ICSE Class 9 Chemistry Chapter 09 Practical Work

Practical Work

Syllabus

Candidates will be asked to observe the effect of reagents and/or of heat on substances supplied to them. The exercises will be simple and may include the recognition and identification of certain gases listed below.

Gases: Hydrogen, Oxygen, Carbon dioxide, Chlorine, Hydrogen chloride, Sulphur dioxide, Hydrogen sulphide, Ammonia, Water vapour, Nitrogen dioxide.

Candidates are expected to have completed the following minimum practical work.

Simple Experiments On:

(i) Action of heat on the following compounds:

(a) copper carbonate, zinc carbonate (b) washing soda, copper sulphate crystals

(c) zinc nitrate, copper nitrate, lead nitrate (d) ammonium chloride, iodine, ammonium dichromate

Make observations, identify the products and make deductions where possible.

(ii) Action of dilute sulphuric acid on the following substances. (warm if necessary)

(a) a metal (b) a carbonate (c) a sulphide (d) a sulphite

Make observations, identify the gas evolved and make deductions.

(iii) Apply the flame test to identify the metal in the unknown substance.

(a) a sodium salt (b) a potassium salt (c) a calcium compound

(iv) Simple experiments based on hard water and soft water - identification of hardness - simple softening - by heating the temporary hard water, using washing soda and advantage of using detergents over soap in hard water.

(v) Find out the sources of pollution of water bodies in the locality. Suggest preventive steps to control it.

I. Recognition And Identification Of Gases

1. Hydrogen

Preparation: Add dilute HCl or dilute H2SO4 to reactive metals (metals above hydrogen in the activity series) like magnesium, zinc, iron, etc.

Active metal + dil Acid - Salt + Hydrogen

Mg + H2SO4 - MgSO4 + H2 up

Zn + 2HCl - ZnCl2 + H2 up

Recognition And Identification Of Gas

(a) The evolved gas is colourless, odourless

(b) Neutral to litmus.

(c) Pure hydrogen burns with a pale blue flame when a burning splint is brought near it.

2H2 + O2 - 2H2O

(d) Hydrogen mixed with air burns with a pop sound when a burning taper is brought near it.

2. Oxygen

Preparation: Heat heavy metallic oxides

Heavy metal oxide - Metal oxide/Metal + Oxygen

2Pb3O4 - 6PbO + O2 up

2PbO2 - 2PbO + O2 up

2HgO - 2Hg + O2 up

Recognition And Identification Of Gas

(a) The gas is colourless, odourless and neutral to litmus.

(b) It rekindles a glowing wooden splinter.

(c) The gas is absorbed in colourless alkaline solution of pyrogallol and turns it dark brown.

3. Carbon Dioxide

Preparation: Heat metallic carbonate (except sodium carbonate and potassium carbonate) or add dilute acid to any carbonate or hydrogen carbonate.

Metallic carbonate - Metal oxide + carbon dioxide

ZnCO3 - ZnO + CO2 up

CuCO3 - CuO + CO2 up

Metal carbonate/+ hydrogen carbonate (dil) - Acid - Salt + Water + Carbon dioxide

CaCO3 + 2HCl - CaCl2 + H2O + CO2 up

NaHCO3 + HCl - NaCl + H2O + CO2 up

Recognition And Identification Of Gas

(a) The gas is colourless and odourless.

(b) It turns moist blue litmus faint red.

(c) When the gas is passed through lime water, it turns milky due to the formation of white precipitate of calcium carbonate.

Ca(OH)2 + CO2 - CaCO3 + H2O

The milkiness disappears when CO2 is passed in excess.

CaCO3 + CO2 + H2O - Ca(HCO3)2

ppt (Soluble)

(d) The gas has no effect on filter paper dipped in acidified K2Cr2O7 or KMnO4.

4. Chlorine

Preparation: Add conc. HCl to oxidizing agents like Pb3O4, PbO2, MnO2, etc.

Oxidizing agent + Conc. HCl - Metal chloride + Water + Chlorine

Pb3O4 + 8HCl - 3PbCl2 + 4H2O + Cl2 up

MnO2 + 4HCl - MnCl2 + 2H2O + Cl2 up

Recognition And Identification Of Gas

(a) The gas is greenish yellow with a sharp pungent choking odour.

