9th Class chemistry

Chapter 10: Environmental Chemistry – 9th Class New Syllabus (Punjab Boards & Lahore Board)

by

Explore Chapter 10 “Environmental Chemistry” from the 9th Class new syllabus for Punjab Boards and Lahore Board. Understand air pollution, greenhouse effect, acid rain, and strategies to protect the environment with easy-to-learn explanations.

Multiple Choice Questions

(i) Which gases are responsible for the greenhouse effect?
Options:
(a) SO₂, NO
(b) CO₂, CO
(c) CO₂, CH₄
(d) O₂, O₃
Correct Answer: (c) CO₂, CH₄
Explanation: Carbon dioxide (CO₂) and methane (CH₄) are major greenhouse gases that trap heat in the Earth’s atmosphere, leading to the greenhouse effect.
Tip: Always associate greenhouse gases with their ability to trap heat (e.g., CO₂, CH₄, N₂O). Gases like O₂ and O₃ don’t significantly contribute to this effect.

(ii) Indicate the source of sulfur which is responsible for the presence of oxides of sulfur in the atmosphere.
Options:
(a) Decomposition of vegetation
(b) Waste gases from digestion of animals
(c) Photochemical smog
(d) Combustion of fossil fuels
Correct Answer: (d) Combustion of fossil fuels
Explanation: Burning fossil fuels (coal, oil, etc.) releases sulfur dioxide (SO₂) and other sulfur oxides into the atmosphere.
Tip: Think of industrial processes and power plants as key contributors to sulfur pollution.

(iii) Concentration of which gas in the atmosphere is decreased by photosynthesis in plants?
Options:
(a) Oxygen
(b) Nitrogen
(c) Carbon dioxide
(d) Water vapor
Correct Answer: (c) Carbon dioxide
Explanation: Plants absorb CO₂ during photosynthesis to produce oxygen and glucose.
Tip: Photosynthesis reduces CO₂, not oxygen or water vapor, in the environment.

(iv) Which substance remains unaffected in the catalytic converter fixed in the exhaust of automobiles?
Options:
(a) CO₂
(b) CO
(c) NO
(d) NO₂
Correct Answer: (a) CO₂
Explanation: Catalytic converters reduce pollutants like CO, NO, and hydrocarbons but do not affect CO₂, which is not harmful in small amounts.
Tip: Remember that catalytic converters aim to reduce harmful emissions but allow CO₂ to pass through.

(v) People of which age groups are the most affected by air pollution?
Options:
(a) Young adults
(b) Cancer patients
(c) Children
(d) Both children and aged people
Correct Answer: (d) Both children and aged people
Explanation: Children and the elderly have weaker immune and respiratory systems, making them more vulnerable to air pollution.
Tip: When asked about susceptibility, think of the most physically vulnerable groups (young and old).

(vi) In which area there is a greater possibility of acid rain?
Options:
(a) Around villages
(b) Around big cities
(c) Around industrial areas
(d) Around water bodies
Correct Answer: (c) Around industrial areas
Explanation: Industrial areas release large amounts of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), which cause acid rain when mixed with water in the atmosphere.
Tip: Acid rain is linked to industrial emissions, so look for industrial-related options.

(vii) Why is smog not felt in summer?
Options:
(a) Because fog is not present in summer
(b) Because due to heat of the earth the smoke rises up
(c) Because in summer smoke and fog cannot mix with each other
(d) Because less fossil fuels are burnt in summer
Correct Answer: (b) Because due to heat of the earth the smoke rises up
Explanation: In summer, higher temperatures cause pollutants to rise higher into the atmosphere, reducing the formation of smog near the ground.
Tip: Smog is more common in winter due to temperature inversion.

(viii) Which catalyst is used in the catalytic converter fixed in the exhaust systems of automobiles?
Options:
(a) Ni
(b) Cu
(c) Pt, Pd and Rh
(d) CaO
Correct Answer: (c) Pt, Pd and Rh
Explanation: Platinum (Pt), Palladium (Pd), and Rhodium (Rh) are used in catalytic converters to reduce harmful vehicle emissions.
Tip: Remember the precious metals (Pt, Pd, Rh) for catalytic converters.

(ix) Which components are essential for the formation of photochemical smog?
Options:
(a) CO, NO₂, CO₂
(b) NO₂, volatile organic compounds, sunlight
(c) CO₂, NO₂, sunlight
(d) Volatile organic compounds, NO₂, CO
Correct Answer: (b) NO₂, volatile organic compounds, sunlight
Explanation: Photochemical smog forms when sunlight reacts with nitrogen oxides (NOₓ) and volatile organic compounds (VOCs).
Tip: Photochemical smog always involves sunlight, NO₂, and VOCs.

