Chapter 7 · Solved paper40 marks

Model Test – Solved ഉത്തരങ്ങൾ

Every question of the model paper, in the same order, with the full answer, the marking scheme, an animated step-by-step solution for the longer questions, and a link to the page where the idea is taught. Try each one again before you open the answer, and mark your own paper honestly.

📐 How the solutions are drawn

Atoms have fixed colours: carbon grey, oxygen red, nitrogen blue, chlorine green, hydrogen white. Under every equation an atom count lists each element on the left and the right: a ✔ in every row means the equation is balanced. In the bar charts, heat = mass × calorific value.

Three questions decide almost every answer: Which gas, and what is its test? (pop – flare – milky – bleach) · Are the atoms equal on both sides? · How are the atoms joined? (four bonds or three, single or triple).

SECTION AAnswer all · 8 × 1 = 8 marks

Which of these is the test for oxygen?

(a) clear lime water turns milky(b) the gas burns with a “pop” at a burning splinter(c) a glowing splinter bursts into flame(d) a moist coloured cloth loses its colour

(1 mark)

💡 Which gas helps things burn?

✅ Answer: (c) a glowing splinter bursts into flame

Oxygen does not burn itself but helps burning – it is a supporter of combustion (ജ്വലനസഹായി), so a glowing splinter bursts into flame. (a) is the test for carbon dioxide, (b) for hydrogen, (d) is the bleaching action of chlorine.

Marking: 1 mark for (c).

📖 Review this on Page 8 · Oxygen & ozone · Page 2 · Gases, apparatus & tests

Different forms of the same element that have different physical properties are called .

(1 mark)

💡 This is the key box of the carbon section.

✅ Answer: allotropes (രൂപാന്തരങ്ങൾ)

Diamond, graphite, graphene and fullerene are allotropes of carbon; the phenomenon is allotropy (രൂപാന്തരത്വം).

Marking: 1 mark.

📖 Review this on Page 5 · Carbon & allotropes

Find the relation and fill in: washing soda : Na₂CO₃·10H₂O :: baking soda :

(1 mark)

💡 One is a carbonate, the other a bicarbonate.

✅ Answer: NaHCO₃ (sodium bicarbonate)

Washing soda (അലക്കുകാരം) is sodium carbonate, Na₂CO₃·10H₂O; baking soda (അപ്പക്കാരം), used for appam, is sodium bicarbonate, NaHCO₃. Both are made using carbon dioxide.

Marking: 1 mark.

📖 Review this on Page 6 · CO₂, CO & carbonates

Nitrogen does not react easily at ordinary conditions because

(a) it is lighter than air(b) its molecule has a strong triple bond(c) it is colourless and odourless(d) it makes up 78 % of air

(1 mark)

💡 Think of the bond between the two atoms.

✅ Answer: (b) its molecule has a strong triple bond

In N₂ the two atoms share three pairs of electrons (N≡N). This bond is very strong and hard to break, so nitrogen is inert (നിഷ്ക്രിയം).

Marking: 1 mark for (b).

📖 Review this on Page 7 · Nitrogen

Pick the odd one out and give the reason: diamond, graphite, lamp black, fullerene

(1 mark)

💡 Recall the two branches of Fig 7.3.

✅ Answer: lamp black – it is amorphous; the others are crystalline

Diamond, graphite and fullerene are crystalline (ക്രിസ്റ്റലീയ) allotropes of carbon. Lamp black (വിളക്കുകരി) is amorphous (അമോർഫസ്), like charcoal, coke and coal.

Marking: Answer ½, reason ½.

📖 Review this on Page 5 · Carbon & allotropes

The most abundant element in the Earth’s crust is

(a) hydrogen(b) nitrogen(c) oxygen(d) carbon

(1 mark)

💡 Rocks and water are full of it.

✅ Answer: (c) oxygen

Oxygen is the most abundant element in the Earth’s crust (hydrogen is the most abundant in the universe; nitrogen is the largest part of air).

Marking: 1 mark for (c).

