Frank Brothers Solutions for ICSE Class 9 Chemistry Chapter 11 Practical Work

PAGE NO :170

Question 1:
Answer: On heating certain solids they don’t melt and directly get converted to their gaseous form, this process of direct conversion of solids to their gaseous form is called as sublimation. For ex: camphor and ammonium chloride undergo sublimation.
This happens because the substance gains enough energy to skip the liquid phase entirely and move straight into a vapor state. You can observe this clearly when solid "dry ice" turns into a foggy gas at room temperature without leaving a wet puddle.
Teacher's Tip: Sublimation is like a "Shortcut" for solids to become gases.
Exam Tip: Mention both "heating" and "direct conversion" in your definition to get full marks.

Question 2:
Answer: Decrepitation is a process of breaking up of solid particles on heating, which makes a crackling sound. Lead nitrate decrepitate on heating.
Pb(NO₃)₂ → {Delta} 2PbO + 4NO₂ + O₂
Lead nitrate
This crackling sound occurs as the internal water or air trapped inside the crystal structure expands and bursts the crystal apart. It is a physical-chemical reaction that shows the intense energy being applied to the solid during decomposition.
Teacher's Tip: Think of "Decrepitation" as "Popcorn Science" because of the distinct crackling sound.
Exam Tip: If asked for an example of decrepitation, always write "Lead Nitrate" as it is the most common laboratory example.

Question 3:
Answer: (a) Zn(NO₃)₂ = white
(b) Cu(NO₃)₂ = blue
(c) CuSO₄ . 5H₂O = blue
(d) CuSO₄ = pale green
(e) K₂Cr₂O₇ = orange
(f) PbS = black
Transition metal salts often have bright, distinct colors that help chemists identify them just by looking at the container. For instance, Copper salts are famous for their beautiful shades of blue and green depending on their chemical environment.
Teacher's Tip: Copper compounds are almost always "Cool Colors" like blue or green.
Exam Tip: Note the difference between hydrated (blue) and anhydrous copper sulphate; examiners often test this specific color change.

Question 4:
Answer:
1. NH₃ (Ammonia): The gas is colourless and pungent odour it turns blue litmus red and it produces white dense fumes when a rod dipped in conc HCl is brought near the gas.
Cl₂ (Chlorine): The gas is greenish yellow and has a pungent smell
2. CO₂ (Carbon Dioxide): It is odourless and colourless, it turns lime water milky and on passing excess of gas through it the milkiness disappears.
SO₂(Sulphur Dioxide): It is colourless and smells like burning sulphur it turns orange potassium dichromate paper green.
3. H₂S (Hydrogen Sulphide): It is colourless and smells like rotten eggs, it turns lead acetate paper black.
SO₂(Sulphur Dioxide): Colourless and it smells like burning sulphur it turns orange potassium dichromate paper green.
4. H₂ (Hydrogen): It will extinguish a lighted splinter and the gas burns with a ‘pop’ sound.
O₂ (Oxygen): It will relight a glowing splinter and absorbed by an alkaline solution of pyrogallol to turn it dark brown.
Identifying gases is a fundamental part of chemistry where we use chemical "fingerprints" to know what was produced. Each gas has a unique set of sensory clues and chemical reactions that make it stand out from others.
Teacher's Tip: If you hear a "Pop" sound, you've definitely found Hydrogen!
Exam Tip: When describing the lime water test for CO₂, always mention that the "milkiness disappears" if excess gas is passed.

Question 5:
Answer:
1. Hard water: By stiring hard water thoroughly with ordinary soap we get scum.
Soft water: By stiring hard water thoroughly with ordinary soap we get foam or lather formation.
2. Temporary hardness: Addition of soap to the boiled water with temporary hardness makes the water soft and then it gives lather easily.
Permanent hardness: Addition of soap to the boiled water with permanent hardness does not make the water soft and its hardness still persist.
3. Soaps: It do not form lather with hard water.
Detergents: It can form lather with hard water.
Hard water contains dissolved minerals like calcium and magnesium which prevent soap from making bubbles. Soft water is much cleaner and allows soap to create a rich, foamy lather immediately, making it better for washing.
Teacher's Tip: Hard water = Scum (bad), Soft water = Suds (good).
Exam Tip: Distinguish clearly between temporary (removed by boiling) and permanent (needs chemicals) hardness for full marks.