(x) Which air pollutant can cause respiratory problems?
Options:
(a) CO₂
(b) Ozone
(c) Smog
(d) CO
Correct Answer: (c) Smog
Explanation: Smog contains harmful pollutants like particulate matter and ground-level ozone, which irritate the respiratory system.
Tip: Respiratory issues are typically associated with smog or particulate pollutants.

Short Questions

1. What is the main objective of environmental education?
To make people aware of environmental problems and teach them how to protect nature and use resources responsibly.

2. How is particulate matter released in the atmosphere?
Particulate matter is released through vehicle emissions, industrial activities, burning of fossil fuels, and construction activities.

3. Which gas is more poisonous, CO₂ or CO?
Carbon monoxide (CO) is more poisonous because it binds to hemoglobin in blood, reducing oxygen supply to the body.

4. How does acid rain affect forests?
Acid rain damages tree leaves, weakens roots, and makes soil acidic, harming plant growth.

5. In what way is sulfur present in fossil fuels dangerous?
When fossil fuels are burned, sulfur is released as sulfur dioxide (SO₂), which can cause acid rain and respiratory problems.

6. Name any three major sources responsible for the greenhouse effect.
The major sources are:

  • Burning of fossil fuels (CO₂)
  • Agriculture (CH₄ from livestock)
  • Deforestation (less CO₂ absorption).

7. How is geothermal energy useful for us?
Geothermal energy is used to generate electricity and provide heating by using heat from inside the Earth.

Constructed Response Questions

i. How is the excessive use of insecticides and pesticides harmful for birds?
Insecticides and pesticides contain harmful chemicals that can enter the food chain. When birds eat insects or seeds contaminated with these chemicals, they can get poisoned. Pesticides can weaken birds’ immune systems, affect their reproduction by thinning eggshells, and even lead to their death. Overuse of such chemicals also reduces the availability of insects that birds feed on, disrupting their natural diet.

ii. Percentage of CO₂ in air is only 0.03%. Then how does it become harmful for the ecosystem?
Even though CO₂ is present in small amounts, it plays a significant role in trapping heat in the Earth’s atmosphere. Excess CO₂ from activities like burning fossil fuels causes global warming, leading to climate change. This disrupts weather patterns, melts glaciers, raises sea levels, and affects ecosystems, harming plants, animals, and humans. It also makes oceans acidic, threatening marine life.

iii. Why only some pollutant gases present in the atmosphere cause greenhouse effect while others do not?
Gases like CO₂, CH₄ (methane), and N₂O absorb heat and trap it in the atmosphere, causing the greenhouse effect. These gases are called greenhouse gases because they allow sunlight to enter the Earth but prevent heat from escaping. Other gases, like nitrogen (N₂) and oxygen (O₂), do not have this property, so they don’t contribute to the greenhouse effect.

iv. How can you reduce the emission of CO present in the gases emitted by the burning of fuel in automobile engines?
To reduce CO emissions:

  1. Use catalytic converters in vehicles to convert CO into less harmful gases.
  2. Use cleaner fuels like compressed natural gas (CNG) or electric vehicles.
  3. Maintain vehicles regularly to ensure efficient fuel burning.
  4. Carpooling and public transport reduce the number of vehicles on the road, lowering emissions.

v. Mention three different ways in which solar energy can be useful for us.

  1. Electricity Generation: Solar panels convert sunlight into electricity for homes, schools, and industries.
  2. Heating: Solar energy can be used to heat water and buildings, reducing the use of gas or electricity.
  3. Cooking: Solar cookers use sunlight to prepare food without using gas or electricity, making them eco-friendly.

Descriptive Questions

i. Describe the harmful effects of the major pollutants present in the air.

  • Carbon monoxide (CO): Reduces oxygen supply to the body, causing dizziness, fatigue, and even death.
  • Sulfur dioxide (SO₂): Causes acid rain, damages plants, and irritates the respiratory system.
  • Nitrogen oxides (NOₓ): Contribute to smog and acid rain and affect lung function.
  • Particulate matter (PM): Tiny particles can enter the lungs, causing breathing issues, asthma, and heart problems.
  • Ozone (O₃): At ground level, it causes respiratory problems and harms crops and materials.

ii. Explain greenhouse effect. How is global warming dangerous for us?
The greenhouse effect is the process by which certain gases (like CO₂, CH₄, and N₂O) trap heat in the Earth’s atmosphere, keeping the planet warm. This is essential for life, but excessive greenhouse gases lead to global warming.
Global warming is dangerous because it:

  • Causes extreme weather events like hurricanes and floods.
  • Melts glaciers, raising sea levels and flooding coastal areas.
  • Disrupts ecosystems, endangering animals and plants.
  • Makes some regions too hot or dry for farming, leading to food shortages.

iii. What is air quality index? What information does it convey?
The Air Quality Index (AQI) measures the quality of air in a specific area and tells us how clean or polluted the air is. It assigns a score or color based on pollutants like CO, NO₂, PM, and O₃.