📖 Review this on Page 8 · Oxygen & ozone

Compounds of chlorine, fluorine and carbon that once were used in refrigerators and ACs, and that damage the ozone layer, are called .

(1 mark)

💡 Their name lists their three elements.

✅ Answer: chlorofluorocarbons (CFCs) (ക്ലോറോഫ്ലൂറോകാർബണുകൾ)

CFCs reach the stratosphere, break up in UV light and release chlorine, which destroys ozone.

Marking: 1 mark.

📖 Review this on Page 8 · Oxygen & ozone

Carbogen (കാർബൊജൻ), used for artificial respiration, is a mixture of

(a) 95 % O₂ and 5 % CO₂(b) 95 % CO₂ and 5 % O₂(c) 78 % N₂ and 22 % O₂(d) CO and H₂

(1 mark)

💡 It is mostly the gas we need for breathing.

✅ Answer: (a) 95 % O₂ and 5 % CO₂

Carbogen is 95 % oxygen and 5 % carbon dioxide. (CO + H₂ is water gas.)

Marking: 1 mark for (a).

📖 Review this on Page 6 · CO₂, CO & carbonates

SECTION BAnswer all · 5 × 2 = 10 marks

Complete and balance: (a) Zn + HCl → ZnCl₂ +
(b) H₂ + Cl₂ → (in sunlight). Name the type of reaction in (b).

(2 marks)

💡 Count the hydrogen and chlorine atoms on both sides.

🎬 Step by step

✅ Answer: (a) Zn + 2HCl → ZnCl₂ + H₂ (b) H₂ + Cl₂ sunlight 2HCl – combination reaction

(a) Zinc displaces hydrogen from the acid: Zn + 2HCl → ZnCl₂ + H₂. Count: Zn 1 = 1, H 2 = 2, Cl 2 = 2 – balanced.

(b) H₂ + Cl₂ sunlight 2HCl. H 2 = 2, Cl 2 = 2. Two substances combine to give one – a combination reaction.

Marking: (a) product ½, balancing ½; (b) equation ½, type ½.

📖 Review this on Page 3 · Hydrogen · Page 2 · Gases, apparatus & tests

10.

Hydrogen is made in industry by passing electricity through water. (a) Write the balanced equation for the electrolysis of water.
(b) Give one use of each gas formed.

(2 marks)

💡 Water splits into its two elements.

🎬 Step by step

✅ Answer: (a) 2H₂O electrolysis 2H₂ + O₂ (b) hydrogen – making ammonia (or fuel, vanaspati); oxygen – artificial respiration (or welding, rocket fuel)

(a) Electrolysis of water (ജലത്തിന്റെ വൈദ്യുതവിശ്ലേഷണം): 2H₂O electrolysis 2H₂ + O₂. H 4 = 4, O 2 = 2.

(b) Hydrogen: making ammonia and methanol, as a fuel, making saturated oils such as vanaspati, as a reducing agent. Oxygen: artificial respiration in hospitals, oxy-acetylene welding, as the oxidiser in rocket fuels.

Marking: (a) 1 mark (formulas ½, balancing ½); (b) ½ + ½.

📖 Review this on Page 3 · Hydrogen · Page 8 · Oxygen & ozone

11.

(a) How does the ozone layer in the stratosphere protect living things?
(b) How do CFCs damage it? Name one substitute used in new refrigerators.

(2 marks)

💡 What does UV do to living cells, and what does chlorine do to ozone?

🎬 Step by step

✅ Answer: (a) it absorbs harmful UV rays (b) CFCs release chlorine which destroys ozone; substitute – HFCs (or liquid hydrocarbons, ammonia, solid CO₂)

(a) Ozone absorbs the harmful ultraviolet radiations (അൾട്രാവയലറ്റ് വികിരണങ്ങൾ) of the Sun. In the stratosphere UV splits O₂ (O₂ UV 2O) and the atoms join O₂ to form ozone (O₂ + O → O₃); ozone absorbs UV and changes back to oxygen. This cycle keeps the ozone steady and stops the UV.