Question 6:
Answer:
a. Chemical test to verify that given piece of metal is zinc not copper:
Take a small quantity of the given piece of metal in a clean and dry test tube. To that 10{ ml} of dilute sulphuric acid is added ,if effervescence are observed and a colourless and odourless gas which burns with a pop sound when a burning splinter is brought near its mouth and some water droplets are formed then the metal is zinc not copper.
b. Chemical test to distinguish between Na₂S and Na₂CO₃.
Na₂S: Take a small quantity of the given piece of metal in a clean and dry test tube. To that 10{ ml} of dilute sulphuric acid is added, a colourless gas is released smelling of rotten egg. The gas released is H₂S.
Na₂S + H₂SO₄ → Na₂SO₄ + H₂S
This gas turns lead acetate paper black.
H₂S + Pb(CH₃COO)₂ → PbS + 2CH₃COOH
Lead acetate (black)
Na₂CO₃: Take a small quantity of the given piece of metal in a clean and dry test tube. To that 10{ ml} of dilute sulphuric acid is added, a colourless gas and odourless gas is released. The gas released is CO₂.
Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂
This gas turns lime water milky.
CO₂+ Ca(OH)₂ → CaCO₃ + H₂O
Lime water (milky)
c. Flame test:
1. Clean the free end of the platinum wire fused in a glass rod, by repeatedly heating it in a flame and again dipping it in conc. hydrochloric acid taken in a watch glass.
2. Place it on non luminous part of the flame if it imparts colour then dip it in HCl again, until it fails to impart colour to the flame.
3. Touch the tip of clean platinum wire with conc. HCl and little substance, and place it in non luminous part of Bunsen burner flame.
4. The colour of the flame is observed by the naked eye, with sodium it shows a golden yellow flame.
These laboratory tests allow us to distinguish between substances that might look identical to the naked eye. The flame test is particularly useful because different metals glow with unique, vivid colors when heated, acting like a chemical rainbow.
Teacher's Tip: Sodium is the "Sun King" because it always burns with a bright golden yellow flame.
Exam Tip: For the flame test, remember to use "Concentrated Hydrochloric Acid" to clean the platinum wire properly to avoid contamination.

Question 7:
Answer: Effect of heat on:
a. Copper carbonate:
CuCO₃ → {Delta} CuO + CO₂
b. Washing soda:
Na₂CO₃ . 10H₂O → {Delta} Na₂CO₃ + 10H₂O
c. Lead nitrate:
Pb(NO₃)₂→ {Delta} 2PbO + 4NO₂ + O₂
d. Ammonium chloride:
NH₄Cl → {Delta} NH₃ + HCl
Applying heat to compounds causes them to decompose or break down into simpler substances. This often releases gases like Oxygen or Carbon Dioxide which we can then capture and test to verify the original substance.
Teacher's Tip: Heating Carbonates usually releases Carbon Dioxide gas.
Exam Tip: Always use the Delta symbol (Delta) over the reaction arrow to signify that heat is being applied in your equations.

Question 8:
Answer: (a) Rotten egg
(b) Burning sulphur
(c) Nitrogen dioxide
(d) Alkali
(e) Ca(OH)₂
(f) Lilac
(g) Brick-red
These descriptors are the "sensory clues" used in chemistry labs to identify reactions. For example, H₂S gas is immediately recognizable by its distinct and unpleasant smell of old eggs, while certain metals create unique flame colors.
Teacher's Tip: Potassium salts always produce a "Lilac" (light purple) flame.
Exam Tip: Use specific color terms like "Brick-red" for Calcium rather than just saying "red" to show precision.

Question 9:
Answer: Point sources of water pollution: Sources of water pollution that discharge directly into the water source are called as point sources of water pollution. For ex: factories.
Non point sources of water pollution: sources of water pollution that remain scattered and don’t have specific locations for the discharge of pollutants into particular water bodies are called as non point sources of water pollution. For ex: run off from fields.
Point source pollution is easier to control because you can put a filter directly on the specific pipe where the waste comes out. Non-point source pollution is much harder to stop because it comes from vast areas like farming communities or city streets.
Teacher's Tip: Point = One specific Pipe. Non-Point = Everywhere at once.
Exam Tip: Give at least one clear example for each type of source to show you understand the environmental impact.

Question 10:
Answer: Parameters which should be determined to test quality of water are:
1. Hardness: It should be between 200-600mg
2. pH: It should be between 7.00-8.00
3. Specific conductance: For 25°C is 300 × 10^-6 mhos
These scientific measurements ensure that the water we drink is safe and won't make us sick or damage our industrial machinery. A pH around 7 means the water is neutral, which is generally the healthiest state for human consumption.
Teacher's Tip: Safe drinking water is usually "almost neutral" (around pH 7).
Exam Tip: Memorize the safe pH range of 7.0 to 8.0 as it is a common question in water quality tests.