  • Good AQI: Air is clean and safe to breathe.
  • Poor AQI: Air is polluted and can harm health, especially for sensitive groups like children and the elderly.

iv. Who are at high risk groups and why is pollution more dangerous for them?
High-risk groups include:

  • Children: Their lungs are still developing, and they breathe faster than adults, so they inhale more polluted air.
  • Elderly people: They often have weaker immune systems and pre-existing health problems like asthma or heart disease.
  • Pregnant women: Pollution can harm the developing baby.
  • People with respiratory issues: They are more sensitive to pollutants, which can worsen their conditions.

v. Describe three strategies to address environmental issues.

  1. Reduce Pollution: Use renewable energy sources (solar, wind) and improve waste management to reduce air, water, and land pollution.
  2. Conservation of Resources: Save water, use energy-efficient appliances, and plant more trees to conserve natural resources.
  3. Raise Awareness: Educate people about the importance of protecting the environment through schools, campaigns, and community programs.

Investigative Questions

i. Major Pakistani cities experience a very high AQI in winter. Point out some of the major causes of high AQI in these cities.

  • Vehicle Emissions: Increased use of vehicles releases pollutants like CO, NOₓ, and particulate matter.
  • Industrial Emissions: Factories release smoke containing SO₂ and other harmful gases.
  • Burning of Crop Residue: Farmers burn crop stubble, which adds to air pollution.
  • Temperature Inversion: In winter, cooler air traps pollutants near the ground, worsening air quality.

ii. Why does AQI not rise in Pakistan in hot days of summer?
In summer, higher temperatures and winds disperse pollutants, preventing them from accumulating near the ground. Unlike winter, there is no temperature inversion to trap pollutants.

iii. How has climate change affected Pakistan during the last five years?

  • Increased Floods: Heavy rains and melting glaciers have caused severe flooding, displacing millions of people.
  • Heatwaves: High temperatures have caused heat-related illnesses and deaths.
  • Droughts: Some regions face water shortages due to irregular rainfall.
  • Crop Damage: Changing weather patterns have affected agricultural production, leading to food insecurity.

Chapter 4 Stoichiometry SLO based solved Exercise 9th class new syllabus

by

Get comprehensive solutions for Chapter 4 Stoichiometry from the 9th Class New Syllabus. This SLO-based solved exercise is designed to help students master the concepts of stoichiometry effectively.

(i) How many atoms are present in one gram of H2O?

1 mole of H2O = 18 g and contains 6.022×1023 molecules.
1 molecule of H2O = 3 atoms (2 H + 1 O).

Thus,
Atoms in 1 gram of H2O: 6.022×1023/18×3≈10.02×1022

Correct answer: (a) 10.02×1022 atoms

(ii) Which is the correct formula of calcium phosphide?

The formula of calcium phosphide is based on the charges of Ca2+ and P3−. To balance charges, the formula is Ca3P2

Correct answer: (c) Ca3P2

(iii) How many atomic mass units (amu) are there in one gram?

1 amu = 1.66×10−24g.

Number of amu in 1 g=1/1.66×10−24≈6.022×1023

Correct answer: (b) 6.022×1023

(iv) Structural formula of 2-hexene is CH3−CH=CH−(CH2)2−CH3. What is its empirical formula?

The molecular formula of 2-hexene is C6H12
Empirical formula is obtained by dividing subscripts by their greatest common divisor (6). Empirical formula=CH2

Correct answer: (b) CH2

(v) How many moles are there in 25 g of H2SO4?

Molar mass of H2SO4 = 2+32+(4×16)=98 g/mol

Moles=Mass/Molar mass=25/98≈0.255 moles.

Correct answer: (c) 0.255 moles

(vi) A necklace has 6 g of diamonds in it. What are the number of carbon atoms in it?

Diamonds are made of carbon.
Molar mass of carbon = 12 g/mol.
Number of moles of carbon: Moles=6/12=0.5 moles

Number of carbon atoms: 0.5×6.022×1023≈3.01×1023

Correct answer: (d) 3.01×1023

(vii) What is the mass of Al in 204 g of aluminum oxide (Al2O3)?

Molar mass of Al2O3 = 2(27)+3(16)=102 g/mol
Mass fraction of Al: 2(27)/102=54/102=0.5294

Mass of Al in 204 g of Al2O3: 0.5294×204≈108 g

Correct answer: (d) 108 g

(viii) Which one of the following compounds will have the highest percentage by mass of nitrogen?

