Madhyamik 2019 Physical Science Solution
Complete English version with detailed solutions for WBBSE Class 10 students
About This Solution
This post gives you the full Madhyamik 2019 Physical Science question paper with solution. It is written in a clear and simple way so that students can easily understand and use it for revision.
All the questions and answers are arranged in the same order as in the real exam paper (Groups A, B, C, and D). This helps you find and study each part quickly and easily.
Question Paper Solutions
1.1
Which among the following gases absorbs long wavelength infrared radiation emitted from the earth's surface?
Answer: (c) CH₄ (Methane)
Explanation: Methane is a greenhouse gas that absorbs long-wavelength infrared radiation emitted by the Earth, trapping heat in the atmosphere.
1.2
At STP, 2.24 L is occupied by: [C = 12, O = 16, S = 32]
Answer: (a) 4.4 g CO₂
Explanation: \(22.4 \text{L}\) at STP = \(1 \) mole.
So \(2.24 \text{L} = \frac{1}{22.4} \times 2.24 = 0.1 \) mole.
As, molar mass of CO₂ = 44 g/mol.
Hence at STP 2.24 L is occupied by → 0.1×44 = 4.4 g CO₂
So \(2.24 \text{L} = \frac{1}{22.4} \times 2.24 = 0.1 \) mole.
As, molar mass of CO₂ = 44 g/mol.
Hence at STP 2.24 L is occupied by → 0.1×44 = 4.4 g CO₂
1.3
How many molecules of CO₂ will be produced when 1 mole of C reacts completely with 1 mole of O₂?
Answer: (a) 6.022 × 10²³
Explanation:
Reaction: C + O₂ → CO₂.
Therefore, from the above balanced chemical reaction, we can say that 1 mole C gives 1 mole CO₂ and 1 mole = Avogadro's number = 6.022×10²³ molecules.
Reaction: C + O₂ → CO₂.
Therefore, from the above balanced chemical reaction, we can say that 1 mole C gives 1 mole CO₂ and 1 mole = Avogadro's number = 6.022×10²³ molecules.
1.4
For a solid, how many types of thermal expansion coefficients are there?
Answer: (c) three
Explanation: Solids have three expansion coefficients commonly discussed: linear (length), area (surface), and volume expansion.
1.5
Which one of the following has the highest wavelength?
Answer: (c) infrared ray
Explanation: Infrared radiation has lower frequency and therefore longer wavelength than X-rays, γ-rays and ultraviolet radiation.
1.6
In case of refraction, if the angle of incidence and the angle of refraction are 45° and 30° respectively, then the angle of deviation is:
Answer: (b) 15°
Explanation: Angle of deviation D = i − r = 45° − 30° = 15°.
1.7
Temperature remaining unchanged, if the potential difference between the two ends of a conductor is V and the current through the conductor is I, which of the following is true?
Answer: (a) V ∝ I
Explanation: By Ohm's law (at constant temperature) V = IR → V is directly proportional to I.
1.8
The relation among electromotive force (V), work (W), and charge (Q) is:
Answer: (b) Q = W/V
Explanation: Work done W = VQ, so rearranging gives Q = W/V.
1.9
For the atom produced by β-particle emission from a radioactive element:
Answer: (b) atomic number increases
Explanation: In β⁻ emission a neutron converts to a proton (plus an electron), so atomic number increases by 1 while mass number stays the same.
1.10
To which group of the long periodic table do the halogen elements belong?
Answer: (c) group 17
Explanation: Halogens (F, Cl, Br, I, At) are placed in Group 17 of the modern periodic table.
1.11
Solid state of which of the following compounds is composed of ions?
Answer: (a) sodium chloride
Explanation: NaCl is an ionic solid made of Na⁺ and Cl⁻ ions arranged in a crystal lattice.
1.12
Which of the following has the highest ability to conduct electricity?
Answer: (d) aqueous solution of acetic acid
Explanation: Among the given choices, the aqueous solution of acetic acid contains ions (though partially dissociated) and conducts electricity better than pure water or sugar solution; (liquid HCl is not an aqueous electrolyte in this option list).
1.13
In the first step of fixation of nitrogen, which of the following compounds is formed as a result of lightning?
Answer: (a) NO
Explanation: The high energy of lightning breaks N₂ and O₂ bonds and forms nitric oxide (NO) which can later form other nitrogen oxides and nitrates in the soil.