(b) CFCs reach the stratosphere, break up and release chlorine, which destroys ozone – ozone layer depletion (ഓസോൺ പാളിയുടെ ശോഷണം). Substitutes: HFCs, liquid hydrocarbons, ammonia, solid carbon dioxide.

Marking: (a) 1 mark; (b) chlorine ½, substitute ½.

📖 Review this on Page 8 · Oxygen & ozone

12.

During a monsoon thunderstorm, lightning makes the nitrogen and oxygen of air combine. (a) Complete: N₂ + O₂ → ; 2NO + O₂ →
(b) How do these products finally help the paddy fields?

(2 marks)

💡 Follow the nitrogen from the air to the roots.

🎬 Step by step

✅ Answer: (a) 2NO; 2NO₂ (b) NO₂ forms nitric acid in rain water → nitrates in the soil, which plants absorb

(a) N₂ + O₂ lightning 2NO and 2NO + O₂ → 2NO₂.

(b) Nitrogen dioxide dissolves in rain water in the presence of oxygen and forms nitric acid: 4NO₂ + 2H₂O + O₂ → 4HNO₃. The acid reaches the soil, reacts with its minerals and forms nitrates – plants absorb these as their nitrogen nutrient.

Marking: (a) ½ + ½; (b) nitric acid ½, nitrates absorbed by plants ½.

📖 Review this on Page 7 · Nitrogen

13.

Give reasons: (a) Hydrogen-filled balloons at a temple festival rise up, but sellers must keep them away from flames.
(b) Nitrogen in air is said to help control combustion.

(2 marks)

💡 Density – and what each gas does to a flame.

🎬 Step by step

✅ Answer: (a) hydrogen is less dense than air; it burns explosively (b) it does not support burning and dilutes the oxygen

(a) Hydrogen is less dense than air, so the balloons float up. Hydrogen burns explosively with air, so a flame near the balloons is dangerous.

(b) Nitrogen (78 % of air) neither burns nor supports burning. It dilutes the oxygen, so things burn steadily, not violently; in pure oxygen they would burn far too fiercely.

Marking: (a) ½ + ½; (b) 1 mark.

📖 Review this on Page 3 · Hydrogen · Page 7 · Nitrogen

SECTION CAnswer all · 4 × 3 = 12 marks

14.

The graph shows the calorific values (kJ/g) of some fuels (Fig 7.2). (a) Which fuel releases the most heat per gram, and which the least?
(b) How much heat is released when 2 g of hydrogen and 2 g of methane burn completely? How much more does hydrogen give?
(c) A scooter needs 480 kJ for a short trip. How many grams of petrol would give this heat? How many grams of ethanol?

(3 marks)

💡 Heat = mass × calorific value; mass = heat ÷ calorific value.

🎬 Step by step

✅ Answer: (a) most – hydrogen; least – carbon monoxide (b) 286 kJ and 112 kJ; 174 kJ more (c) 10 g of petrol, 16 g of ethanol

Calorific value (കലോറിക മൂല്യം) = heat energy released when unit mass (1 g) of a fuel burns completely. So heat = mass × calorific value.

(a) Most: hydrogen, 143 kJ/g. Least: carbon monoxide, 10 kJ/g.

(b) Hydrogen: 2 × 143 = 286 kJ. Methane: 2 × 56 = 112 kJ. Difference: 286 − 112 = 174 kJ more from hydrogen.

(c) Petrol: 480 ÷ 48 = 10 g. Ethanol: 480 ÷ 30 = 16 g – more fuel is needed because its calorific value is lower.

Marking: (a) ½ + ½; (b) ½ for both heats, ½ for the difference; (c) ½ + ½.

📖 Review this on Page 4 · Hydrogen as a fuel

15.

Figures A and B show how the carbon atoms are joined in two allotropes of carbon. (a) Identify A and B. To how many other carbon atoms is each carbon joined in each?
(b) Explain from the structure why diamond is used to cut glass but graphite is used as a lubricant.
(c) Equal masses of diamond and graphite, burnt in oxygen, give only carbon dioxide. What does this show? Write the equation.

(3 marks)

💡 Count the bonds around one atom in each picture.

