly heating copper carbonate, light green amorphous powder changes to black and a colourless odourless gas is evolved that extinguishes a burning wooden splinter and turns lime water milky but has no effect on acidified K₂Cr₂O₇ solution suggesting that it is CO₂.

(d) ammonium chloride and iodine :

On heating ammonium chloride, white crystalline solid sublimates to form a basic gas (NH₃) and acidic gas (HCl). The dense white fumes are noticed that form a white mass on the cooler parts of the test tube.

On heating iodine, violet crystalline solid sublimates to form violet vapours. These vapours settle down on cooler parts of test tube to form violet crystals.

(a) Sodium sulphite and sodium carbonate :

On adding dilute sulphuric acid to sodium sulphite and warming, a colourless gas with suffocating smell of burning sulphur is evolved. The evolved gas turns lime water milky and changes the colour of acidified K₂Cr₂O₇ solution from orange to green suggesting that it is SO₂.

Na₂SO₃ + H₂SO₄ (dil.) ⟶ Na₂SO₄ + H₂O + SO₂↑

On adding dilute sulphuric acid to sodium carbonate and warming, a colourless gas is evolved with brisk effervescence. The evolved gas extinguishes a burning wooden splinter and turns lime water milky but has no effect on acidified K₂Cr₂O₇ solution suggesting that it is CO₂.

Na₂CO₃ + H₂SO₄ (dil.) ⟶ Na₂SO₄ + H₂O + CO₂↑

(b) Copper and magnesium :

Magnesium on reaction with dil. sulphuric acid produces a colourless, odourless gas with brisk effervescence. The gas evolved is hydrogen as it burns with a pale blue flame producing a pop sound.

$\text{Mg} + \underset{\text{ dil.}}{\text{H}_2\text{SO}_4}\longrightarrow \text{MgSO}_4+ \text{H}_2$

Copper does not react with dil. sulphuric acid liberating hydrogen as it is lower in metal reactivity series than hydrogen.

(c) Sodium sulphide and sodium sulphite :

When dil. sulphuric acid is added to sodium sulphide and heated, hydrogen sulphide gas is evolved which has the smell of rotten eggs.

Na₂S + H₂SO₄ ⟶ Na₂SO₄ + H₂S ↑

On adding dilute sulphuric acid to sodium sulphite and warming, a colourless gas with suffocating smell of burning sulphur is evolved. The evolved gas turns lime water milky and changes the colour of acidified K₂Cr₂O₇ solution from orange to green suggesting that it is SO₂.

Na₂SO₃ + H₂SO₄ (dil.) ⟶ Na₂SO₄ + H₂O + SO₂↑

(a) Ammonium dichromate — Orange red crystalline solid, on heating, swells up and decomposes violently with flashes of light leaving greenish residue. It also gives off steamy fumes, which condense on the cooler parts of the test tube to form tiny droplets of water.

$\underset{\text{[Ammonium dichromate (orange)]}}{(\text{NH}_4)_2\text{Cr}_2\text{O}_7} \xrightarrow{\space \Delta \space} \underset{\text{ [Chromic oxide(green)]}}{\text{Cr}_2\text{O}_3} + 4\text{H}_2\text{O} + \text{N}_2$

(b) Copper nitrate — Bluish green crystalline solid, on heating, melts to form a bluish green mass and gives off steamy vapours that condense on the cooler parts of test tube.

${\text{Cu(NO}_3)_2}.6\text{H}_2\text{O} \xrightarrow{\space \Delta \space} {\text{Cu(NO}_3)_2} + 6\text{H}_2\text{O}$

On further heating, the bluish green mass changes to a black residue of copper (II) oxide and brown coloured nitrogen dioxide gas is evolved along with oxygen gas.

$\underset{\text{[Copper nitrate (blue)]}}{2\text{Cu(NO}_3)_2} \xrightarrow{\space \Delta \space} \underset{\text{[copper oxide - black]}}{2\text{CuO}} + \underset{\text{[Nitrogen dioxide]}}{4\text{NO}_2\uparrow} + \underset{\text{[Oxygen]}}{\text{O}_2\uparrow}$

(c) Copper carbonate — Light green amorphous powder changes to black. Carbon dioxide gas is given off which turns lime water milky.

$\underset{\text{[Copper carbonate (light green)]}}{\text{CuCO}_3} \overset{\Delta}{\longrightarrow} \underset{\text{[copper oxide - black]}}{\text{CuO}} + \underset{\text{[Carbon dioxide]}}{\text{CO}_2\uparrow}$

(d) Zinc carbonate — On strong heating, light amorphous white solid changes to pale yellow. Carbon dioxide gas is given off which turns lime water milky.

The residue, on cooling, changes to a white colour. i.e., residue is yellow when hot and white when cold.