1.14
Which of the following is the formula of bauxite, ore of aluminium?
Answer: (c) Al₂O₃·2H₂O
Explanation: Bauxite is primarily hydrated aluminium oxide; one common formula representation is Al₂O₃·2H₂O (shows aluminium oxide combined with water of crystallization).
1.15
Which of the following is the alkyl group containing two carbon atoms?
Answer: (b) ethyl
Explanation: The ethyl group (–C₂H₅) contains two carbon atoms; methyl has one, propyl has three, and isopropyl is a branched three-carbon group.
Group B – Very Short Answer (1×21 = 21)
2.1
Write down the unit of calorific value of fuel.
Answer: Kilojoule per kilogram (kJ/kg)
Explanation: Calorific value means the amount of heat produced by complete burning of 1 kg of fuel. Hence, its unit is kilojoule per kilogram (kJ/kg).
2.1 (Or)
Does the temperature increase or decrease with an increase in altitude in the stratosphere?
Answer: Temperature increases with altitude in the stratosphere.
Explanation: Ozone present in the stratosphere absorbs ultraviolet radiation from the Sun, which warms the upper layers and causes temperature to increase with height.
2.2
Which radiation, coming from the sun, is prevented by the ozone layer from falling on the earth’s surface?
Answer: Ultraviolet (UV) radiation
Explanation: The ozone layer absorbs most harmful ultraviolet radiation (mainly UV-B and UV-C) from the Sun, preventing it from reaching the Earth’s surface.
2.3
State whether the following statement is true or false: The volume of gas molecules is taken into consideration in Avogadro’s law.
Answer: False
Explanation: Avogadro’s law assumes that gas molecules have negligible volume and there is no force of attraction between them. So, their actual volume is not considered.
2.4
The product of volume and pressure of how many grams of N₂ gas is 224 litre in the atmosphere at STP? [N = 14]
Answer: 280 g of N₂
Explanation: 22.4 L of gas at STP = 1 mole.
So, 224 L = 10 moles.
Since, Molar mass of N₂ = 28 g/mol.
Therefore, the required mass = 10 × 28 g = 280 g.
So, 224 L = 10 moles.
Since, Molar mass of N₂ = 28 g/mol.
Therefore, the required mass = 10 × 28 g = 280 g.
2.5
State whether the following statement is true or false: The constituent particles of a material change position during conduction of heat through it.
Answer: False
Explanation: During heat conduction, particles only vibrate and transfer energy to neighboring particles. Their mean positions remain fixed.
2.5 (Or)
The width and the cross-section of a conductor remaining unchanged, what is the relation between the thermal resistance and thermal conductivity of that conductor?
Answer: Thermal resistance (R) is inversely proportional to thermal conductivity (k), i.e., R ∝ 1/k.
Explanation: Thermal resistance is given by R = L/(kA). If length and area are constant, higher conductivity means lower resistance.
2.6
What will be the angle of incidence when a ray of light passes through the centre of curvature of a concave mirror?
Answer: 0° (zero degree)
Explanation: A ray passing through the centre of curvature strikes the mirror along its normal, hence the angle of incidence is zero.
2.7
How many rectangular surfaces are there in a prism?
Answer: Equal to the number of sides of the base (e.g., 3 in a triangular prism)
Explanation: A prism has one rectangular face for each edge of its base. So, if the base is triangular, it has 3 rectangular surfaces.
2.8
Give an example of a semiconductor.
Answer: Silicon (Si)
Explanation: Silicon is a semiconductor widely used in electronic devices like transistors and diodes.
2.9
A thin wire and a thick wire of the same conducting material have the same length. Which one will carry more current when connected to the same potential difference?
Answer: The thick wire will carry more current.
Explanation: Resistance is inversely proportional to cross-sectional area. A thicker wire has less resistance, so it allows more current to flow.
2.10
Mention one misuse of nuclear fission reaction.
Answer: Use of nuclear fission in atomic bombs.
Explanation: The energy released by nuclear fission can be used destructively to make nuclear weapons, causing massive damage.
2.10 (Or)
Which law explains the release of huge amounts of energy in nuclear fusion?
Answer: Einstein’s mass–energy equivalence, E = mc²
Explanation: A small amount of mass is converted into a large amount of energy during fusion, as explained by Einstein’s equation.
