Practical Chemistry

Question 1

Give a chemical test to distinguish between the following gases:

(a) Hydrogen and oxygen

(b) Carbon dioxide and sulphur dioxide

(c) Hydrogen chloride and hydrogen sulphide

(d) Chlorine and nitrogen dioxide

(e) Ammonia and hydrogen chloride

(f) Sulphur dioxide and chlorine

Answer

(a) Hydrogen and oxygen

Burning wooden splinter is extinguised in hydrogen whereas Oxygen rekindles a glowing wooden splinter.

(b) Carbon dioxide and sulphur dioxide

Carbon dioxide has no effect on acidified KMnO₄ or K₂Cr₂O₇.

Sulphur dioxide turns KMnO₄ from pink to clear colourless and K₂Cr₂O₇ from orange to clear green.

2KMnO₄ + 2H₂O + 5SO₂ ⟶ K₂SO₄ + 2MnSO₄ + 2H₂SO₄

K₂Cr₂O₇ + H₂SO₄ + 3SO₂ ⟶ K₂SO₄ + Cr₂(SO₄)₃ + H₂O

(c) Hydrogen chloride and hydrogen sulphide

Hydrogen chloride — Forms a curdy white precipitate on passage through AgNO₃ solution.

AgNO₃ [aq.] + HCl ⟶ AgCl ↓ [curdy white ppt.] + HNO₃

Hydrogen sulphide — Turns moist lead acetate paper silvery black.

Pb(CH₃COO)₂ [colourless] + H₂S ⟶ PbS ↓ [black] + 2CH₃COOH

(d) Chlorine and nitrogen dioxide

Chlorine reacts with silver nitrate solution to form a white precipitate of silver chloride.

4AgNO₃ [aq.] + 2Cl₂ (g) ⟶ 4AgCl ↓ [curdy white ppt.] + 2N₂O₅ (g) + O₂ (g)

Nitrogen dioxide does not react with silver nitrate solution, hence, no white precipitate is formed.

(e) Ammonia and hydrogen chloride

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

(f) Sulphur dioxide and chlorine

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

Question 2

On heating which of the following substances i.e., copper carbonate, zinc carbonate, washing soda, copper sulphate, zinc nitrate, copper nitrate, lead nitrate, ammonium chloride and ammonium dichromate - relate to the reactions given below.

(a) A white substance which leaves an amphoteric oxide as a residue [whose colour varies in the heated and in the cold state] and evolves a gas which turns lime water milky.

(b) An efflorescent substance which leaves a residue having the same colour as the substance and evolves a gas which changes the colour of cobalt chloride paper.

(c) A white solid which evolves two colourless gases which on cooling combine and condense on the cooler parts of the test tube.

(d) A coloured substance which decomposes violently leaving a coloured residue and evolving two neutral gases one of which is unreactive or inert in nature.

(e) A coloured substance which leaves a black residue and evolves two gases one of which is acidic and the other neutral and colourless.

(f) A coloured substance which leaves on strong heating a black residue and evolves two colourless gases one of which is acidic and other neutral.

(g) A white crystalline solid which decrepitates on heating leaving a residue which fuses with the glass and evolves two gases one of which is coloured and acidic.

(h) An amorphous substance which turns from pale green to black on strong heating evolving a colourless, acidic gas as the only gaseous product.

Answer

(a) A white substance which leaves an amphoteric oxide as a residue [whose colour varies in the heated and in the cold state] and evolves a gas which turns lime water milky — Zinc carbonate.

(b) An efflorescent substance which leaves a residue having the same colour as the substance and evolves a gas which changes the colour of cobalt chloride paper — Washing soda.

(c) A white solid which evolves two colourless gases which on cooling combine and condense on the cooler parts of the test tube — Ammonium chloride.

(d) A coloured substance which decomposes violently leaving a coloured residue and evolving two neutral gases one of which is unreactive or inert in nature — Ammonium dichromate.

(e) A coloured substance which leaves a black residue and evolves two gases one of which is acidic and the other neutral and colourless — Copper nitrate.

(f) A coloured substance which leaves on strong heating a black residue and evolves two colourless gases one of which is acidic and other neutral — Coper sulphate.

(g) A white crystalline solid which decrepitates on heating leaving a residue which fuses with the glass and evolves two gases one of which is coloured and acidic — Lead nitrate.

(h) An amorphous substance which turns from pale green to black on strong heating evolving a colourless, acidic gas as the only gaseous product — Copper carbonate.

