Practical Work

The chapter on Practical Work serves as a foundational introduction to the hands-on aspect of the subject. It emphasizes the importance of the laboratory as a space for observing chemical phenomena firsthand, moving beyond theoretical knowledge. The chapter begins by outlining essential safety protocols, such as wearing protective gear, handling chemicals with care, and knowing the location of emergency equipment like fire extinguishers and first-aid kits. It also stresses the need for meticulousness—reading instructions carefully, keeping the workspace clean, and properly disposing of waste. This foundation ensures that students develop a responsible and cautious approach from the outset, which is critical for all future scientific inquiry.

The chapter then delves into the familiarization with common laboratory apparatus. Students learn about the uses of various glassware and tools, including beakers, test tubes, Bunsen burners, tripod stands, wire gauze, and measuring instruments like graduated cylinders and balances. It explains basic techniques such as heating substances safely, preparing solutions, and filtering mixtures. A significant portion is dedicated to mastering fundamental skills like accurate measurement of liquids and solids, which is vital for obtaining reliable results. This hands-on familiarity with equipment builds confidence and prepares students for executing specific experiments, turning abstract concepts into tangible experiences.

Finally, the practical work chapter typically introduces a few simple but classic experiments. These might involve processes like crystallization of salts from their solutions, demonstrating the concept of purification, or simple paper chromatography to separate ink components, illustrating separation techniques. Other common activities include conducting tests for the presence of water or identifying gases like hydrogen or oxygen through characteristic tests. Through these activities, the chapter aims to cultivate observational skills, precise recording of steps and results in a practical notebook, and the ability to draw logical conclusions. Ultimately, it bridges theory and practice, helping students appreciate chemistry as a dynamic experimental science.

Exercise 9 (A)

Question  1. 

(a) Give only one suitable chemical test to identify the following gases.

1. Ammonia
2. Sulphur dioxide
3. Hydrogen Chloride
4. Chlorine
5. Carbon Dioxide
6. Oxygen
7. Hydrogen

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

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

(d) State the gas responsible for bleaching action.

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

Ans:

(a) Chemical Tests for Gases

• Ammonia: Turns moist red litmus paper blue.
• Sulphur dioxide: Turns acidified potassium dichromate paper from orange to green.
• Hydrogen chloride: Gives a white precipitate with silver nitrate solution (AgNO₃).
• Chlorine: Bleaches moist litmus paper.
• Carbon dioxide: Turns limewater (calcium hydroxide solution) milky.
• Oxygen: Relights a glowing splint.
• Hydrogen: Burns with a characteristic pop sound when a lighted splint is brought near it.

(b) Basic Gas and How Its Nature is Suspected

• Basic gas: Ammonia.
• How suspected: Its basic nature is indicated by its ability to turn moist red litmus paper blue.

(c) Acidic Gases and How Their Nature is Suspected

• Acidic gases: Sulphur dioxide, Hydrogen chloride, Carbon dioxide.
• How suspected: Their acidic nature is indicated by their ability to turn moist blue litmus paper red. (Note: Carbon dioxide does this slowly due to the formation of carbonic acid.)

(d) Gas Responsible for Bleaching Action

• Gas: Chlorine.

(e) Gas That Turns Blue Cobalt Chloride Paper Light Pink

• Gas: Ammonia.

Question 2. 

What is observed in performing the following :

	Hydrogen	Oxygen	Carbon dioxide	Chlorine
Litmus test
Apply a burning splint to the gas
Colour of gas	colourless	colourless	colourless	greenish-yellow
Odour of gas

Ans:

Test / Property	Hydrogen (H2​)	Oxygen (O2​)	Carbon Dioxide (CO2​)	Chlorine (Cl2​)
Litmus test	Neutral. No change on moist blue or red litmus paper.	Neutral. No change on moist blue or red litmus paper.	Acidic. Turn moist blue litmus paper red. (Forms carbonic acid, H2​CO3​).	Acidic and Bleaching. Turns moist blue litmus paper red, then quickly bleaches it white.
Apply a burning splint to the gas	The gas itself burns with a pale blue flame and a pop sound (explosive ignition). The splint is extinguished.	Does not burn but vigorously relights a glowing splint, showing it supports combustion.	Does not burn and extinguishes a burning splint.	Does not burn but usually extinguishes a burning splint.
Colour of gas	Colourless	Colourless	Colourless	Greenish-yellow
Odour of gas	Odourless	Odourless	Odourless	Pungent, choking smell (like bleach)

Question 3. 