2.11
Match the right column with the left column: 4
Left Column:
2.11.1 Oxide layer protects from attack by water vapour
2.11.2 Group 1 element of the long periodic table having the least reducing property
2.11.3 When the metal remains exposed to air, the metal slowly develops green patches on its surface
2.11.4 Group 2 element of the long periodic table having the least atomic radius
Right Column:
(a) Cu (b) Be (c) Al (d) Li
Left Column:
2.11.1 Oxide layer protects from attack by water vapour
2.11.2 Group 1 element of the long periodic table having the least reducing property
2.11.3 When the metal remains exposed to air, the metal slowly develops green patches on its surface
2.11.4 Group 2 element of the long periodic table having the least atomic radius
Right Column:
(a) Cu (b) Be (c) Al (d) Li
Answer:
Correct Matching:
2.11.1 → (c) Al
2.11.2 → (d) Li
2.11.3 → (a) Cu
2.11.4 → (b) Be
Explanation:
• (c) Aluminium (Al) forms a protective oxide layer (Al₂O₃) that prevents reaction with water vapour.
• (d) Lithium (Li) is the least reducing element among Group 1 metals because of its small atomic size and high ionization energy.
• (a) Copper (Cu) slowly develops green patches of basic copper carbonate when exposed to moist air.
• (b) Beryllium (Be) has the smallest atomic radius in Group 2 due to strong nuclear attraction on outer electrons.
2.11.1 → (c) Al
2.11.2 → (d) Li
2.11.3 → (a) Cu
2.11.4 → (b) Be
Explanation:
• (c) Aluminium (Al) forms a protective oxide layer (Al₂O₃) that prevents reaction with water vapour.
• (d) Lithium (Li) is the least reducing element among Group 1 metals because of its small atomic size and high ionization energy.
• (a) Copper (Cu) slowly develops green patches of basic copper carbonate when exposed to moist air.
• (b) Beryllium (Be) has the smallest atomic radius in Group 2 due to strong nuclear attraction on outer electrons.
2.12
Draw the Lewis dot structure of the N₂ molecule. (Atomic number of N is 7)
Answer: :N≡N:
Explanation: Each nitrogen has 5 valence electrons. Two N atoms share three pairs (a triple bond). Each N also has one lone pair. The Lewis structure is written as
:N≡N:
2.13
Which kind of electricity is used in electrolysis?
Answer: Direct current (DC)
Explanation: Electrolysis requires a unidirectional flow of charge so DC is used; alternating current would reverse the reactions and prevent steady electrolysis.
2.13 (Or)
Write down the cathode reaction in the electrolysis of acidulated water using platinum electrodes.
Answer: 2H⁺(aq) + 2e⁻ → H₂(g)
Explanation: At the cathode hydrogen ions are reduced to form hydrogen gas; platinum is inert and only provides the surface for the reaction.
2.14
In electroplating gold on brass, what is the electrolyte used?
Answer: A gold salt plating solution (commonly a gold cyanide complex bath, e.g., KAu(CN)₂ or similar commercial gold plating baths)
Explanation: Gold ions in a soluble gold complex provide Au⁺/Au³⁺ in solution; during plating these ions are reduced at the cathode to deposit a thin layer of gold on the brass surface.
2.15
What colour is formed in the reaction of ammonia with Nessler’s reagent?
Answer: Yellow to brown (brownish) colour / brown precipitate
Explanation: Nessler’s reagent gives a yellow to brown coloration (depending on concentration) when ammonium is present; a deeper brown indicates higher ammonia/ammonium concentration.
2.16
Write down the formula of the precipitate formed when H₂S gas is passed through an aqueous solution of silver nitrate.
Answer: Ag₂S (silver sulfide)
Explanation: H₂S provides S²⁻ ions which react with Ag⁺ to form insoluble silver sulfide, Ag₂S, as a black/brown precipitate.
2.16 (Or)
Write the name of the compound which is formed by the reaction of nitrogen with magnesium metal at a high temperature.
Answer: Magnesium nitride, Mg₃N₂
Explanation: At high temperature magnesium reacts with nitrogen to form magnesium nitride with formula Mg₃N₂.
2.17
What is the value of the H–C–H bond angle in methane?
Answer: 109.5°
Explanation: Methane has a tetrahedral shape; the H–C–H bond angles are approximately 109.5°.
2.17 (Or)
Write the IUPAC name of CH₃CH₂COOH.