  • For CO(NH2)2(urea):
    Molar mass = 12+16+2(14)+4(1)=60
    Nitrogen mass = 28
    Percentage of N = 28/60×100=46.67%
  • For NH4NO3:
    Molar mass = 14+4+14+3(16)=80
    Nitrogen mass = 28.
    Percentage of N = 28/80×100=35%

Correct answer: (a) CO(NH2)2

(ix) When one mole of each of the following compounds is reacted with oxygen, which will produce the maximum amount of CO2?

The number of moles of CO2 depends on the number of carbon atoms in the compound. Among the given options:

  • Methane (CH4): Produces 1 mole of CO2.
  • Ethane (C2H6): Produces 2 moles of CO2.
  • Diamond (C): Produces 1 mole of CO2

Correct answer: (b) Ethane

(x) What mass of 95% Ca(OH)2 will be required to neutralize 50 cm3 of 0.5 M H2SO4?

  • Reaction: Ca(OH)2+H2SO4→CaSO4+2H2O
    1 mole of H2SO4 reacts with 1 mole of Ca(OH)2.
    Moles of H2SO4: 0.5×0.05=0.025 moles
    Mass of pure Ca(OH)2: 0.025×(40+2(16+1))=0.025×74=1.85 g.
  • Mass of 95% pure Ca(OH)2: 1.850.95≈1.95 g

Correct answer: (d) 1.95 g

2. Questions for Short Answers

i. Write down the chemical formula of barium nitride.
The chemical formula of barium nitride is Ba₃N₂.
Explanation: Barium (Ba) is an alkaline earth metal with a +2 oxidation state, and nitride (N) is a nonmetal with a -3 charge. To balance the charges, three barium ions combine with two nitride ions, forming Ba₃N₂.

ii. Find out the molecular formula of a compound whose empirical formula is CH₂O and its molar mass is 180.
Step 1: Calculate the molar mass of the empirical formula (CH₂O).
Molar mass of CH₂O = 12 (C) + 2(1) (H) + 16 (O) = 30 g/mol.

Step 2: Determine the ratio of the molecular formula to the empirical formula.
Ratio=Molar mass of compound / Molar mass of empirical formula=180/30=6.

Step 3: Multiply the empirical formula by the ratio.
Molecular formula = (CH2O)6=C6H12O6 = C₆H₁₂O₆.
The molecular formula of the compound is C₆H₁₂O₆ (glucose).

iii. How many molecules are present in 1.5 g H₂O?
Step 1: Calculate the molar mass of water (H₂O).
Molar mass = 2(1) + 16 = 18 g/mol.

Step 2: Find the number of moles in 1.5 g of H₂O.
Moles = Mass/Molar mass=1.5/18=0.0833 mol

Step 3: Calculate the number of molecules using Avogadro’s number (6.022×1023 molecules/mol)
Number of molecules = 0.0833×6.022×1023=5.02×1022
Thus, there are approximately 5.02×1022

iv. What is the difference between a mole and Avogadro’s number?

  • Mole: A mole is a unit in chemistry that represents a quantity of substance. One mole of any substance contains the same number of particles (atoms, molecules, or ions) as there are in 12 g of carbon-12.
  • Avogadro’s Number: Avogadro’s number is a constant that defines the number of particles in one mole of a substance. It is equal to 6.022×1023 particles/mol

Difference: The mole is a quantity (like a dozen), while Avogadro’s number provides the specific count of particles in one mole.

v. Write down the chemical equation for the given reaction.
Copper + Sulphuric acid → Copper sulphate + Sulphur dioxide + Water
Balanced chemical equation:
Cu+2H₂SO₄→CuSO₄+SO₂+2H₂O

3. Constructed Response Questions

i. Different compounds will never have the same molecular formula but can have the same empirical formula. Explain.

  • The empirical formula represents the simplest whole-number ratio of elements in a compound, whereas the molecular formula shows the exact number of atoms of each element in a molecule.
  • Example: Acetic acid (C₂H₄O₂) and glucose (C₆H₁₂O₆) have the same empirical formula (CH₂O), but their molecular formulas are different.
  • Explanation: Compounds with different molecular structures can share the same empirical formula because the molecular formula is a multiple of the empirical formula.

ii. Write down the chemical formulas of the following compounds.