1. Give a chemical test to distinguish between the following gases. H2 and CO2 

2. Give a chemical test to distinguish between the following gases. H2 and O2

3. Give a chemical test to distinguish between the following gases. CO2 and SO2

4. Give a chemical test to distinguish between the following gases. HCl and H2S

5. Give a chemical test to distinguish between the following gases. HCl and Cl2

6. Give a chemical test to distinguish between the following gases. NH3 and HCl

7. Give a chemical test to distinguish between the following gases. SO2 and Cl2 

8. Give a chemical test to distinguish between the following gases. NH3 and SO2

Ans:

1. H₂ and CO₂:
   Bring a burning splint near each gas. Hydrogen (H₂) will burn with a pop sound, while carbon dioxide (CO₂) will extinguish the flame.
2. H₂ and O₂:
   Insert a glowing splint into each gas. Oxygen (O₂) will reignite the glowing splint, while hydrogen (H₂) will either give a pop sound or burn quietly if ignited.
3. CO₂ and SO₂:
   Bubble each gas through acidified potassium permanganate (KMnO₄) solution. SO₂ will decolorize the purple solution, while CO₂ will not. Alternatively, pass each gas through limewater first — both turn it milky, but only SO₂ will also decolorize KMnO₄.
4. HCl and H₂S:
   Expose each gas to a paper moistened with lead(II) acetate solution. H₂S will turn the paper black (due to PbS formation), while HCl gas will produce white fumes of ammonium chloride when brought near ammonia.
5. HCl and Cl₂:
   Bring a moist blue litmus paper close to each gas. Both may turn it red initially, but chlorine (Cl₂) will eventually bleach it white, while HCl gas will only turn it red and not bleach it.
6. NH₃ and HCl:
   Bring a moist red litmus paper close to each gas. Ammonia (NH₃) will turn it blue, while HCl gas will turn moist blue litmus paper red.
7. SO₂ and Cl₂:
   Pass each gas through acidified potassium dichromate (K₂Cr₂O₇) solution. SO₂ will change the orange solution to green, while Cl₂ will not. Also, Cl₂ has a stronger bleaching action on moist litmus than SO₂.
8. NH₃ and SO₂:
   Test with a moist pH paper or litmus paper. NH₃ will turn red litmus blue (alkaline), while SO₂ will turn blue litmus red (acidic). Alternatively, ammonia gives white fumes with HCl gas; SO₂ does not.

Question 4. 

1. Name the gas that. Turn moist starch iodide paper blue black. 

2. Name the gases which turn moist red litmus blue. 

3. Name the gas that: Does not affect acidified K2Cr2O7 paper but turns lime water milky. 

4. The name affects the acidified K2Cr2O7 paper and also turns lime water dirty milky.

Ans:

1. The gas that turns moist starch iodide paper blue-black is chlorine (Cl₂). This occurs because chlorine, being a strong oxidizing agent, displaces the iodine from potassium iodide in the paper. The liberated iodine then reacts with the starch to form a characteristic blue-black complex.

2. Gases which turn moist red litmus blue are basic or alkaline in nature. The most common examples are ammonia (NH₃) and methylamine (CH₃NH₂). Ammonia dissolves in the moisture to form ammonium hydroxide, which is alkaline and changes the litmus color.

3. The gas that does not affect acidified potassium dichromate (K₂Cr₂O₇) paper but turns lime water milky is carbon dioxide (CO₂). Carbon dioxide is not an oxidizing agent, so it does not reduce the orange dichromate paper. However, it reacts with calcium hydroxide in lime water to form insoluble calcium carbonate, causing the milky appearance.

4. The gas that turns acidified potassium dichromate paper green and also turns lime water milky (often described as dirty milky due to the formation of a soluble product) is sulfur dioxide (SO₂). SO₂ reduces the orange dichromate ion to green chromium(III) ions. When passed through lime water, it first forms a milky precipitate of calcium sulfite, which can dissolve in excess SO₂ to form clear calcium hydrogen sulfite.

Question 5. 

1. What do you observe when CO2 is passed through lime water first and then a little in excess. 

2. What do you observe when HCI is passed through a silver nitrate solution?

3. What do you observe when H2S is passed through a lead nitrate solution? 

4. What do you observe when Cl2 is passed through potassium iodide (KI) solution. 

5. What do you observe when Cobalt chloride paper is introduced in water vapour.

Ans:

1. Carbon Dioxide and Lime Water:
When carbon dioxide (CO₂) is first bubbled through clear lime water (calcium hydroxide solution), a white, milky precipitate of calcium carbonate forms, turning the solution cloudy. This occurs due to the formation of insoluble calcium carbonate. However, when excess CO₂ is passed through the milky mixture, the precipitate dissolves completely, and the solution becomes clear again. This happens because the calcium carbonate reacts further with excess CO₂ and water to produce soluble calcium bicarbonate.

2. Hydrogen Chloride and Silver Nitrate Solution:
Passing hydrogen chloride gas (HCl) through a silver nitrate solution results in the immediate formation of a thick, curdy white precipitate. This precipitate is silver chloride, which is insoluble in water. The reaction is a characteristic test for chloride ions, as the silver ions from silver nitrate combine with chloride ions from HCl to create the visible white solid.