Answer: Propanoic acid
Explanation: The compound has three carbon atoms and a carboxylic acid group, so its IUPAC name is propanoic acid (commonly known as propionic acid).
2.18
What is the industrial source of CNG?
Answer: Natural gas (mainly methane) obtained from petroleum/natural gas fields
Explanation: Compressed Natural Gas (CNG) is primarily methane produced from natural gas reservoirs and processed at gas plants; it is then compressed for use as fuel.
Group C – Short Answer (2×9 = 18)
3.1
What is the concept of sustainable development?
Answer: Sustainable development means meeting the needs of the present without stopping future generations from meeting their needs.
Explanation: It balances economic growth, care for the environment, and social well-being — for example using resources wisely, protecting nature, and planning so people in the future also have food, clean water, and jobs.
3.2
1 g of a gas at 7°C and 2 atm pressure occupies a volume of 410 mL. Determine the molar mass of the gas. (R = 0.082 litre atm mol⁻¹ K⁻¹)
Answer: Molar mass ≈ 28.0 g mol⁻¹
Explanation: Use ideal gas law \(PV = nRT\).
Given, V = 410 mL = 0.410 L
T = 7°C = (7 + 273) K= 280 K.
\(n = \dfrac{PV}{RT} = \dfrac{2 \times 0.410}{0.082 \times 280} \approx 0.0357\) mol.
Therefore, Molar mass \(M = \dfrac{\text{mass}}{n} = \dfrac{1}{0.0357} \approx 28.0\ \text{g mol}^{-1}\).
Given, V = 410 mL = 0.410 L
T = 7°C = (7 + 273) K= 280 K.
\(n = \dfrac{PV}{RT} = \dfrac{2 \times 0.410}{0.082 \times 280} \approx 0.0357\) mol.
Therefore, Molar mass \(M = \dfrac{\text{mass}}{n} = \dfrac{1}{0.0357} \approx 28.0\ \text{g mol}^{-1}\).
3.2 (Or)
A fixed mass of gas occupies a volume of 273 cm³ at STP. At what pressure will the above gas occupy a volume of 300 cm³ at 27°C?
Answer: 1 atm
Explanation:
Use combined gas relation \( \dfrac{P_1V_1}{T_1}=\dfrac{P_2V_2}{T_2}\).
Take STP as \(P_1=1\ \text{atm},\ T_1=273\ \text{K},\ V_1=273\ \text{cm}^3\)
For final state \(V_2=300\ \text{cm}^3,\ T_2=27°C=(273+27)=300\ \text{K}\) So \(P_2 = P_1 \dfrac{V_1 T_2}{V_2 T_1} = 1 \times \dfrac{273\times300}{300\times273} = 1\ \text{atm}\)
Use combined gas relation \( \dfrac{P_1V_1}{T_1}=\dfrac{P_2V_2}{T_2}\).
Take STP as \(P_1=1\ \text{atm},\ T_1=273\ \text{K},\ V_1=273\ \text{cm}^3\)
For final state \(V_2=300\ \text{cm}^3,\ T_2=27°C=(273+27)=300\ \text{K}\) So \(P_2 = P_1 \dfrac{V_1 T_2}{V_2 T_1} = 1 \times \dfrac{273\times300}{300\times273} = 1\ \text{atm}\)
3.3
What is the refractive index of a medium?
Answer: The refractive index is the ratio of the speed of light in vacuum to the speed of light in the medium.
Explanation: If light travels at speed c in vacuum and v in the medium, refractive index \(n = c/v\). It also equals \(\dfrac{\sin i}{\sin r}\) for an interface (Snell’s law).
3.3 (Or)
Which type of defect of vision is rectified by a convex lens?
Answer: Hypermetropia (farsightedness)
Explanation: A convex (converging) lens helps focus light from near objects onto the retina, correcting farsighted eyes whose focal point would otherwise lie behind the retina.
3.4
Two resistances r₁ and r₂, when connected separately to the same potential difference, it was seen that the current flowing through r₁ was six times the current flowing through r₂. Determine the ratio of r₁ and r₂.
Answer: r₁ : r₂ = 1 : 6
Explanation: For same voltage \(V\), \(I=V/R\).
Given \(I_1 = 6I_2\).