🎬 Step by step

✅ Answer: (a) A – diamond, 4; B – graphite, 3 (b) diamond: rigid 3-D network of strong bonds; graphite: layers held by weak forces slide (c) both are pure carbon – allotropes; C + O₂ → CO₂

(a) A – diamond (വജ്രം): each carbon is covalently bonded to 4 other carbon atoms in a 3-D network. B – graphite (ഗ്രാഫൈറ്റ്): each carbon is bonded to 3 others in flat layers of hexagons.

(b) In diamond every atom is held by four strong covalent bonds in all directions, so it is the hardest natural substance and can cut glass. In graphite the layers are held together only by weak forces, so they slide over one another – it is soft and slippery, a lubricant (സ്നേഹകം).

(c) Both contain only carbon atoms – they are allotropes of the same element, differing only in how the atoms are arranged: C + O₂ → CO₂.

Marking: 1 mark for each part ((a) names ½, numbers ½).

📖 Review this on Page 5 · Carbon & allotropes

16.

Three gas jars P, Q and R contain hydrogen, oxygen and carbon dioxide, but they are not labelled. The tests and results are given.

Jar	Test	What is seen
P	a burning splinter is brought to the mouth	the gas burns with a “pop” sound
Q	a glowing splinter is put in	the splinter bursts into flame
R	the gas is shaken with clear lime water	the lime water turns milky

(a) Identify P, Q and R.
(b) Write the balanced equation for preparing the gas in Q in the laboratory from potassium permanganate.
(c) Jars of two of these gases are kept with the mouth up, but the jar of one gas is kept mouth down. Which one, and why?

(3 marks)

💡 Pop – flare – milky.

🎬 Step by step

✅ Answer: (a) P hydrogen, Q oxygen, R carbon dioxide (b) 2KMnO₄ heat K₂MnO₄ + MnO₂ + O₂ (c) P (hydrogen) – it is less dense than air

(a) The “pop” (പോപ്പ് ശബ്ദം) shows P – hydrogen; the glowing splinter bursting into flame shows Q – oxygen; lime water (ചുണ്ണാമ്പുവെള്ളം) turning milky shows R – carbon dioxide.

(b) 2KMnO₄ heat K₂MnO₄ + MnO₂ + O₂. K 2 = 2, Mn 2 = 1 + 1, O 8 = 4 + 2 + 2.

(c) Hydrogen (P) is less dense than air: in a jar standing mouth up it would rise out and escape, so its jar is kept mouth down. Oxygen and carbon dioxide are denser than air and stay in a jar kept mouth up.

Marking: 1 mark for each part ((a) ½ if only two are right).

📖 Review this on Page 2 · Gases, apparatus & tests · Page 3 · Hydrogen · Page 8 · Oxygen & ozone · Page 6 · CO₂, CO & carbonates

17.

(a) A family slept in a closed room with a charcoal stove burning. Name the poisonous gas that may form, write the equation for its formation, and say why it is dangerous.
(b) Which two properties of carbon dioxide make it useful in fire extinguishers?
(c) The solution of a white powder X gives a white precipitate with barium chloride solution; the precipitate dissolves in dilute hydrochloric acid with effervescence. What kind of salt is X? Give one example used at home.

(3 marks)

💡 Too little oxygen gives which oxide of carbon?

🎬 Step by step

✅ Answer: (a) carbon monoxide; 2C + O₂ → 2CO; forms carboxyhaemoglobin (b) does not support combustion; denser than air (c) a carbonate – washing soda

(a) With little oxygen the carbon burns incompletely to carbon monoxide: 2C + O₂ → 2CO. CO joins haemoglobin to form carboxyhaemoglobin (കാർബോക്സീഹീമോഗ്ലോബിൻ), so the blood carries less oxygen – it can cause death. Never burn charcoal in a closed room.

(b) Carbon dioxide does not support combustion and is denser than air, so it settles over the fire like a blanket and cuts off the oxygen.