3. Hydrogen Sulphide and Lead Nitrate Solution:
When hydrogen sulphide gas (H₂S) is passed through a colourless lead nitrate solution, a striking colour change occurs, producing a shiny black precipitate. This black substance is lead sulphide. The formation of this dark precipitate is a standard test for the presence of sulphide ions or lead ions, resulting from a double decomposition reaction.

4. Chlorine and Potassium Iodide Solution:
Passing chlorine gas (Cl₂) through a potassium iodide (KI) solution causes the colourless or pale yellow solution to turn a distinct brownish-yellow colour. This happens because chlorine, being more reactive, displaces iodine from the potassium iodide. The liberated iodine dissolves in the solution, imparting the characteristic brown colour. In some cases, a black precipitate may also form if the concentration is high.

5. Cobalt Chloride Paper in Water Vapour:
Introducing blue cobalt chloride paper into water vapour triggers a clear and reversible colour change. The paper, which is originally blue in its anhydrous (dry) state, gradually turns pink. This change occurs because the cobalt chloride combines with water molecules to form a hydrated compound. This property makes cobalt chloride paper a useful indicator for testing the presence of moisture.

Question 6. 

1. Name: Two carbonates do not produce carbon dioxide on heating.

2. Name: Two nitrates that do not produce nitrogen dioxide on heating. 

3. Name: A brown gas 

4. Name: A greenish-yellow gas. 

5. Name: Gas with rotten egg smell.

Ans:

1. Among carbonates, sodium carbonate (Na₂CO₃) and potassium carbonate (K₂CO₃) are notable for their exceptional stability. Unlike most other carbonates, which decompose to release carbon dioxide gas when heated strongly, these two do not break down under normal laboratory heating conditions. This high thermal stability is due to the strong electropositive character of the sodium and potassium ions.
2. Similarly, in the case of nitrates, sodium nitrate (NaNO₃) and potassium nitrate (KNO₃) behave differently from others like copper or lead nitrate. Upon heating, they do not produce nitrogen dioxide. Instead, they decompose to yield oxygen gas and the corresponding nitrite (e.g., sodium nitrite, NaNO₂), making them exceptions to the general rule of nitrate decomposition.
3. Nitrogen dioxide (NO₂) is a well-known gas with a distinctive deep brown colour. It is often produced during the thermal decomposition of many heavy metal nitrates and is a prominent pollutant, contributing to the brown haze in smog.
4. Chlorine (Cl₂) is a highly reactive gaseous element recognized by its pale greenish-yellow colour. It has a pungent, irritating odour and finds use in water purification and the manufacture of various chemicals, though it is toxic in high concentrations.
5. The gas famously associated with the smell of rotten eggs is hydrogen sulfide (H₂S). This colourless gas is produced by the bacterial breakdown of organic matter in oxygen-poor environments and is also notable for its high toxicity and flammable nature.

Exercise 9 (B)

Question 1. 

1. Distinguish by heating the following in dry test tubs. Zinc carbonate, Copper carbonate and lead carbonate. 

2. Distinguish by heating the following in dry test tubs. Zinc nitrate and copper nitrate. 3. Distinguish by heating the following in dry test tubs. Copper Sulphate and copper carbonate 

4. Distinguish by heating the following in dry test tubs. Ammonium Chloride and iodine.

Ans:

1. Distinguishing Zinc Carbonate, Copper Carbonate, and Lead Carbonate by Heating

• Zinc Carbonate: On heating, it decomposes to form zinc oxide (yellow when hot, white when cold) and carbon dioxide gas (turns limewater milky). No further change.
• Copper Carbonate: Decomposes with a color change from green to black residue (copper(II) oxide). Carbon dioxide is evolved (limewater turns milky).
• Lead Carbonate: Decomposes to form a yellow residue (lead(II) oxide, litharge) when hot, which may turn paler yellow on cooling. Carbon dioxide is released.

Distinction: Observe the color of the residue formed: white (ZnO), black (CuO), or yellow (PbO).

2. Distinguishing Zinc Nitrate and Copper Nitrate by Heating

• Zinc Nitrate: Decomposes to give zinc oxide (yellow when hot, white when cold), nitrogen dioxide (reddish-brown fumes), and oxygen.
• Copper Nitrate: Decomposes vigorously to give a black residue (copper(II) oxide), and copious reddish-brown fumes of nitrogen dioxide mixed with oxygen.

Distinction: Both produce brown NO₂ gas, but the final residue color differs: white (ZnO) vs. black (CuO).

3. Distinguishing Copper Sulphate and Copper Carbonate by Heating

• Copper Sulphate (Hydrated): On heating, it first loses water of crystallization, turning from blue to white (anhydrous copper sulphate). On very strong heating, it decomposes further to black copper(II) oxide and sulphur trioxide.
• Copper Carbonate: Decomposes directly on heating, changing from green to a black residue (copper(II) oxide) while releasing carbon dioxide (turns limewater milky).