$\begin{matrix} \text{ZnCO}_3 &\xrightarrow\Delta & \text{ZnO} & + & \text{CO}_2\uparrow \\ \text{[Zinc} & & \text{[Zinc oxide]} & & \text{[Carbon} \\ \text{carbonate]} & & \text{[yellow - hot]} & & \text{dioxide]} \\ \text{[white]} & & \text{[white - cold]} \end{matrix}$

(e) Ammonium chloride — On heating ammonium chloride, white crystalline solid sublimates to form a basic gas (NH₃) and acidic gas (HCl). The dense white fumes are noticed that form a white mass on the cooler parts of the test tube. No residue is left behind.

$\text{NH}_4\text{Cl} \xrightleftharpoons[\text{cool}]{\text{heat}} \text{NH}_3 \uparrow + \text{HCl} \uparrow$

acidified potassium dichromate paper

Reason — There is no effect of CO₂ gas on potassium dichromate whereas SO₂ turns acidified potassium dichromate from orange to clear green.
K₂Cr₂O₇ + H₂SO₄ + 3SO₂ ⟶ K₂SO₄ + Cr₂(SO₄)₃ + H₂O

Both A and R are true and R is the correct explanation of A.

Explanation — Chlorine reacts with water to form hydrochloric acid (HCl) and hypochlorous acid (HClO).

Cl₂ + H₂O $\rightleftharpoons$ HCl + HClO

These acids make the solution acidic, turning moist blue litmus red and due to the oxidising nature of HClO, the dye in the litmus paper is bleached, leading to discolourisation. Hence, both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

Both A and R are true but R is not the correct explanation of A.

Explanation — When a glass rod is dipped in ammonia solution and brought near the gas, dence white fumes of ammonium chloride are formed. Hence, the assertion (A) is true.

NH₃ (aq) + HCl ⟶ NH₄Cl

Hydrochloric acid (HCl) is strongly acidic in nature. Hence, the reason (R) is true.

Reason (R) doesn't explain assertion because the formation of white fumes is specifically due to the reaction between NH₃ and HCl forming NH₄Cl, not merely because HCl is acidic. Hence, the reason (R) is not the correct explanation of assertion (A).

A is false but R is true.

Explanation — On heating zinc nitrate melts and forms a white sticky mass. The colourless gas, brown gas and yellow residue is formed only when the zinc nitrate is heated strongly. Hence, the assertion (A) is false.

Zinc nitrate on heating strongly decomposes to give ZnO, NO₂ and O₂. Hence, the reason (R) is true.

A is true but R is false.

Explanation — Sodium chloride and potassium chloride can be distinguished using flame text. Sodium salt produces golden yellow flame and potassium salt produces lilac flame. Hence, the assertion (A) is true.

Sodium salt produces golden yellow flame rather than red flame and potassium salt produces lilac flame. Hence, the reason (R) is false.

(a) NH₃, HCl, SO₂, H₂S, CO₂, NO₂, Cl₂.

(b) Ammonia [NH₃]

(c) Water vapour, hydrogen, oxygen

(d) Sulphur dioxide [SO₂]

(a) When dil. sulphuric acid is added to sodium sulphide and heated, hydrogen sulphide gas is evolved which has the smell of rotten eggs and turns lead acetate paper black.

Na₂S + H₂SO₄ ⟶ Na₂SO₄ + H₂S

(b) When dilute sulphuric acid is added to sodium carbonate a colourless gas [CO₂] is evolved with brisk effervescence that turns lime water milky and has no effect on acidified potassium dichromate paper.

Na₂CO₃ + H₂SO₄ ⟶ Na₂SO₄ + H₂O + CO₂ (g)

(c) When dilute sulphuric acid is added to granulated zinc, hydrogen gas is evolved with an effervescence.

Zn + H₂SO₄ (dil) ⟶ ZnSO₄ + H₂ (g)

The procedure to detect the colour imparted by flame test is as follows:

A thin platinum wire is first thoroughly cleaned by dipping it in conc. hydrochloric acid. It is then heated in the non-luminous flame of the burner. The process is repeated. When the wire imparts no colour to the flame, it is ready for use.

Now, wire is first dipped in conc. hydrochloric acid and then in the salt to be tested. It is then introduced into the non-luminous part of the flame and the colour imparted to the flame is observed.

We can distinguish between the three salts [sodium chloride, potassium chloride and calcium chloride] with the help of flame test as described below:

Method

1. Thin platinum wire is thoroughly cleaned and then heated in a non-luminous flame of a burner.
2. When the wire imparts no colour it is dipped in conc. HCl and then into the substance to be identified.
3. The wire is then reintroduced into the non-luminous flame and the colour imparted is noted.

Observation —

1. The golden yellow flame confirms the presence of sodium [Na¹⁺] ion.
2. Lilac colour of flame confirms the presence of Potassium [K¹⁺]
3. Brick red colour of flame, confirms the presence of Calcium [Ca²⁺] ion.

Differentiating hard water from soft water —

1. Two unknown samples 'X' and 'Y' containing hard water and soft water are taken separately in a trough or beaker.
2. Ordinary soap is rubbed by the hands inside each sample.