(c) The white precipitate (barium carbonate) dissolving with effervescence (CO₂) in dilute HCl shows that X is a carbonate (കാർബണേറ്റുകൾ) – e.g. washing soda, Na₂CO₃·10H₂O.

Marking: 1 mark for each part ((a) gas and equation ½, danger ½).

📖 Review this on Page 6 · CO₂, CO & carbonates

SECTION DAnswer all · 2 × 5 = 10 marks

18.

Read the passage and answer the questions.

After the August 2018 floods, the wells of many homes near Chengannur and Aluva were filled with muddy flood water. Health workers asked families to clean out the wells and then add bleaching powder. Near the treated wells there was a strong, pungent smell. A coloured cloth soaked in a strong bleaching-powder solution slowly lost its colour. At the school laboratory the teacher added silver nitrate solution to a sample of the well water.

(a) Bleaching powder is made by passing dry chlorine over dry slaked lime. Write the balanced equation.
(b) Name the gas that gives the smell. Write two of its physical properties.
(c) Write the equations that show how chlorine and water give nascent oxygen. How does nascent oxygen remove the colour of the cloth?
(d) The silver nitrate gave a curdy white precipitate that dissolved in ammonium hydroxide. What does this show? Write the equation, taking sodium chloride as the salt.
(e) In the laboratory, chlorine from potassium permanganate and concentrated hydrochloric acid is collected by upward displacement of air, inside a fume cupboard. Give a reason for each.

(5 marks)

💡 Chlorine and water give HOCl, and HOCl gives nascent oxygen.

🎬 Step by step

✅ Answer: (a) 2Ca(OH)₂ + 2Cl₂ → Ca(OCl)₂ + CaCl₂ + 2H₂O (b) chlorine – greenish-yellow, pungent smell, denser than air (c) H₂O + Cl₂ → HCl + HOCl; HOCl → HCl + [O]; [O] oxidises the colour (d) a chloride is present; NaCl + AgNO₃ → AgCl + NaNO₃ (e) chlorine is denser than air; it is poisonous

(a) 2Ca(OH)₂ + 2Cl₂ → Ca(OCl)₂ + CaCl₂ + 2H₂O. Ca 2 = 1 + 1, O 4 = 2 + 2, H 4 = 4, Cl 4 = 2 + 2. The product Ca(OCl)₂ is bleaching powder (ബ്ലീച്ചിങ് പൗഡർ).

(b) Chlorine. It is a greenish-yellow gas with a pungent smell and it is denser than air (any two).

(c) H₂O + Cl₂ → HCl + HOCl, then HOCl → HCl + [O]. The hypochlorous acid (ഹൈപ്പോക്ലോറസ് ആസിഡ്) gives nascent oxygen (നവജാത ഓക്സിജൻ), [O]. It oxidises the coloured substances to colourless ones – the cloth fades. The same oxidising action kills germs, so bleaching powder is a disinfectant (അണുനാശിനി). That is why moisture is needed: dry chlorine does not bleach.

(d) A chloride is present: the curdy white precipitate is silver chloride, and it dissolves in NH₄OH. NaCl + AgNO₃ → AgCl + NaNO₃.

(e) Upward displacement of air – chlorine is denser than air, so it sinks to the bottom of the jar and pushes the air out at the top. Fume cupboard – chlorine is poisonous (toxic); the fumes must not be breathed. This is a teacher’s demonstration only.

Marking: 1 mark for each part ((c) equations ½, explanation ½; (d) conclusion ½, equation ½; (e) ½ + ½).

📖 Review this on Page 9 · Chlorine · Page 12 · Applications

19.

Read the passage and answer the questions.

FACT (The Fertilisers and Chemicals Travancore Ltd) at Udyogamandal near Kochi makes ammonia by combining nitrogen with hydrogen, and from it nitrogenous fertilisers for Kerala’s farms. Ramettan, a paddy farmer in Palakkad, uses this fertiliser and also cow-dung manure, and after the harvest he sows payar (green gram) in the same field. His son works at a tyre shop that fills tyres with nitrogen, and the snack packets in the shop are also filled with nitrogen.