Distinction: Initial color change is key. Copper sulphate goes blue → white, while copper carbonate goes green → black. Both can form a black residue on prolonged strong heating.

4. Distinguishing Ammonium Chloride and Iodine by Heating

• Ammonium Chloride: Undergoes sublimation. It forms a dense white fumes/sublimate of ammonium chloride crystals on the cooler parts of the test tube. The fumes have a characteristic pungent smell.
• Iodine: Undergoes sublimation to produce dense violet vapors which condense into shiny grey-black crystals on the cooler parts. The vapor has a distinctive sharp odor.

Distinction: Directly observe the color of the vapor and sublimate: white fumes/crystals (NH₄Cl) vs. violet vapors/grey-black crystals (I₂).

Question 2. 

Match the following:

Column A	Column B
(a) Pb(NO3)2	(i) rotten eggs smell
(b) CO2	(ii) burns with a pop sound
(c) (NH4)2Cr2O7	(iii) the suffocating smell of sulphur
(d) HCI	(iv) lime water turns milky
(e) NO2	(v) crackling sound
(f) O2	(vi) residue swells up
(g) H2	(vii) brown gas
(i) H2S	(viii) supports combustion
(j) SO2	(ix) fumes with NH3 solution

Ans:

Column A (Compound/Reaction)	Match	Column B (Observation/Property)
(a) Pb(NO3​)2​ (Lead Nitrate)	(v)	crackling sound (Observed when solid lead nitrate is strongly heated, often referred to as decrepitation)
(b) CO2​ (Carbon Dioxide)	(iv)	lime water turns milky (The standard test for CO2​ is reaction with Ca(OH)2​ to form insoluble CaCO3​)
(c) (NH4​)2​Cr2​O7​ (Ammonium Dichromate)	(vi)	residue swells up (Decomposes vigorously upon heating, leaving behind voluminous Cr2​O3​ residue, resembling a volcano)
(d) HCl (Hydrogen Chloride/Hydrochloric Acid)	(ix)	fumes with NH3​ solution (HCl fumes react with ammonia fumes to form dense white fumes of NH4​Cl)
(e) NO2​ (Nitrogen Dioxide)	(vii)	brown gas (Nitrogen dioxide is a reddish-brown, pungent gas)
(f) O2​ (Oxygen)	(viii)	supports combustion (The standard test for O2​ is that it relights a glowing splint)
(g) H2​ (Hydrogen)	(ii)	burns with a pop sound (The standard test for H2​ is that it burns with a small explosion when ignited)
(i) H2​S (Hydrogen Sulfide)	(i)	rotten eggs smell (H2​S is known for its distinctive and unpleasant smell)
(j) SO2​ (Sulfur Dioxide)	(iii)	the suffocating smell of sulphur (SO2​ has a sharp, irritating, and suffocating odor, often associated with burnt sulfur)

Question 3. 

1. Distinguish by dilute sulphuric acid. Sodium Sulphite and sodium carbonate?

2. Distinguish by dilute sulphuric acid. Copper and magnesium?

3. Distinguish by dilute sulphuric acid. Sodium sulphide and sodium sulphite?

Ans:

1. Distinguishing Sodium Sulphite and Sodium Carbonate using Dilute Sulphuric Acid

When dilute sulphuric acid is added to sodium sulphite, a vigorous reaction occurs with the immediate evolution of a colourless gas that has a pungent, choking smell of burning sulphur. This gas is sulphur dioxide. In contrast, adding the acid to sodium carbonate also produces effervescence of a colourless and odourless gas, which is carbon dioxide. The key distinguishing test is to pass the evolved gases through specific reagents. Sulphur dioxide from sodium sulphite will turn acidified potassium dichromate paper from orange to green. Carbon dioxide from sodium carbonate, when bubbled through limewater (calcium hydroxide solution), will turn it milky due to the formation of insoluble calcium carbonate.

2. Distinguishing Copper and Magnesium using Dilute Sulphuric Acid

The distinction here is based on the reactivity of the metals. Magnesium, being a reactive metal above hydrogen in the reactivity series, reacts vigorously with dilute sulphuric acid. This produces rapid effervescence of hydrogen gas and the magnesium metal dissolves, forming a colourless solution of magnesium sulphate. To confirm the gas as hydrogen, it produces a characteristic ‘pop’ sound when a burning splint is brought near it. Copper, being less reactive than hydrogen, shows no reaction with dilute sulphuric acid. There is no effervescence, no dissolution of the metal, and the solution remains unchanged.