So \(\dfrac{V}{r_1} = 6\dfrac{V}{r_2} \Rightarrow \dfrac{1}{r_1} = \dfrac{6}{r_2}\). Thus \(r_2 = 6r_1\)
\(\therefore r_1:r_2 = 1:6\).
Given \(I_1 = 6I_2\).
So \(\dfrac{V}{r_1} = 6\dfrac{V}{r_2} \Rightarrow \dfrac{1}{r_1} = \dfrac{6}{r_2}\). Thus \(r_2 = 6r_1\)
\(\therefore r_1:r_2 = 1:6\).
3.5
How did Kossel explain the formation of the ionic bond?
Answer: Kossel explained that atoms transfer electrons to achieve noble-gas electron configuration; the resulting oppositely charged ions attract each other electrostatically to form an ionic bond.
Explanation: For example, sodium donates one electron to chlorine; Na⁺ and Cl⁻ form and stick together by strong electrical attraction, giving an ionic compound like NaCl.
3.5 (Or)
Liquid hydrogen chloride cannot conduct electricity, but molten sodium chloride can conduct electricity—Explain.
Answer: Liquid HCl is molecular (no free ions) so it does not conduct; molten NaCl has free mobile ions (Na⁺ and Cl⁻) so it conducts.
Explanation: Conductivity needs charged particles. In liquid HCl the molecules remain neutral; in molten NaCl ionic bonds break and ions move, allowing electric current.
3.6
Distinguish between sodium chloride and naphthalene by two physical properties.
Answer: (1) Solubility — NaCl is soluble in water; naphthalene is insoluble in water. (2) Electrical behaviour — solid NaCl is ionic and does not conduct (but its aqueous solution or molten form does); naphthalene is a molecular solid and does not conduct in any form.
Explanation: Also NaCl has a high melting point and crystalline ionic structure, while naphthalene has a low melting point and is made of neutral molecules (used as mothballs).
3.7
Between two aqueous solutions, one is ferric chloride and the other is aluminium chloride. How would you identify the ferric chloride solution using an aqueous solution of ammonia? Answer with a balanced chemical equation.
Answer: Add aqueous ammonia — ferric chloride gives a reddish-brown precipitate of Fe(OH)₃:
\(\text{Fe}^{3+}(aq) + 3\text{OH}^-(aq) \rightarrow \text{Fe(OH)}_3(s)\)
Explanation: On adding NH₄OH (or NaOH) to Fe³⁺ solution you get a brownish Fe(OH)₃ precipitate, while Al³⁺ gives a white Al(OH)₃ precipitate (which behaves differently on excess reagent). This colour difference identifies ferric chloride.
3.8
Why zinc blende can be called both a mineral and ore of zinc?
Answer: Zinc blende (sphalerite, ZnS) is a naturally occurring mineral and it contains zinc in usable amounts, so it is also an ore of zinc.
Explanation: A mineral is a natural solid substance; when that mineral is rich enough in a metal to be mined for extraction, it is called an ore. ZnS satisfies both.
3.8 (Or)
Mention two ways of preventing rusting of iron.
Answer: (1) Painting or coating with oil/paint. (2) Galvanization (coating with zinc) or using sacrificial anodes.
Explanation: Paint/oil blocks air and moisture. Zinc coating protects iron because zinc corrodes first (sacrificial protection) and keeps iron safe.
3.9
Write with a balanced chemical equation what happens when methane is burnt in oxygen.
Answer: Complete combustion:
\(\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}\)
\(\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}\)
Explanation: Methane reacts with oxygen to form carbon dioxide and water, releasing heat (this is why methane is a fuel).
3.9 (Or)
Mention one use of each of acetic acid and ethyl alcohol.
Answer: Acetic acid — used as vinegar in food and as a chemical reagent; Ethyl alcohol (ethanol) — used as a fuel/additive, solvent, and antiseptic.
Explanation: Acetic acid in dilute form is common vinegar. Ethanol is used in beverages (dilute), in hand sanitizers, as a solvent in labs and industries, and as a fuel or fuel additive.
Group D – Long Answer (3×12 = 36)
4.1
What is meant by the molar volume of a gas? Mention two reasons for the deviation of real gases from the behaviour of ideal gases.
Answer:
Molar volume: The volume occupied by one mole of a gas at STP (Standard Temperature and Pressure) is called its molar volume.
At STP, molar volume = 22.4 L mol⁻¹.