Question 3

Give balanced equations for the following conversions affected by heat alone on the substances:

(a) Copper carbonate to copper oxide

(b) Hydrated copper sulphate to sulphur dioxide

(c) Copper nitrate to nitrogen dioxide

(d) Ammonium dichromate to nitrogen

(e) Zinc carbonate to zinc oxide

(f) Zinc nitrate to nitrogen dioxide

Answer

(a) Copper carbonate to copper oxide

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

(b) Hydrated copper sulphate to sulphur dioxide

$\underset{\text{[Copper sulphate (hydrous - blue)]}}{\text{CuSO}_4.5\text{H}_2\text{O}}\longrightarrow \underset{\text{ [Copper sulphate (anhydrous - white)]}}{\text{CuSO}_4} + \underset{\text{ [water vapour]}}{5\text{H}_2\text{O}}$

On strong heating

$2\text{CuSO}_4 \longrightarrow \underset{\text{[ Copper oxide - black] }}{2\text{CuO}} + \underset{\text{[Sulphur dioxide]}}{2{\text{SO}_2}} + \underset{\text{ [oxygen]}}{\text{O}_2}$

(c) Copper nitrate to nitrogen dioxide

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

(d) Ammonium dichromate to nitrogen

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

(e) Zinc carbonate to zinc oxide

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

(f) Zinc nitrate to nitrogen dioxide

$\underset{\text{[zinc nitrate (white)]}}{2\text{Zn(NO}_3)_2} \overset{\Delta}{\longrightarrow} \underset{\text{[yellow - hot, white - cold]}}{2\text{ZnO}} + \underset{\text{[Nitrogen dioxide]}}{4\text{NO}_2} + \underset{\text{[Oxygen]}}{\text{O}_2}$

Question 4

Using dilute sulphuric acid how would you differentiate between :

(a) Copper and magnesium

(b) Sodium carbonate, sodium sulphide and sodium sulphite

How would you identify the gaseous product evolved.

Answer

(a) 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.

(b) Sodium carbonate — When dil. sulphuric acid is added to sodium carbonate, the gas evolved turns lime water milky but has no effect on potassium permanganate solution or potassium dichromate solution. This confirms that the gas evolved is Carbon dioxide.

$\text{Na}_2\text{CO}_3 + \underset{\text{ [dil.]}}{\text{H}_2\text{SO}_4}\longrightarrow \text{Na}_2\text{SO}_4+ \text{H}_2\text{O} + \text{CO}_2$

$2\text{NaHCO}_3 + \underset{\text{ [dil.]}}{\text{H}_2\text{SO}_4}\longrightarrow \text{Na}_2\text{SO}_4+ 2\text{H}_2\text{O} + 2\text{CO}_2$

Sodium sulphide — When dil. sulphuric acid is added to sodium sulphide, colourless gas is evolved with a rotten egg smell that turns moist lead acetate paper silvery black. This confirms that the gas evolved is hydrogen sulphide.

$\text{Na}_2\text{S} + \underset{\text{ [dil.]}}{\text{H}_2\text{SO}_4}\longrightarrow \text{Na}_2\text{SO}_4+ \text{H}_2\text{S}$

Sodium sulphite — When dil. sulphuric acid is added to sodium sulphite, colourless gas is evolved with a suffocating odour. It turns lime water milky and pink potassium permanganate solution colourless. It also turns orange potassium dichromate solution clear green, confirming the presence of sulphur dioxide.

$\text{Na}_2\text{SO}_3 + \underset{\text{ [dil.]}}{\text{H}_2\text{SO}_4}\longrightarrow \text{Na}_2\text{SO}_4+ \text{H}_2\text{O} + \text{SO}_2$

Question 5

Using a platinum wire, conc. hydrochloric acid and a bunsen burner how would you distinguish between the three salts i.e., sodium chloride, potassium chloride and calcium chloride. Explain in brief the method used for the same.

Answer

We can distinguish between the three salts 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.

Question 6

Using given samples of temporary and permanent hard water, soft water, ordinary soap, detergent and washing soda how would you:

(a) Distinguish between hard and soft water.

(b) Distinguish between temporary hard water and permanent hard water.

(c) Remove temporary hardness from water without using a chemical compound.

(d) Remove temporary hardness and permanent hardness from water using a chemical compound.

(e) Prove the advantage of detergent over soap.

Answer

(a) 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.

(b) 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.

(c) Remove temporary hardness from water without using a chemical compound.

1. Temporary hard water is taken in a beaker and heated slowly.
2. After the gases escape out, the water is filtered through a filter paper.
3. Ordinary soap is rubbed inside the filtered solution.

Observation — The boiled and filtered sample of temporary hard water lathers readily with soap.

Result — Temporary hard water can be softened by heating.

$\underset{\text{[in temporary hard water]}}{\text{Ca(HCO}_3)_2} \longrightarrow \underset{\text{[ppt. filtered out]}}{\text{CaCO}_3\downarrow} + \text{CO}_2 + \underset{\text{ [filtered water is soft]}}{{\text{H}_2\text{O}}}$

(d) Remove temporary hardness and permanent hardness from water using a chemical compound.