(b) It turns moist blue litmus paper red and finally bleaches, i.e. decolorizes it.

Cl2 + H2O - HCl + HClO

HClO - HCl + [O]

Colouring matter + [O] - Colourless or bleached product

(c) It turns moist starch iodide paper (KI + starch solution) blue black.

Cl2 + 2KI - 2KCl + I2

Starch + I2 - Blue black colour

(d) Pass the gas through silver nitrate solution, a white precipitate (of silver chloride) is formed.

5. Hydrogen Chloride

Preparation: Add conc. H2SO4 to metal chlorides like NaCl, KCl, etc.

Metal chloride + Conc. sulphuric acid - Metal sulphate + Hydrogen chloride

NaCl + H2SO4 - NaHSO4 + HCl up

KCl + H2SO4 - KHSO4 + HCl up

Recognition And Identification Of Gas

(a) The gas is colourless and has a pungent choking odour.

(b) The gas turns moist blue litmus paper red.

(c) If a rod dipped in ammonia solution is brought near the gas, dense white fumes of ammonium chloride are formed.

NH3(aq) + HCl - NH4Cl

(d) When the gas is passed through silver nitrate solution, a white precipitate of silver chloride is formed.

AgNO3(aq) + HCl - AgCl + HNO3

The precipitate dissolves in excess NH4OH.

6. Sulphur Dioxide

Preparation: Add dil. HCl or dil. H2SO4 to metallic sulphites.

Metal\Hydrogen sulphite + dil. Acid - Salt + Water + Sulphur dioxide

Na2SO3 + 2HCl - 2NaCl + H2O + SO2 up

Na2SO3 + H2SO4 - Na2SO4 + H2O + SO2 up

2NaHSO3 + H2SO4 - Na2SO4 + 2H2O + 2SO2 up

Recognition And Identification Of Gas

(a) The gas is colourless with smell of burning sulphur, (a suffocating odour).

(b) Burning wooden splinter is extinguished in sulphur dioxide.

(c) It turns moist blue litmus red and finally bleaches it. This bleaching is temporary.

(d) The gas turns lime water turbid (milky). (Similar to carbon dioxide).

Ca(OH)2 + SO2 - CaSO3 + H2O

Milkiness disappears on passage of excess of sulphur dioxide.

CaSO3 + H2O + SO2 - Ca(HSO3)2 (soluble)

(d) It decolorizes pink acidified potassium permanganate solution.

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

(pink) (colourless)

(e) It changes orange/yellow solution of acidified potassium dichromate green.

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

(orange) (green)

(f) It has no effect on lead acetate paper.

7. Hydrogen Sulphide

Preparation: Add dil. HCl or dil. H2SO4 to metallic sulphides like ZnS, FeS, etc.

Metal sulphide + dil. Acid - Salt + Hydrogen sulphide

FeS + H2SO4 - FeSO4 + H2S up

ZnS + 2HCl - ZnCl2 + H2S up

Recognition And Identification Of Gas

(a) The gas is colourless with a foul smell as of rotten eggs.

(b) It turns moist blue litmus red.

(c) It turns lead acetate paper black.

(CH3COO)2Pb + H2S - PbS + 2CH3COOH

(colourless) (black)

(d) It also turns lead nitrate solution black.

Pb(NO3)2 + H2S - PbS + 2HNO3

(colourless) (black)

Note: H2S also turns acidified potassium permanganate from pink to colourless and acidified potassium dichromate from orange to green but as yellow sulphur is also present the solution is not clear.

8. Ammonia

Preparation: Add alkali to ammonium salt like ammonium chloride or ammonium sulphate.

Ammonium salt + Alkali - Salt + Water + Ammonia

2NH4Cl + Ca(OH)2 - CaCl2 + 2H2O + 2NH3 up

NH4Cl + NaOH - NaCl + H2O + NH3 up

Recognition And Identification Of Gas

(a) The gas is colourless with a characteristic sharp, pungent smell.

(b) It turns moist red litmus blue.