Reasons for deviation:
1. Real gas molecules have a definite volume (they are not point particles).
2. There are forces of attraction between real gas molecules (not neglected as in ideal gases).
Reasons for deviation:
1. Real gas molecules have a definite volume (they are not point particles).
2. There are forces of attraction between real gas molecules (not neglected as in ideal gases).
4.2
How many grams of Al are required to prepare 558 g of Fe by the reduction of Fe₂O₃ with Al at high temperature? How many moles of Fe₂O₃ will be required in the reaction? [Fe = 55.8, Al = 27, O = 16]
Answer:
Reaction: Fe₂O₃ + 2Al → 2Fe + Al₂O₃
Moles of Fe = 558 ÷ 55.8 = 10 mol.
From equation, 2 mol Fe need 1 mol Fe₂O₃ → 10 mol Fe need 5 mol Fe₂O₃.
Also, 2 mol Fe need 2 mol Al → 10 mol Fe need 10 mol Al.
Mass of Al = 10 × 27 = 270 g.
Answer: Al required = 270 g; Fe₂O₃ required = 5 mol.
4.2 (Or)
By heating 32.1 g ammonium chloride with calcium hydroxide, 10.2 g NH₃, 33.3 g CaCl₂, and 10.8 g H₂O are obtained. How many grams of calcium hydroxide take part in the reaction? How many moles of NH₃, and how many litres of NH₃ at STP are formed in the reaction? [N = 14, H = 1]
Answer:
Reaction:
2NH₄Cl + Ca(OH)₂ → 2NH₃ + 2H₂O + CaCl₂
Moles of NH₃ = 10.2 ÷ 17 = 0.6 mol.
From equation, 2 mol NH₃ need 1 mol Ca(OH)₂ → 0.6 mol NH₃ need 0.3 mol Ca(OH)₂.
Mass = 0.3 × 74 = 22.2 g Ca(OH)₂.
Volume of NH₃ = 0.6 × 22.4 = 13.44 L at STP.
2NH₄Cl + Ca(OH)₂ → 2NH₃ + 2H₂O + CaCl₂
Moles of NH₃ = 10.2 ÷ 17 = 0.6 mol.
From equation, 2 mol NH₃ need 1 mol Ca(OH)₂ → 0.6 mol NH₃ need 0.3 mol Ca(OH)₂.
Mass = 0.3 × 74 = 22.2 g Ca(OH)₂.
Volume of NH₃ = 0.6 × 22.4 = 13.44 L at STP.
4.3
Which quantities remain fixed in the definition of the volume expansion coefficient of a gas? Name a non-metal which is a good conductor of heat.
Answer:
In the definition of volume expansion coefficient, pressure and mass of gas remain constant.
Good conductor non-metal: Graphite (an allotrope of carbon) conducts heat and electricity efficiently.
Good conductor non-metal: Graphite (an allotrope of carbon) conducts heat and electricity efficiently.
4.3 (Or)
What is meant by ‘linear expansion coefficient of copper is 17×10⁻⁶/°C’? Why does the value remain the same even in kelvin scale?
Answer:
It means if the temperature of copper is raised by 1°C, its length increases by 17×10⁻⁶ times its original length.
The value is same in kelvin because a change of 1°C equals a change of 1 K — both scales have equal intervals.
The value is same in kelvin because a change of 1°C equals a change of 1 K — both scales have equal intervals.
4.4
What type of mirror is used by dentists? Why does a ray of light not deviate as a result of refraction through a glass slab?
Answer:
Dentists use a concave mirror because it gives a magnified, upright image when held close.
A ray does not deviate through a glass slab because the emergent ray is parallel to the incident ray; lateral displacement occurs, but the total deviation angle is zero.
A ray does not deviate through a glass slab because the emergent ray is parallel to the incident ray; lateral displacement occurs, but the total deviation angle is zero.
4.5
When an object is placed 20 cm away from a convex lens, no image is obtained on either side of the lens. What is the focal length of the lens? If the refractive index of glass with respect to air is 1.5, what is the refractive index of air with respect to glass?
Answer:
No image on either side means the object is at the focus, so focal length f = 20 cm.
Refractive index of air w.r.t glass = 1 / 1.5 = 0.67.
Refractive index of air w.r.t glass = 1 / 1.5 = 0.67.
4.5 (Or)
The length of an object is 5 cm. An image of length 10 cm is obtained when it is placed at a distance of 2 cm in front of a convex lens. What is the linear magnification and image distance?