  1. Calcium phosphate: Ca₃(PO₄)₂
  2. Aluminium nitride: AlN
  3. Sodium acetate: CH₃COONa
  4. Ammonium carbonate: (NH₄)₂CO₃
  5. Bismuth sulphate: Bi₂(SO₄)₃

iii. Why does Avogadro’s number have immense importance in chemistry?
Avogadro’s number (6.022×1023 is essential in chemistry because:

  • It establishes a connection between the microscopic (atoms/molecules) and macroscopic (grams/moles) scales.
  • It allows chemists to calculate the number of particles in a given mass of substance.
  • It is crucial for determining stoichiometric relationships in chemical reactions, enabling precise calculations for reactants and products.

iv. When 8.657 g of a compound were converted into elements, it gave 5.217 g of carbon, 0.962 g of hydrogen, and 2.478 g of oxygen. Calculate the percentage of each element present in this compound.
Step 1: Calculate the total mass of the compound.
Total mass = 5.217+0.962+2.478=8.657 g

Step 2: Calculate the percentage of each element.

  • Carbon: 5.2178.657×100=60.3%
  • Hydrogen: 0.9628.657×100=11.1%
  • Oxygen: 2.4788.657×100=28.6%

Thus, the percentages are:

  • Carbon: 60.3%
  • Hydrogen: 11.1%
  • Oxygen: 28.6%.

4. Descriptive Questions

i. Which conditions must be fulfilled before writing a chemical equation for a reaction?
To write a chemical equation, the following conditions must be fulfilled:

  1. Correct identification of reactants and products: Ensure the substances involved in the reaction are accurately identified.
  2. Balancing the equation: The equation must follow the law of conservation of mass, where the number of atoms for each element is the same on both sides.
  3. Physical states of substances: Indicate the physical states of reactants and products (solid (s), liquid (l), gas (g), or aqueous (aq)).
  4. Reaction conditions: Specify the conditions such as temperature, pressure, or the use of catalysts if required for the reaction.

ii. Explain the concepts of Avogadro’s number and mole.

  • Avogadro’s Number: Avogadro’s number (6.022×1023 is the number of particles (atoms, molecules, or ions) in one mole of a substance. It bridges the gap between the microscopic scale and the macroscopic scale.
  • Mole: A mole is a unit of measurement used in chemistry to represent 6.022×1023 particles of a substance. It is the amount of a substance that contains as many entities as there are atoms in 12 grams of carbon-12.

iii. How many grams of CO₂ will be produced when we react 10 g of CH₄ with excess of O₂ according to the following equation?
CH4+2O2→CO2+2H2OCH₄ + 2O₂ → CO₂ + 2H₂O

  1. Molar masses:
    • CH₄ = 16 g/mol
    • CO₂ = 44 g/mol
  2. Moles of CH₄:
    Moles = Mass/Molar mass=10/16=0.625 mol
  3. Moles of CO₂:
    From the equation, 1 mole of CH₄ produces 1 mole of CO₂.
    Moles of CO₂ = 0.625 mol.
  4. Mass of CO₂:
    Mass = Moles × Molar mass = 0.625×44=27.5 g

Answer: 27.5 g of CO₂ will be produced.

iv. How many moles of coal are needed to produce 10 moles of CO according to the following equation?
3C+O2+H2O→H2+3CO3C + O₂ + H₂O → H₂ + 3CO

From the equation, 3 moles of carbon (C) produce 3 moles of CO.
Thus, to produce 10 moles of CO: Moles of coal=103=3.33 moles.\text{Moles of coal} = \frac{10}{3} = 3.33 \, \text{moles}.

Answer: 3.33 moles of coal are needed.

v. How much SO₂ is needed in grams to produce 10 moles of sulphur?
2H2S+SO2→2H2O+3S

From the equation, 1 mole of SO₂ produces 1.5 moles of sulphur.
To produce 10 moles of sulphur: Moles of SO₂=10/1.5=6.67 moles

Molar mass of SO₂ = 32 (S) + 2(16) = 64 g/mol.
Mass = Moles × Molar mass = 6.67×64=426.88 g

Answer: 426.88 g of SO₂ are needed.

vi. How much ammonia is needed in grams to produce 1 kg of urea fertilizer?
2NH3+CO2→CO(NH2)2+H2O

  1. Molar masses:
    • NH₃ = 17 g/mol
    • CO(NH₂)₂ (urea) = 60 g/mol
  2. Moles of urea:
    Moles = Mass/Molar mass=1000/60=16.67 mol
  3. Moles of NH₃:
    From the equation, 2 moles of NH₃ produce 1 mole of urea.
    Moles of NH₃ = 16.67×2=33.34 mol.
  4. Mass of NH₃:
    Mass = Moles × Molar mass = 33.34×17=566.78 g

Answer: 566.78 g of ammonia is needed.

vii. Calculate the number of atoms in the following:

(a) 3 g of H₂:
Moles of H₂ = Mass/Molar mass=3/2=1.5 mol
Number of molecules = 1.5×6.022×1023=9.033×1023
Each molecule of H₂ contains 2 atoms, so:
Number of atoms = 9.033×1023×2=1.8066×1024
Number of molecules = 3.4×6.022×1023=2.0475×1024
Each molecule of N₂ contains 2 atoms, so:
Number of atoms = 2.0475×1024×2=4.095×1024
Molar mass of C₆H₁₂O₆ = 6(12)+12(1)+6(16)=180 g/mol
Number of molecules = 0.0556×6.022×1023=3.348×1022
Each molecule contains 6+12+6=246 + 12 + 6 = 24 atoms.
Number of atoms = 3.348×1022×24=8.035×1023

5. Investigative Questions

i. How many moles of water are needed for a single adult?
Volume of water per glass = 400 cm3
Density of water = 1 g/cm3
Mass of water = 400×8=3200 g

Molar mass of water = 18 g/mol.
Moles of water = Mass/Molar mass=3200/18=177.78 mol

Answer: 177.78 moles of water are needed.

ii. How many moles of SiO₂ are present in the glass?
Assuming the mass of glass is provided, moles = Mass of glass/Molar mass of SiO₂

where Molar mass of SiO₂=28+2(16)=60 g/mol

Exercise Solutions Atomic Structure – Chapter 2, 9th Punjab Board New Book (2025)

by

The detailed, step-by-step solutions to Chapter 2, “Atomic Structure,” from the new Punjab Board 9th Class textbook (2025 edition) by Ahsan Publishers. Aligned with the SLO-based syllabus, this guide provides clear explanations, solved exercises, and conceptual insights tailored to help students excel in their studies. Perfect for students preparing for exams and teachers seeking comprehensive teaching material.

(i) How many electrons can be accommodated at the most in the third shell of the elements?

  • Options:
    (a) 8
    (b) 18
    (c) 10
    (d) 32
  • Answer: (b) 18
  • Explanation:
    The maximum number of electrons in any shell is determined by the formula 2n2, where n is the shell number. For the third shell = 2(9) = 18 Thus, the third shell can accommodate a maximum of 18 electrons.

(ii) What information was obtained from discharge tube experiments?

  • Options:
    (a) Structure of atom was discovered.
    (b) Neutrons and protons were discovered.
    (c) Electrons and protons were discovered.
    (d) Presence of nucleus in an atom was discovered.
  • Answer: (d) Presence of nucleus in an atom was discovered.
  • Explanation:
    Rutherford’s experiments using a discharge tube and his gold foil experiment showed that atoms have a small, dense, positively charged nucleus at their center. These discoveries laid the foundation for understanding atomic structure.

(iii) Why have isotopes not been shown in the periodic table?

  • Options:
    (a) Periodic table cannot accommodate a large number of isotopes of different elements.
    (b) Some of the isotopes are unstable and they give rise to different elements.
    (c) All the isotopes have the same atomic number; so there is no need to give them separate places.
    (d) Isotopes do not show periodic behavior.
  • Answer: (c) All the isotopes have the same atomic number; so there is no need to give them separate places.
  • Explanation:
    Isotopes of an element have the same number of protons but differ in the number of neutrons. Since the periodic table is based on atomic number, isotopes are not given separate places—they occupy the same position.

(iv) Which particle is present in a different number in the isotopes?

  • Options:
    (a) Electron
    (b) Neutron
    (c) Proton
    (d) Both neutron and electron
  • Answer: (b) Neutron
  • Explanation:
    Isotopes are variants of the same element that have the same number of protons and electrons but differ in the number of neutrons. This difference in neutron number affects their atomic mass but not their chemical properties.

(v) Predict the boiling point of heavy water (D2O).

  • Options:
    (a) 101.4°C
    (b) 98.2°C
    (c) 100°C
    (d) 105.4°C
  • Answer: (a) 101.4°C
  • Explanation:
    Heavy water (D2O) has deuterium atoms instead of regular hydrogen atoms. Because deuterium is heavier, heavy water has slightly stronger intermolecular forces, resulting in a higher boiling point (approximately 101.4°C) compared to normal water (100°C).

(vi) What will be the relative atomic mass of hydrogen given the abundances of its two isotopes, 99.9844% and 0.0156%?

  • Options:
    (a) 1.0078
    (b) 1.0784
    (c) 1.0800
    (d) 1.0700
  • Answer: (a) 1.0078
  • Explanation:
    The relative atomic mass is calculated using the formula: Relative atomic mass=(m1⋅f1)+(m2⋅f2) , m1=1.0078, f1=0.999844 m2=2.0140 f2=0.000156 : (1.0078⋅0.999844)+(2.0140⋅0.000156)≈1.0078

(vii) How is radiocarbon dating useful for archeologists?