3. Distinguishing Sodium Sulphide and Sodium Sulphite using Dilute Sulphuric Acid

Both compounds react with the acid to produce distinct gaseous products. Sodium sulphide reacts to produce hydrogen sulphide gas, which is colourless but has a strong, offensive odour of rotten eggs. This gas can be confirmed by its ability to turn moist lead acetate paper silvery black due to the formation of lead sulphide. Sodium sulphite, as before, produces sulphur dioxide gas with its sharp, choking smell. A reliable chemical test is to use acidified potassium permanganate solution; sulphur dioxide will decolourise the purple pink solution, while hydrogen sulphide will turn it colourless but may also produce a yellow precipitate of sulphur. The difference in smell alone is often a primary indicator.

Question 4. 

1. Write your observation and a balanced equation in the case of the following substances being heated. Ammonium dichromate.

2. Write your observation and a balanced equation in the case of the following substances being heated. Copper nitrate. 

3. Write your observation and a balanced equation in the case of the following substances being heated. Copper carbonate.

4. Write your observation and a balanced equation in the case of the following substances being heated. Zinc carbonate.

5. Write your observation and a balanced equation in the case of the following substances being heated. Ammonium chloride

Ans:

1. Ammonium dichromate upon heating:
   Observation: The solid undergoes a vigorous decomposition reaction resembling a volcanic eruption. It produces sparks, flames, and a voluminous green residue, accompanied by the release of colorless gas and water vapor.
   Balanced equation:
   (NH₄)₂Cr₂O₇(s) → Cr₂O₃(s) + N₂(g) + 4H₂O(g)
2. Copper nitrate upon heating:
   Observation: The blue solid turns black as it decomposes. Brown fumes of nitrogen dioxide are emitted, and a colorless gas (oxygen) may also be released.
   Balanced equation:
   2Cu(NO₃)₂(s) → 2CuO(s) + 4NO₂(g) + O₂(g)
3. Copper carbonate upon heating:
   Observation: The green powder changes to a black solid, and carbon dioxide gas is evolved, often observed as bubbling or effervescence if tested.
   Balanced equation:
   CuCO₃(s) → CuO(s) + CO₂(g)
4. Zinc carbonate upon heating:
   Observation: The white solid turns yellow while hot, reverting to white upon cooling. Carbon dioxide gas is released, which can be detected by passing it through limewater.
   Balanced equation:
   ZnCO₃(s) → ZnO(s) + CO₂(g)
5. Ammonium chloride upon heating:
   Observation: The white solid sublimes, producing dense white fumes that condense on cooler surfaces as a solid coating, indicating a reversible process.
   Balanced equation:
   NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

Question 5. 

1. State the original colour of the following substance and colour of residue obtained after heating. Ammonium dichromate 

2. State the original colour of the following substance and colour of residue obtained after heating. Copper carbonate 

3. State the original colour of the following substance and colour of residue obtained after heating. Lead nitrate 

4. State the original colour of the following substance and colour of residue obtained after heating. Zinc carbonate

Ans:

1. Ammonium dichromate
   • Original colour: Orange-red.
   • Colour of residue after heating: Green.
2. Copper carbonate
   • Original colour: Green.
   • Colour of residue after heating: Black.
3. Lead nitrate
   • Original colour: White.
   • Colour of residue after heating: Yellow.
4. Zinc carbonate
   • Original colour: White.
   • Colour of residue after heating: White.

Exercise 9 (C)

Question 1. 

1. What do you observe when dilute sulphuric acid is added to the following: Sodium Sulphide 

2. What do you observe when dilute sulphuric acid is added to the following: Sodium carbonate

3. What do you observe when dilute sulphuric acid is added to Zinc granules?

Ans:

Here is a detailed description for each case:

1. Sodium Sulfide: Upon adding dilute sulfuric acid to sodium sulfide, immediate effervescence occurs as bubbles of gas are rapidly released. This gas has a distinctive rotten egg smell, characteristic of hydrogen sulfide. The mixture may also become slightly warm due to the exothermic nature of the reaction. In some cases, if the sodium sulfide is in solution, a faint yellow coloration might appear, though the primary observation is the foul-smelling gas evolution.
2. Sodium Carbonate: When dilute sulfuric acid is introduced to sodium carbonate, brisk fizzing is observed right away. This effervescence is caused by the production of carbon dioxide gas, which is colorless and odorless. The reaction often appears vigorous, with bubbles forming throughout the mixture. If performed in a container, you might notice a slight hissing sound. Additionally, the solid sodium carbonate, if present, dissolves progressively as the reaction proceeds.
3. Zinc Granules: Adding dilute sulfuric acid to zinc granules results in the gradual appearance of gas bubbles on the surface of the zinc. These bubbles are hydrogen gas, which is non-toxic and odorless. The zinc granules slowly dissolve over time, and the mixture may become warm to the touch. If the gas is collected and ignited, it produces a characteristic popping sound, confirming its identity. The solution eventually turns colorless, indicating the formation of zinc sulfate.

These observations are based on typical laboratory experiments and highlight the unique behaviors of each reaction. Always exercise caution, especially when dealing with gases like hydrogen sulfide, which is toxic in confined spaces.

Question 2. 