Answer:
Linear magnification m = image height/object height = 10 ÷ 5 = 2.
m = v/u → v = 2u = 2×2 = 4 cm.
Magnification = 2; Image distance = 4 cm.
m = v/u → v = 2u = 2×2 = 4 cm.
Magnification = 2; Image distance = 4 cm.
4.6
Write in brief the basic principle of hydroelectric power generation.
Answer:
Hydroelectric power works by converting the potential energy of stored water in dams into kinetic energy, which turns turbines. The turbines rotate generators to produce electricity — converting mechanical energy into electrical energy.
4.7
A current of 1A flows when an electric bulb is connected to 220 V mains. What would be the current when the same bulb is connected to 110 V mains?
Answer:
Resistance of bulb, R = V/I = 220/1 = 220 Ω.
For 110 V: I = V/R = 110/220 = 0.5 A.
Answer: The current will be 0.5 A.
For 110 V: I = V/R = 110/220 = 0.5 A.
Answer: The current will be 0.5 A.
4.7 (Or)
Find the ratio of resistances for two bulbs of 220V–60W and 110V–60W.
Answer:
R = V²/P.
For 220V bulb: R₁ = (220²)/60 = 806.7 Ω.
For 110V bulb: R₂ = (110²)/60 = 201.7 Ω.
Ratio R₁:R₂ = 806.7 : 201.7 ≈ 4 : 1.
For 220V bulb: R₁ = (220²)/60 = 806.7 Ω.
For 110V bulb: R₂ = (110²)/60 = 201.7 Ω.
Ratio R₁:R₂ = 806.7 : 201.7 ≈ 4 : 1.
4.8
Explain why a new element is formed by α-particle emission but no new element is formed by γ-ray emission from a radioactive element.
Answer:
α-particle emission removes 2 protons and 2 neutrons, decreasing atomic number by 2 and mass number by 4, forming a new element.
γ-rays have no charge or mass; they only release energy. So atomic and mass numbers remain unchanged, no new element forms.
4.9
Write down Dobereiner’s law of triads. Arrange Cl, Br, I, F in increasing order of their oxidising power.
Answer:
Law of triads: When elements are arranged in groups of three, the atomic mass of the middle element is approximately the average of the other two.
Order of oxidising power: I < Br < Cl < F.
Order of oxidising power: I < Br < Cl < F.
4.9 (Or)
What is the important conclusion of Moseley’s experiment? What is the importance of this conclusion in regard to the periodic table?
Answer:
Conclusion: Moseley showed that the atomic number (not atomic mass) determines the properties of an element.
Importance: The periodic table was rearranged according to increasing atomic number, removing irregularities of Mendeleev’s table.
Importance: The periodic table was rearranged according to increasing atomic number, removing irregularities of Mendeleev’s table.
4.10
Write two differences between the conduction of electricity through a metallic wire and an electrolyte during electrolysis. In electrolytic refining of copper, impure copper rod is used as which electrode?
Answer:
Differences:
1. In metallic wire — electrons flow; in electrolyte — ions flow.
2. Metals conduct without chemical change; electrolytes conduct with chemical reactions.
In refining of copper, impure copper is the anode.
1. In metallic wire — electrons flow; in electrolyte — ions flow.
2. Metals conduct without chemical change; electrolytes conduct with chemical reactions.
In refining of copper, impure copper is the anode.
4.11
Write the names of the chemicals used and balanced chemical equation in the industrial production of urea.
Answer:
Chemicals used: Ammonia (NH₃) and Carbon dioxide (CO₂).
Equation: 2NH₃ + CO₂ → NH₂CONH₂ + H₂O
Explanation: Urea (NH₂CONH₂) is formed by reacting ammonia with carbon dioxide under high temperature and pressure.
Equation: 2NH₃ + CO₂ → NH₂CONH₂ + H₂O
Explanation: Urea (NH₂CONH₂) is formed by reacting ammonia with carbon dioxide under high temperature and pressure.
4.12
(A) and (B) are two unsaturated hydrocarbons, each containing 2 carbon atoms. On reaction with bromine, (A) adds one molecule of bromine per molecule and (B) adds two molecules of bromine per molecule. Write structural formulas of (A) and (B). Write a balanced chemical equation of the reaction of (B) with bromine.