(c) If a rod dipped in conc. HCl is brought near the gas, dense white fumes of ammonium chloride are formed.

(d) The gas turns colourless Nesslers reagent, i.e. (K2HgI4) potassium mercuric iodide, brown.

(e) On passing the gas through a solution of copper sulphate, pale blue precipitate is formed, on passing excess ammonia this precipitate dissolves to form a dark blue solution.

9. Water Vapour

Preparation: Heat a crystalline substance like hydrated sodium carbonate or hydrated copper sulphate.

Hydrated salt - Anhydrous salt + Water vapour

Na2CO3-10H2O - Na2CO3 + 10H2O

CuSO4-5H2O - CuSO4 + 5H2O

(blue) (white)

Recognition And Identification Of Gas

(a) Colourless and odourless vapours condense to form a clear liquid on the cooler part of the test-tube.

(b) The liquid is neutral to litmus.

(c) This liquid turns anhydrous copper sulphate from white to blue.

CuSO4 + 5H2O - CuSO4-5H2O

(white) (blue)

(d) It turns blue cobalt chloride paper pink.

CoCl2 + 2H2O - CoCl2-2H2O

(blue) (pink)

10. Nitrogen Dioxide

Preparation: Heat a heavy metal nitrate like copper nitrate or lead nitrate.

Metal nitrate - Metal oxide + Oxygen + Nitrogen dioxide

2Cu(NO3)2 - 2CuO + O2 + 4NO2 up

2Pb(NO3)2 - 2PbO + O2 + 4NO2 up

Note: Sodium nitrate and potassium nitrate do not produce nitrogen dioxide when they are heated.

Recognition And Identification Of Gas

(a) The gas is brown in colour, has an irritating (pungent) odour and is non-combustible.

(b) It turns moist blue litmus paper red.

(c) It liberates iodine (violet vapours) from potassium iodide (KI) solution or turns potassium iodide paper brown.

2KI + 2NO2 - 2KNO2 + I2 up

(d) It turns green acidified ferrous sulphate solution brown.

Teacher's Note

Gas identification is like detective work - each gas has unique fingerprints (color, smell, and reaction) that help us recognize it, just as we identify friends by their appearance and voice in daily life.

Exercise 9 (A)

1. (a) Give a chemical test to identify the following gases.

(i) Ammonia (ii) Sulphur dioxide (iii) Hydrogen chloride (iv) Chlorine (v) Carbon dioxide (vi) Oxygen (vii) Hydrogen

(b) Select a basic gas mentioned in Q.1 (a). How is the basic nature verified?

(c) Select acidic gases from the gases mentioned in Q.1(a). How is the acidic nature verified?

(d) State the gas responsible for the bleaching action.

(e) Which gas turns blue cobalt chloride paper light pink?

2. What is observed on performing the following:

3. Give a chemical test to distinguish between the following gases.

(a) H2 and CO2 (b) H2 and O2 (c) CO2 and SO2 (d) HCl and H2S

(e) HCl and Cl2 (f) NH3 and HCl (g) SO2 and Cl2 (h) NH3 and SO2

4. Name the gas that

(a) turns moist starch iodide paper blue black.

(b) turns moist red litmus blue.

(c) does not affect acidified K2Cr2O7 paper but turns lime water milky.

(d) affects acidified K2Cr2O7 paper and also turns lime water dirty milky.

5. What do you observe when

(a) CO2 is passed through lime water first and then a little in excess.

(b) HCl is passed through silver nitrate solution.

(c) H2S is passed through lead nitrate solution.

(d) Cl2 is passed through potassium iodide (KI) solution.

(e) Cobalt chloride paper is introduced in water vapour.

Write balanced equations for each of the above.

6. Name:

(a) Two carbonates that do not produce carbon dioxide on heating.

(b) Two nitrates that do not produce nitrogen dioxide on heating.

(c) A brown gas.

(d) A greenish yellow gas.

(e) A gas with rotten egg smell.

Teacher's Note

Just as doctors use different tests to diagnose diseases, chemists use specific reactions and observations to identify unknown gases - recognizing patterns is key to both medicine and chemistry.

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