Match column A with column B. 

Column A	Column B
(a) Blue salt changes to white and then black	(i) Ammonium dichromate
(b) Orange coloured compounds change to green.	(ii) Iodine
(c) The red compound changes to brown and then yellow.	(iii) Zinc Nitrate
(d) White to yellow when hot and white when cold.	(iv) Copper sulphate
(e) Violet solid changes to violet vapours.	(v) Red Lead

Ans:

Column A (Observation)	Match	Column B (Compound)
(a) Blue salt changes to white and then black.	(iv)	Copper sulphate (CuSO4​⋅5H2​O)
(b) Orange coloured compounds change to green.	(i)	Ammonium dichromate ((NH4​)2​Cr2​O7​)
(c) The red compound changes to brown and then yellow.	(v)	Red Lead (Pb3​O4​)
(d) White to yellow when hot and white when cold.	(iii)	Zinc Nitrate (Decomposes to Zinc Oxide)
(e) Violet solid changes to violet vapours.	(ii)	Iodine (I2​)

Question 3. 

1. How is a flame test performed? 

2. How will you distinguish sodium chloride, potassium chloride, and calcium chloride? 3. How will you distinguish between soft water and hard water? 

4. How will you distinguish between temporary hard water and permanent hard water?

Ans:

1. How is a flame test performed?
A flame test is performed to identify the presence of certain metal ions based on the characteristic color they impart to a flame. First, a clean platinum or nichrome wire is prepared by dipping it in concentrated hydrochloric acid and then heating it in the non-luminous (blue) part of a Bunsen burner flame until it no longer colors the flame. This cleaning process is repeated until the flame remains colorless. Once clean, the wire’s tip is dipped into the concentrated acid and then into the solid sample to be tested, picking up a tiny amount. The coated wire is then introduced into the hot, blue flame. The metal ion in the sample volatilizes and excites, emitting light of a specific color. Observing this characteristic color, such as golden yellow for sodium or lilac for potassium, allows for the identification of the metal cation present.

2. How will you distinguish sodium chloride, potassium chloride, and calcium chloride?
These three chloride salts can be effectively distinguished using the flame test. A small sample of each compound is subjected to the flame test procedure separately. Sodium chloride will produce an intense and persistent golden-yellow flame. Potassium chloride will impart a lilac (pale violet) color to the flame, which is often best observed through cobalt blue glass that filters out the yellow sodium emission if any impurity is present. Calcium chloride will give a brick-red or reddish-orange flame. In addition to the flame test, calcium chloride can be further confirmed by dissolving a portion in distilled water and adding ammonium oxalate solution, which will produce a white precipitate of calcium oxalate. Sodium and potassium salts do not form this precipitate.

3. How will you distinguish between soft water and hard water?
Soft water and hard water are distinguished using a simple soap test. Take two separate, clean test tubes or beakers. Add an equal volume of the water sample to be tested into each container. To both, add an equal and measured amount of a good quality soap solution (like a liquid soap). Shake or stir both vigorously for the same amount of time. Soft water will produce a rich, stable lather or foam almost immediately with very little scum. Hard water, on the other hand, will initially form a thick, curdy white precipitate (scum) instead of lather. This scum forms because the calcium and magnesium ions in hard water react with the soap. Only after all these ions have precipitated out as scum will further addition of soap produce a lather, requiring significantly more soap compared to soft water.

4. How will you distinguish between temporary hard water and permanent hard water?
To distinguish temporary hardness from permanent hardness, the boiling test followed by the soap test is used. First, take two equal volumes of the hard water sample. Boil one portion vigorously for about 5-10 minutes and then allow it to cool. The second portion is left unboiled. Now, perform the soap test on both. If the water is temporarily hard, boiling will have removed the hardness by decomposing dissolved calcium or magnesium bicarbonate into insoluble carbonate, which precipitates out. Therefore, the boiled sample will form a good lather with soap much more easily than the unboiled sample. If the water is permanently hard, boiling has no effect on the dissolved chlorides or sulfates of calcium and magnesium. Consequently, both the boiled and unboiled samples will behave identically, forming scum and requiring a large amount of soap to lather, showing no improvement after boiling. Permanent hardness can only be removed by chemical methods like adding washing soda (sodium carbonate).

Question 4. 

1. What do you understand about Temporary hardness? 

2. What do you understand about soft water? 

3. What do you understand by permanent hardness? 

4. How the temporary and permanent hardness is removed?

Ans:

1. Understanding Temporary Hardness

Temporary hardness refers to the presence of dissolved minerals, specifically calcium and magnesium bicarbonate, in water. This type of hardness is called “temporary” because it can be reduced or eliminated simply by boiling the water. When heated, the soluble bicarbonates decompose into insoluble carbonates, which precipitate out as a solid scale (like the white crust inside kettles). This process removes the minerals that cause the hardness. Water with temporary hardness still lathers poorly with soap and leads to scale formation in pipes and appliances, but this issue can be addressed through heating.

