Practical Chemistry

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 coloured 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 the 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 coloured 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 — Copper 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.

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

A burning wooden splinter is extinguished in hydrogen, whereas oxygen rekindles a glowing wooden splinter. Hydrogen burns with a pale blue flame producing a 'pop' sound.

(b) Carbon dioxide and sulphur dioxide

Carbon dioxide has no effect on acidified KMnO₄ or K₂Cr₂O₇.

Sulphur dioxide turns KMnO₄ from pink to 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 turns moist blue litmus paper red and then bleaches it. It also turns moist starch iodide paper blue-black.

Cl₂ + 2KI ⟶ 2KCl + I₂

Starch + I₂ ⟶ Blue-black colour

Nitrogen dioxide is a reddish-brown gas. It turns KI paper brown by liberating iodine.

2KI + 2NO₂ ⟶ 2KNO₂ + I₂

(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 [aq.] + 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

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

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

(b) Hydrated copper sulphate to sulphur dioxide

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

On strong heating

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

(c) Copper nitrate to nitrogen dioxide

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

(d) Ammonium dichromate to nitrogen

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

(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

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

Using a platinum wire, conc. hydrochloric acid and a bunsen burner, explain how would you distinguish between the three salts i.e., sodium chloride, potassium chloride and calcium chloride.

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.

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

Distinguish between —
(a) hard & soft water
(b) temporary hard water & permanent hard water.

How would you remove —
(a) temporary hardness from water without using a chemical compound
(b) temporary hardness & permanent hardness from water using a chemical compound

How would you 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.

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.

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

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 to each sample. 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 soda.

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.

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

Explain the term 'water pollution'. How would you find the 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 its 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 dishwashing contain phosphates, mainly sodium triphosphate. Phosphates are a major source of water pollution and may cause eutrophication.
2. Domestic sewage — It is the wastewater 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.
3. Industrial wastes — Waste from industrial sources may contain toxic chemicals such as lead, mercury, acids and alkalis, which pollute water bodies.
4. Other sources — Agricultural run-off containing fertilisers, insecticides and herbicides, oil spills, petroleum products, garbage and run-off water from streets and drains may also pollute water bodies.

State 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 wastewater system for sewage consists of a network of wastewater pipes.
   Sewer lateral pipes — These wastewater pipes collect sewage from households.
   Sewer main pipe — It is the larger pipe on the main street which connects with the sewer lateral pipes. The wastewater from this pipe is then led to the water treatment plant.
2. Water treatment plant — In a water treatment plant, the wastewater passes through the following stages to remove pollutants and impurities, thereby controlling water pollution:
   1. Screening
   2. Coagulation
   3. Sedimentation
   4. Filtration
   5. Disinfection
3. Other preventive steps — Household-generated pollutants should be monitored. Hazardous wastes such as motor oils, old batteries, paints and cleaning solvents should be recycled or disposed of properly and should not be poured down the sink.

The gas which turns moist starch iodide paper blue black :

1. NH₃
2. Cl₂
3. SO₂
4. H₂S

Answer

Cl₂

Reason — Chlorine is a strong oxidising agent. It oxidises iodide ions from potassium iodide into iodine. The iodine produced reacts with starch and forms a blue-black starch–iodine complex.

Cl₂ + 2KI ⟶ 2KCl + I₂

Ammonia does not oxidise potassium iodide present in moist starch iodide paper. Sulphur dioxide and Hydrogen sulphide are reducing agents. They do not oxidise iodide ions to iodine.

The gas which turns KI paper brown :

1. CO₂
2. SO₂
3. HCl
4. NO₂

Answer

NO₂

Reason — NO₂ acts as an oxidising agent. It liberates iodine from KI, producing a brown colour.

2KI + 2NO₂ ⟶ 2KNO₂ + I₂

CO₂ is neither a strong oxidising nor reducing agent. It does not liberate iodine from KI paper. SO₂ is a reducing agent. It does not oxidise iodide ions to iodine. HCl is an acidic gas but not an oxidising agent. It cannot liberate iodine from potassium iodide.

The gas which turns lead acetate paper silvery black :

1. SO₂
2. NH₃
3. H₂S
4. Cl₂

Answer

H₂S

Reason — Hydrogen sulphide reacts with lead acetate present on the paper to form black lead sulphide. The black lead sulphide formed turns the lead acetate paper silvery black.

Pb(CH₃COO)₂ + H₂S ⟶ PbS↓ + 2CH₃COOH

The gas which turns Nessler's reagent pale brown :

1. Cl₂
2. HCl
3. NO₂
4. NH₃

Answer

NH₃

Reason — When ammonia reacts with Nessler’s reagent, a pale brown colour (or reddish brown precipitate) of iodide of Millon's base is produced. The gas that turns Nessler’s reagent pale brown is NH₃.

The gas which turns moist blue litmus faint red:

1. CO₂
2. SO₂
3. H₂S
4. NO₂

Answer

CO₂

Reason — Carbon dioxide dissolves in water to form weak carbonic acid. Hence, it turns moist blue litmus faint red.

CO₂ + H₂O ⟶ H₂CO₃

SO₂, H₂S and NO₂ are also acidic gases, but CO₂ is specifically identified by its faint red colour change on moist blue litmus.

The substance which evolves an inert gas on strong heating.

1. Pb(NO₃)₂
2. (NH₄)₂Cr₂O₇
3. CuSO₄.5H₂O
4. NH₄Cl

Answer

(NH₄)₂Cr₂O₇

Reason — On strong heating, ammonium dichromate decomposes to form nitrogen gas, water vapour and green chromic oxide. Nitrogen is unreactive or inert in nature.

(NH₄)₂Cr₂O₇ $\xrightarrow{\Delta}$ Cr₂O₃ + N₂ + 4H₂O

Using dilute sulphuric acid how would you differentiate between - Sodium carbonate, sodium sulphide and sodium sulphite. How would you identify the gaseous product evolved.

Answer

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.

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

Give a reason why a sample of water containing calcium & magnesium, chlorides & sulphates cannot be softened by heating, followed by filtration.

Answer

Water containing calcium & magnesium, chlorides & sulphates cannot be softened by heating, followed by filtration because water containing calcium and magnesium chlorides and sulphates has permanent hardness. On heating, these salts do not decompose to form insoluble precipitates. Therefore, calcium and magnesium ions remain dissolved in water and cannot be removed by filtration.

Give a reason why water affected by eutrophication may be passed through a water treatment plant & checked by use of a pH paper to determine its quality.

Answer

Eutrophication causes excessive growth of algae and decomposition of organic matter, which changes the chemical nature of water and may make it acidic or alkaline. Therefore, the water is passed through a treatment plant to remove impurities and harmful substances, and its quality is checked using pH paper to determine whether the water has a safe pH and is suitable for use.

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

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

Answer

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

Complete the table given below:

Answer

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.