  • Options:
    (a) It helps determine the age of organic matter.
    (b) It helps determine the composition of matter.
    (c) It helps determine the usefulness of matter.
    (d) It helps determine whether the matter is radioactive or not.
  • Answer: (a) It helps determine the age of organic matter.
  • Explanation:
    Radiocarbon dating is based on measuring the decay of carbon-14, a radioactive isotope, to determine the age of organic materials such as bones, wood, or fossils. This technique is widely used in archeology to estimate the age of artifacts.

(viii) What does keep the particles present in the nucleus intact?

  • Options:
    (a) Particles are held together by strong nuclear force.
    (b) Particles are held together by weak nuclear force.
    (c) Particles are held together by electrostatic force.
    (d) Particles are held together by dipolar force.
  • Answer: (a) Particles are held together by strong nuclear force.
  • Explanation:
    The strong nuclear force is the fundamental force that binds protons and neutrons in the nucleus, overcoming the electrostatic repulsion between positively charged protons. This force operates only at very short ranges.

(ix) How do electrons keep themselves away from the oppositely charged nucleus?

  • Options:
    (a) By keeping themselves stationary
    (b) By revolving around the nucleus
    (c) Due to their wave-like nature
    (d) A magnetic field around the nucleus keeps them away
  • Answer: (b) By revolving around the nucleus
  • Explanation:
    Electrons revolve around the nucleus due to the balance between the electrostatic attraction to the positively charged nucleus and the centrifugal force from their motion. This concept is based on Bohr’s atomic model.

(x) Rubidium consists of two isotopes 85 and 87 . The percent abundance of the light isotope is 72.2%. What is the percent abundance of the heavier isotope? Its atomic mass is 85.47.

  • Options:
    (a) 15%
    (b) 28%
    (c) 37%
    (d) 72%
  • Answer: (b) 28%
  • Explanation:
    The percent abundances of isotopes must add up to 100%. Given that the light isotope has an abundance of 72.2%, the heavier isotope will have: 100%−72.2%=28%100

2. Questions for Short Answers

(i) Why is it said that almost all the mass of an atom is concentrated in its nucleus?
The nucleus of an atom contains protons and neutrons, which are much heavier than electrons. Since electrons are very light and located outside the nucleus, nearly all the mass of an atom comes from the protons and neutrons in its nucleus.

(ii) Why are elements different from one another?
Elements are different because they have different numbers of protons in their nuclei. The number of protons (also called the atomic number) is unique to each element and determines its properties and behavior.

(iii) How many neutrons are present in 83Bi 210
The isotope Bi 210 has a mass number (number of protons and number of neutrons) of 210 and an atomic number (number of protons) of 83. Number of neutrons=Mass number−Atomic number=209−83=126

So, it has 126 neutrons.

(iv) Why is tritium a radioactive element?
Tritium is radioactive because its nucleus, which contains one proton and two neutrons, is unstable. This instability causes it to emit radiation as it breaks down into a more stable form.

(v) How can an atom absorb and evolve energy?
Atoms absorb energy when electrons move to higher energy levels (excited state) after gaining energy. They release energy when electrons return to lower energy levels (ground state). This process is observed as the emission or absorption of light or other forms of energy.

3. Constructed Response Questions

(i) Why does the energy of an electron increase as we move from the first shell to the second shell?
The energy of an electron increases as we move from the first shell (closer to the nucleus) to the second shell (farther from the nucleus) due to the concept of electrostatic attraction. Electrons in the first shell are held more tightly by the positive charge of the nucleus because they are closer to it. To move an electron from the first shell to the second shell, it must overcome the strong attractive force of the nucleus, which requires energy. Additionally, electrons in higher shells have higher potential energy because they are less bound to the nucleus and are more “free” to move.

(ii) Why is it needed to lower the pressure of the gas inside the discharge tube?
In a discharge tube, lowering the pressure reduces the number of gas particles per unit volume. This is important because high-pressure gas would cause frequent collisions between gas particles, preventing the movement of free electrons and ions. At low pressure, the gas becomes less dense, allowing electrons to move freely through the tube and collide with gas atoms. These collisions excite the gas atoms, which then emit light as they return to their ground state, creating a visible glow or discharge. This principle was crucial in experiments like J.J. Thomson’s cathode ray experiments, which led to the discovery of the electron.