2. Understanding Soft Water

Soft water is water that contains very low concentrations of calcium and magnesium ions. It produces a rich lather easily with soap without forming scum, and it does not create significant limescale deposits in plumbing systems, water heaters, or kettles. Soft water can occur naturally in areas where the water supply flows over hard, impervious rocks like granite, or it can be produced artificially by removing hardness minerals from hard water through various treatment processes. While beneficial for cleaning and appliance longevity, very soft water can sometimes be more corrosive to metal pipes and may have a slightly different taste due to higher sodium content if produced via ion-exchange softening.

3. Understanding Permanent Hardness

Permanent hardness is caused by the presence of dissolved calcium and magnesium in the form of chlorides and sulfates. Unlike temporary hardness, this type of hardness cannot be removed by simply boiling the water. The minerals remain in solution even after heating. This persistent hardness interferes with soap’s ability to lather effectively and also contributes to scale buildup, but the scale forms through evaporation rather than thermal decomposition. Permanent and temporary hardness often coexist in a water supply, combining to give the “total hardness.”

4. Removal of Temporary and Permanent Hardness

The methods for removing hardness differ due to the chemical nature of the dissolved minerals.

• Removing Temporary Hardness:

1. Boiling: The simplest method. Heating the water causes the soluble calcium/magnesium bicarbonate to decompose into insoluble carbonate, which settles out.
2. Clark’s Process (Lime Treatment): Adding calculated amounts of slaked lime (calcium hydroxide) to the water. The lime reacts with the bicarbonates to form insoluble calcium carbonate, which precipitates.

• Removing Permanent Hardness:

1. Ion-Exchange Process: This is the most common domestic and industrial method. Water is passed through a column containing a resin saturated with sodium ions. The resin swaps these sodium ions for the calcium and magnesium ions in the water, effectively removing them. The resin is later regenerated with a brine (salt) solution.
2. Adding Washing Soda (Sodium Carbonate): Used in laundry. The carbonate ions from washing soda react with the calcium and magnesium ions to form insoluble carbonates that precipitate out, preventing them from interfering with the detergent.
3. Distillation: Boiling water and condensing the steam produces pure, soft water, but this is energy-intensive and not practical for large-scale supply.
4. Reverse Osmosis: Forcing water through a semi-permeable membrane under pressure, which filters out the hardness ions along with other contaminants.

To remove both types of hardness simultaneously (total hardness) from a water supply, the ion-exchange process is the most effective and widely used technique.

Question 5. 

1. What are soaps and detergents? 

2. Why do they differ in their actions? 

3. Explain their cleansing action?

Ans:

1. What are Soaps and Detergents?

Soaps and detergents are chemical cleansing agents used to remove dirt, grease, and oils from surfaces, most commonly fabrics and skin. While they share a common purpose, their origins and chemical makeups differ.

• Soaps are natural, biodegradable cleaners produced through a centuries-old process called saponification. This involves a chemical reaction between a fat or oil (like olive oil or tallow, which are triglycerides) and a strong alkali (like sodium hydroxide or lye). The result is a salt of a fatty acid—soap molecules—alongside glycerol. Traditional bar soaps, Castile soap, and some liquid hand soaps are examples.
• Detergents are synthetic, man-made cleansing agents developed in the 20th century. They are crafted from petrochemical derivatives or other raw materials. Their key component is a surfactant (surface-active agent) that is engineered for specific cleaning tasks. Detergents encompass a wide range of products, including laundry powders, dishwasher tablets, shampoos, and most liquid laundry detergents.

2. Why Do They Differ in Their Actions?

The fundamental difference in their action stems from their molecular structure and how they interact with minerals in water, particularly hard water.

• Soap in Hard Water: Hard water contains high concentrations of dissolved calcium and magnesium ions. Soap molecules react with these ions to form an insoluble, greasy substance called “scum.” This scum creates two problems: it reduces the soap’s cleaning power as the active molecules are wasted, and it leaves a filmy residue on clothes (making them stiff) and on bathtubs. Soaps also tend to be less effective in acidic water.
• Detergents in Hard Water: The surfactants in modern detergents are intentionally designed to avoid this reaction. Their molecular “head” is often a sulfonate or sulfate group, which does not form insoluble precipitates with calcium or magnesium ions. Consequently, detergents lather and clean efficiently in hard, soft, cold, or even acidic water without forming scum. This makes them universally effective for household cleaning.

In essence, soaps are simple salts of fatty acids that struggle with hard water, while detergents are complex, engineered surfactants built to perform under a wide variety of water conditions.

3. Explaining Their Cleansing Action

Despite their differences, both soaps and detergents clean using the same fundamental mechanism, driven by their unique molecular structure. Each molecule has a dual nature:

1. A hydrophilic (“water-loving”) head that is attracted to water molecules.
2. A hydrophobic (“water-fearing”) tail that is repelled by water but attracted to oil, grease, and dirt (which are generally non-polar substances).