1. Temporary and permanent hard water are taken in separate beakers and washing soda is added in it. Then the solutions are filtered to remove the precipitate.
2. Ordinary soap is rubbed inside the filtered solution.

Observation — The filtered sample of temporary and permanent hard water lathers readily with ordinary soap.

Result — Temporary hard water and permanent hard water can be softened by using washing soap.

(e) Advantage of detergent over soap

1. A sample of hard water 'X' is taken and ordinary soap is rubbed inside the water.
2. Another sample of hard water 'Y' is taken and detergent is rubbed inside the water. [Detergents are sodium salts of sulphonic acid and do not form scum].

Observation — Lather forms in 'Y' but not in 'X'.

Result — Detergents form lather even with hard water, while ordinary soap is wasted due to formation of scum.

$\underset{\text{[in hard water]}}{\text{Ca(HCO}_3)_2} + \underset{\text{[soap]}}{2\text{NaSt}} \longrightarrow \underset{\text{[scum]}}{\text{Ca(St)}_2} + 2\text{NaHCO}_3$

Question 7

State the meaning of the term 'water pollution'. State how would you find out the different sources of pollution of water bodies in the locality.

Answer

Water pollution is an undesirable change in the chemical, physical and biological conditions of water due to the presence of foreign substances in water.
It leads to degradation in the quality of water and makes it unsuitable for it's designated use.

The main sources of pollution of water bodies in the locality are:

1. Household Detergents — Household chemical cleaning organic compounds used for laundering and dish washing contain phosphates, mainly sodium triphosphate. Phosphates are a major source of water pollution.
2. Domestic Sewage — It is the waste water generated from household activities. Domestic sewage water contains organic materials from food and vegetables and inorganic materials like phosphates and nitrates from soaps and detergents.

Question 8

State in brief, the preventive steps to control the pollution of water bodies in the locality.

Answer

The following preventive steps can control the pollution of water bodies in the locality —

1. Collection and disposal of domestic sewage — A typical waste water system for sewage consists of a network of waste water pipes.
   Sewer Lateral Pipes — These waste water pipes collect sewage from household.
   Sewer Main Pipe — It is the larger pipe on the main street which connects from sewer lateral pipe. The waste water from this pipe is then led to the Water treatment plant.
2. Water treatment plant — In a water treatment plant, the waster water goes through the below stages to remove the pollutants and impurities from it, thereby controlling water pollution:
   1. Screen
   2. Coagulation
   3. Sedimentation
   4. Filtration
   5. Disinfection

Question 1

Select the correct gas from A to F which matches with the description 1 to 5.

    A: CO₂
    B: SO₂
    C: NH₃
    D: Water vapour
    E: Cl₂
    F: H₂S

1. Turns moist blue litmus red and then bleaches it.
2. Turns moist red litmus paper blue.
3. Turns lime water milky and blue litmus paper slightly pink.
4. Turns cobalt chloride paper from blue to pink.
5. Turns lead acetate paper from white to silvery black.

Answer

1. Turns moist blue litmus red and then bleaches it - E: Cl₂
2. Turns moist red litmus paper blue - C: NH₃
3. Turns lime water milky and blue litmus paper slightly pink - A: CO₂
4. Turns cobalt chloride paper from blue to pink - D: Water vapour
5. Turns lead acetate paper from white to silvery black - F: H₂S

Question 2

Select the correct salt from list II which on thermal decomposition exhibits the change in colour from list I - 1 to 5

Answer

Question 3

State which of the substances given below evolves oxygen gas on thermal decomposition.

1. Zinc carbonate
2. Washing Soda
3. Lead nitrate
4. Ammonium dichromate
5. Trilead tetroxide
6. Zinc nitrate
7. Mercury [II] oxide
8. Anhydrous copper sulphate

Answer

Substances that evolve oxygen gas on thermal decomposition are:

1. Lead nitrate
2. Trilead tetroxide
3. Zinc nitrate
4. Mercury [II] oxide
5. Anhydrous copper sulphate

Question 4

Complete the table given below:

Answer

Question 5

Select the correct answer from the words in bracket

1. Hard and soft water can be distinguished using ............... [dil. acid / ordinary soap / detergent].
2. Household detergents contain mainly ............... [carbonates/phosphates] and pollute water bodies.
3. The type of water softened by addition of washing soda is ............... [permanent / temporary / both types].
4. Unpolluted water has ............... [high/low] amount of dissolved oxygen.
5. The sample of hard water which lathers with soap after boiling and filtration contains ............... [calcium chloride / calcium bicarbonate]

Answer

1. Hard and soft water can be distinguished using ordinary soap.
2. Household detergents contain mainly phosphates and pollute water bodies.
3. The type of water softened by addition of washing soda is both types.
4. Unpolluted water has high amount of dissolved oxygen.
5. The sample of hard water which lathers with soap after boiling and filtration contains calcium bicarbonate.