Observation —

1. One sample of water 'X' lathers with soap
2. The sample of water 'Y' does not lather with soap.

Result —

1. The sample 'X' which lathers with soap is soft water.
2. The sample 'Y' which does not lather with soap is hard water.

Distinguish between temporary hard water and permanent hard water —

1. Two unknown samples 'A' and 'B' containing temporary and permanent hard water are taken separately in a trough or beaker.
2. The water is boiled slowly, gases allowed to escape out and then the water is filtered.
3. Ordinary soap is rubbed by the hands inside each filtered sample.

Observation —

1. One sample of water 'A' lathers with soap.
2. The sample of water 'B' does not lather with soap.

Result —

1. The boiled and filtered sample 'A' which lathers is temporary hard water whose hardness is removed by boiling.
2. Sample 'B' is permanent hard water whose hardness cannot be removed by boiling.

(a) (i) Temporary hardness — Hardness of water due to the presence of calcium and magnesium bicarbonates — Ca(HCO₃)₂, Mg(HCO₃)₂ — that can be removed by boiling and filtration is called temporary hardness.

(ii) Soft water — Water that does not contain dissolved calcium and magnesium salts is called soft water. This water forms lather with ordinary soap.

(iii) Permanent hardness — Hardness of water due to the presence of calcium and magnesium chlorides and sulphates — CaCl₂, MgCl₂, CaSO₄, MgSO₄ — that cannot be removed by boiling is called permanent hardness.

(b) Temporary hardness can be removed by boiling followed by filtration and permanent hardness can be removed by addition of washing soda or caustic soda.

(a) Soap — It is the sodium or potassium salt of an organic fatty acid. It reacts with hard water forming scum, which is why ordinary soap is wasted.

CaSO₄ + 2NaSt (soap) ⟶ CaSt₂ (scum) + Na₂SO₄

Detergents — They are the sodium salts of alkyl sulphonic acids. Detergents contain a sulphonic acid group (-SO₃H) instead of a carboxylic group (-COOH).

(b) Soaps reacts with hard water forming scum, which is why ordinary soap is wasted.

Detergents can lather even with hard water. Due to the solubility of their calcium and magnesium salts in water, they do not form scum.

(c) Soap or detergent molecules form clusters called micelles when dissolved in water. The molecules arrange themselves with their tails inward and their heads outward. During the cleansing process, the hydrocarbon tail of the detergent attaches to oil and dirt. When water is stirred, the oil and dirt separate into smaller fragments. This allows other tails to attach to the fragmented oil and dirt. The detergent molecules surround these small oil and dirt globules. The negatively charged heads in the water prevent the globules from recombining into larger aggregates. As a result, when clothes are rinsed with water, the oil and dirt are effectively removed.

Soaps react with hard water forming scum which is why ordinary soap is wasted whereas detergents can lather even with hard water. Due to the solubility of their calcium and magnesium salts in water, they do not form scum.

(i) Gas A is HCl, When the HCl is passed through silver nitrate solution, a white precipitate of silver chloride is formed.

AgNO₃ (aq) + HCl ⟶ AgCl ↓ + HNO₃

(ii) Gas B is SO₂ , it decolorises purplish pink acidified potassium permanganate solution.

(iii) Gas B is SO₂ and Gas C is CO₂, they are colourless, acidic gases which can turn lime water milky (turbid).

(iv) Gas B (SO₂) decolorises purplish pink acidified potassium permanganate solution and it can change orange/yellow solution of acidified potassium dichromate green. Gas C (CO₂) has no effect on acidified potassium permanganate solution and acidified potassium dichromate solution.

(v) Only Q,

MnO₂ + 4HCl ⟶ MnCl₂ + H₂O + Cl₂

(vi)

CO₂ in limited Quantity:

$\text{Ca(OH)}_2 + \text{CO}_2 \longrightarrow \text{CaCO}_3 \downarrow + \text{ H}_2\text{O}$

CO₂ passed in excess:

$\underset{\text{ppt}}{\text{CaCO}_3}+{\text{CO}_2}+{\text{H}_2{\text{O}}}\longrightarrow \underset{\text{soluble}}{{\text{Ca(HCO}_3)_2}}$

SO₂ in limited Quantity:

$\text{Ca(OH)}_2 + \text{SO}_2 \longrightarrow \text{CaSO}_3 \downarrow + \text{ H}_2\text{O}$

SO₂ passed in excess:

$\underset{\text{ppt}}{\text{CaSO}_3}+{\text{SO}_2}+{\text{H}_2{\text{O}}}\longrightarrow \underset{\text{soluble}}{{\text{Ca(HSO}_3)_2}}$

(vii) Only P

When a glass rod dipped in ammonia solution and brought near the gas A i.e., HCl, dense white fumes of ammonium chloride are formed.

NH₃ (aq) + HCl ⟶ NH₄Cl

(a) A → Ammonia

(b) B → Hydrogen sulphide

(c) C → Sulphur dioxide

(d) D → Nitrogen dioxide