Answer:
(A) Ethene (C₂H₄); (B) Ethyne (C₂H₂).
Reaction: C₂H₂ + 2Br₂ → C₂H₂Br₄
Ethyne adds two bromine molecules forming tetrabromoethane.
Reaction: C₂H₂ + 2Br₂ → C₂H₂Br₄
Ethyne adds two bromine molecules forming tetrabromoethane.
4.12 (Or)
Write a balanced chemical equation of the reaction of sodium hydroxide with acetic acid. Which one between jute and polythene is environment-friendly for packaging and why?
Answer:
Reaction: CH₃COOH + NaOH → CH₃COONa + H₂O
Environment-friendly: Jute, because it is biodegradable and does not pollute the environment, while polythene is non-biodegradable and harmful.
Environment-friendly: Jute, because it is biodegradable and does not pollute the environment, while polythene is non-biodegradable and harmful.
Group E – For External Candidates Only
Very Short Answer Questions (1×4 = 4 marks)
5.1
Give an example of a non-conventional energy with the help of which electricity can be generated.
Answer:
Solar energy is an example of non-conventional energy used to generate electricity with solar panels.
5.2
Write the SI unit of electric power.
Answer:
The SI unit of electric power is watt (W).
5.3
Which acid is manufactured utilising catalytic oxidation of ammonia?
Answer:
Nitric acid (HNO₃) is produced by catalytic oxidation of ammonia in the Ostwald process.
5.4
At STP, an ideal gas occupies a volume of 273 cm³. What volume will the gas occupy at 76 cm Hg pressure and at a temperature of 273°C?
Answer:
Using the gas law:
\( \dfrac{P_1V_1}{T_1} = \dfrac{P_2V_2}{T_2} \)
\( P_1 = 76\text{ cm}\), \(T_1 = 273 K,\) \(V_1 = 273\text{ cm}^3,\) \(T_2 = 273 + 273 = 546 K. \)
\( V_2 = \dfrac{P_1V_1T_2}{P_2T_1} = \dfrac{76×273×546}{76×273} = 546\text{ cm}^3. \)
Answer: The gas will occupy 546 cm³ at 273°C.
\( \dfrac{P_1V_1}{T_1} = \dfrac{P_2V_2}{T_2} \)
\( P_1 = 76\text{ cm}\), \(T_1 = 273 K,\) \(V_1 = 273\text{ cm}^3,\) \(T_2 = 273 + 273 = 546 K. \)
\( V_2 = \dfrac{P_1V_1T_2}{P_2T_1} = \dfrac{76×273×546}{76×273} = 546\text{ cm}^3. \)
Answer: The gas will occupy 546 cm³ at 273°C.
5.5
What is the functional group present in ethyl alcohol?
Answer:
The functional group in ethyl alcohol (ethanol) is the hydroxyl group (-OH).
Short Answer Questions (2×3 = 6 marks)
6.1
What is a superconductor?
Answer:
A superconductor is a material that offers zero electrical resistance and allows current to flow without any energy loss when cooled below a certain critical temperature.
6.2
Mention one use of each of ultraviolet rays and gamma rays.
Answer:
Ultraviolet rays: Used for sterilising medical instruments and drinking water.
Gamma rays: Used for treating cancer and for sterilising medical equipment.
Gamma rays: Used for treating cancer and for sterilising medical equipment.
6.3
Write the name and formula of one ore of iron.
Answer:
Name: Hematite
Formula: Fe₂O₃
Formula: Fe₂O₃
6.4
Write mentioning the product, how the polymerization reaction of ethylene is carried out.
Answer:
Ethylene (CH₂=CH₂) undergoes polymerization under high pressure and temperature with a catalyst to form polyethylene (polythene).
Reaction: nCH₂=CH₂ → (–CH₂–CH₂–)â‚™
Reaction: nCH₂=CH₂ → (–CH₂–CH₂–)â‚™
Key Takeaways for Students
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MCQs are Concept Heavy: Group A tests your fundamental understanding of definitions and laws. Revise your textbook chapters thoroughly.
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Practice Numerical Problems: Questions on gas laws, moles, and electricity are scoring if you practice the numericals.
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Chemical Equations are Crucial: Make sure you can write balanced chemical equations for important reactions (like electrolysis, combustion, etc.).
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Define and Differentiate: Groups C and D require you to define terms clearly and write descriptive differences between concepts.
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