The cleansing process, known as emulsification, unfolds in steps:

• Lowering Surface Tension: When added to water, the surfactant molecules orient themselves at the water’s surface, weakening the attraction between water molecules. This lowers water’s surface tension, allowing it to wet the fabric or surface more thoroughly.
• Surrounding the Grease: The hydrophobic tails begin to seek out and embed themselves into the oily dirt droplet, while the hydrophilic heads remain pointed outward into the surrounding water.
• Formation of Micelles: Eventually, the molecules completely surround the grease, forming spherical structures called micelles. The grease is now trapped in the center of these spheres, shielded by the water-loving heads on the outside.
• Lifting and Suspension: The micelle, with its water-soluble exterior, is now able to be lifted away from the surface of the fabric. It becomes suspended in the water, forming an emulsion.
• Rinsing Away: Agitation (like hand-scrubbing or the tumbling of a washing machine) helps break the grease into smaller droplets for micelles to form. Finally, rinsing with clean water washes the suspended micelles away, leaving the surface clean.

Question 6. 

Compare the effect of soaps and detergents on hard water.

Ans:

How Hard Water Causes Problems
Hard water contains dissolved minerals, mainly calcium and magnesium. When you try to clean with it, these minerals react with certain cleaning agents, changing how well they work.

Effect on Soaps
Soaps are made from natural fats or oils and an alkali. In hard water, the calcium and magnesium ions bind with the soap molecules to form an insoluble, sticky substance often called “soap scum.” You might see it as a gray film on clothes, a ring around the bathtub, or a cloudy layer on dishes.

Because of this reaction:

• A lot of soap is wasted just to overcome the minerals before it can start cleaning.
• It doesn’t lather well until the minerals are neutralized.
• The leftover scum leaves residues on fabrics, making them feel stiff and look dull.

Effect on Synthetic Detergents
Modern detergents are manufactured from chemical compounds. Their molecules are engineered to not react with calcium and magnesium ions.

As a result:

• They lather and clean effectively in hard water right away, with no initial waste.
• They do not form soap scum, so there’s no bathtub ring or stiff residue on laundry.
• Many contain added “builders” that actively soften the water by trapping the minerals, which boosts cleaning power.

Practical Outcome
If you have hard water, using traditional bar soap or soap flakes for laundry or dishes is inefficient. You’ll use more product, get poor results, and deal with stubborn residues. A synthetic detergent—whether liquid, powder, or pod—will perform consistently, clean thoroughly, and generally make the task easier.

Question 7. 

Copy and complete the following table which refers to the action of heat on some carbonates:

Carbonate	Colour of residue on cooling
Zinc Carbonate
Lead carbonate
Copper carbonate

Ans:

Carbonate	Chemical Equation (Decomposition)	Colour of Residue on Heating (Hot)	Colour of Residue on Cooling
Zinc Carbonate (ZnCO3​)	ZnCO3​Heat​ZnO+CO2​	Yellow	White
Lead Carbonate (PbCO3​)	PbCO3​Heat​PbO+CO2​	Dark Yellow/Brown	Yellow (Lemon Yellow)
Copper Carbonate (CuCO3​)	CuCO3​Heat​CuO+CO2​	Black	Black

Question 8. 

1. Identify the following substance: An alkaline gas A which gives dense white fumes with hydrogen chloride. 

2. Identify the following substance: Gas B that has an offensive smell as rotten eggs. 

3. Identify the following substance: Gas C that is colourless and can be used as a bleaching agent. 

4. Identify the following substance: A brown gas D with the irritating smell.

Ans:

1. Ammonia (NH₃)
2. Hydrogen sulfide (H₂S)
3. Chlorine (Cl₂)
4. Nitrogen dioxide (NO₂)

Question 9. 

Complete the following table and write your observations.

	Hydrogen sulphide	Ammonia	Sulphur dioxide	Hydrogen chloride
Shake the gas with red litmus solution
Shake the gas with blue litmus solution
Apply a burning splint to the gas

Ans:

Test / Property	Hydrogen Sulphide (H2​S)	Ammonia (NH3​)	Sulphur Dioxide (SO2​)	Hydrogen Chloride (HCl)
Shake the gas with red litmus solution	No Change (Gas is slightly acidic but very weak acid)	Turns Blue (Gas is basic/alkaline)	No Change (Gas is acidic)	No Change (Gas is acidic)
Shake the gas with blue litmus solution	Turns Red (Forms weak acid, H2​S)	No Change (Gas is basic)	Turns Red (Forms weak acid, H2​SO3​)	Turns Red (Forms strong acid, HCl)
Apply a burning splint to the gas	Burns with a blue flame and produces SO2​ (sulfur dioxide)	Does not burn	Does not burn	Does not burn