(a) Write the balanced equation for making ammonia. From where does the factory get its nitrogen?
(b) Nitrogen needs special conditions in the factory before it reacts. Why does it not react easily?
(c) How does growing payar add nitrogen to the soil of the field?
(d) Give one merit and one limitation each of organic manure and chemical fertiliser. What is the best practice for Ramettan?
(e) Why is nitrogen used to fill tyres and snack packets?

(5 marks)

💡 Air → factory → ammonia → fertiliser; air → Rhizobium → soil.

🎬 Step by step

✅ Answer: (a) N₂ + 3H₂ → 2NH₃; from air (78 % nitrogen) (b) strong triple bond N≡N (c) Rhizobium in root nodules fixes nitrogen (d) Table 7.4; balanced, scientific use (e) inert – does not support burning; keeps out oxygen

(a) N₂ + 3H₂ → 2NH₃. N 2 = 2, H 6 = 6. Nitrogen comes from air, which is 78 % nitrogen; hydrogen can be made by the electrolysis of water.

(b) The two nitrogen atoms are held by a very strong triple bond (N≡N), which is hard to break, so nitrogen is inert (നിഷ്ക്രിയം) at ordinary conditions.

(c) Rhizobium bacteria in the root nodules of pulse plants take nitrogen from the air and change it into compounds the plants can absorb – nitrogen fixation (നൈട്രജൻ സ്ഥിരീകരണം). When the plants decay, these compounds enrich the soil for the next paddy crop.

(d) Organic manure (ജൈവവളം): merit – eco-friendly, keeps the natural character of the soil, less pollution; limitation – gives nutrients slowly, needed in large amounts. Chemical fertiliser (രാസവളം): merit – more yield, gives N, P, K quickly; limitation – increases soil acidity, harms soil life, pollutes. Best practice: a scientific, balanced use of both – manure for soil health plus measured amounts of fertiliser.

(e) Nitrogen is inert: it does not support burning and does not react with the rubber. In snack packets it keeps out oxygen, so the oily snacks do not spoil.

Marking: 1 mark for each part ((a) equation ½, source ½; (d) ½ for the two merits/limitations, ½ for the best practice).

📖 Review this on Page 7 · Nitrogen · Page 3 · Hydrogen · Page 12 · Applications

Marks summary

Section	Questions	Marks each	Total	Topics
A	1–8	1	8	test for oxygen, allotropes, baking soda, inert nitrogen, crystalline and amorphous carbon, oxygen in the Earth’s crust, CFCs, carbogen
B	9–13	2	10	balancing the hydrogen and HCl equations, electrolysis of water and uses, the ozone layer and CFCs, lightning and nitrates, reasons (balloons, nitrogen and combustion)
C	14–17	3	12	calorific values from a chart, diamond and graphite structures and properties, identifying H₂, O₂ and CO₂ by their tests, CO danger, fire extinguishers and the carbonate test
D	18–19	5	10	Kerala case studies: bleaching powder for wells after the 2018 floods (bleaching, chloride test, chlorine safety); FACT fertilisers, nitrogen fixation, organic vs chemical fertilisers
Total			40

⏱️ Time management (15 min cool-off + 90 minutes)

Cool-off (15 min) – read the whole paper. Mark the equations to write or balance (Q9, Q10, Q12, Q15, Q16, Q17, Q18, Q19) and the calculation (Q14).
Section A – about 8 minutes. One word, formula or letter each.
Section B – about 20 minutes. Write the formulas first, then count atoms to balance.
Section C – about 27 minutes. In Q14 write “heat = mass × calorific value”; in Q15 count the bonds round one atom.
Section D – about 25 minutes. Underline every substance named in the passage; answer each part separately.
Checking – keep 10 minutes: is every equation balanced (count each element)? Did you name the test and what is seen? Units (kJ, g) on every number?

🎉 Finished the chapter?

Count your marks: 32 or more – excellent; 24–31 – good, revise the pages linked under the questions you missed; below 24 – go back through pages 3 to 9 with their labs and do the scored quiz again. Then return to the chapter home and tick off what you have learned.