(iii) What is the classical concept of an electron? How has this concept changed with time?
The classical concept of an electron, based on the early atomic models such as J.J. Thomson’s and Bohr’s models, described electrons as small particles orbiting the nucleus in fixed circular paths (orbits), similar to planets orbiting the sun. However, with the development of quantum mechanics, this concept changed. The modern quantum mechanical model describes electrons as existing in regions called orbitals, where there is a high probability of finding them. Unlike fixed paths, orbitals represent three-dimensional regions around the nucleus, and electrons exhibit both particle-like and wave-like behavior. This understanding was made possible by advancements in the Schrödinger equation and Heisenberg’s uncertainty principle.

(iv) Why are the nuclei of radioactive elements unstable?
The nuclei of radioactive elements are unstable because of an imbalance in the number of protons and neutrons. A stable nucleus requires an optimal ratio of protons to neutrons, but in radioactive elements, this balance is disrupted. This imbalance results in excessive repulsive forces between protons, or insufficient binding forces among neutrons, causing the nucleus to emit radiation (alpha, beta, or gamma rays) in an attempt to reach a more stable state. The larger the nucleus, the harder it is to maintain stability, which is why heavier elements are often radioactive.

(v) During discharge tube experiments, how did scientists conclude that the same type of electrons and protons are present in all the elements?
Scientists observed that the cathode rays produced in a discharge tube always exhibited the same properties, regardless of the gas used in the tube. These rays were found to consist of negatively charged particles (electrons) with a fixed charge-to-mass ratio, independent of the type of atom or element present. Similarly, the positive rays (protons) generated in these experiments had consistent properties across different gases. This uniformity demonstrated that electrons and protons are fundamental components of all atoms, and their properties do not vary from one element to another.

4. Descriptive Questions

(i) Explain the structure of a hydrogen atom.
A hydrogen atom is the simplest atom, consisting of a single proton in the nucleus and one electron revolving around it. The nucleus, which contains the proton, is positively charged, while the electron is negatively charged. The electron occupies a specific energy level (or shell) around the nucleus. In the modern quantum mechanical model, the electron is described as existing in a spherical orbital around the nucleus, where it is most likely to be found. Since hydrogen has no neutrons, its mass is almost entirely due to the proton.

(ii) How does the theory of atomic structure explain the ionization of atoms by a radioactive isotope?
Ionization occurs when an atom loses or gains an electron, resulting in a charged particle called an ion. Radioactive isotopes emit high-energy particles or radiation (alpha, beta, or gamma rays) that can knock electrons out of atoms. For example, beta particles emitted by radioactive isotopes have enough energy to remove electrons from nearby atoms, creating ions. This process is the basis for many applications of radioactive isotopes, such as radiation therapy and smoke detectors.

(iii) What is radioactivity? Explain any three applications of radioactive isotopes.
Radioactivity is the spontaneous emission of particles or energy from the unstable nucleus of an atom. This occurs as the nucleus seeks to become more stable.
Applications:

  1. Medical Applications: Radioactive isotopes like iodine-131 are used in diagnosing and treating thyroid diseases, and cobalt-60 is used in cancer radiation therapy.
  2. Archaeology: Carbon-14 dating is used to determine the age of ancient objects like fossils or artifacts by measuring the decay of carbon-14 in organic materials.
  3. Energy Production: Uranium-235 and plutonium-239 are used as fuel in nuclear reactors to generate electricity through controlled nuclear fission reactions.

(iv) Find out the relative atomic mass of mercury from the given data.
To calculate the relative atomic mass, use the formula:

Ar=∑ (Isotope mass × Relative abundance fraction)

Substitute the given values:

Ar=(199×0.00146)+(198×0.1002)+(200×0.1322)+(201×0.0685)+(202×0.1634)+(204×0.2313)+(206×0.298)

After performing the calculations, the final relative atomic mass is obtained. This process demonstrates how isotopes contribute to the average atomic mass of an element.

5. Investigative Questions

(i) How can scientists synthesize elements in the laboratory?
Scientists synthesize new elements by bombarding the nuclei of existing elements with high-speed particles such as protons, neutrons, or heavier ions. This process often takes place in particle accelerators, where the nuclei are forced to collide with enough energy to fuse and form a new element. For example, synthetic elements like uranium-235 and transuranium elements like americium and einsteinium were created in this way. These processes require precise conditions and advanced technology.

(ii) A system just like our solar system exists in an atom. Comment on this statement.
The early atomic model, proposed by Niels Bohr, compared the atom to a miniature solar system, with electrons revolving around the nucleus in circular orbits, similar to how planets orbit the sun. While this analogy helped visualize atomic structure, modern quantum mechanics has shown that electrons do not follow fixed orbits. Instead, they exist in probabilistic regions of space called orbitals, which are shaped by their energy levels and interactions. Unlike planets, electrons also exhibit wave-particle duality, a concept that cannot be explained using the solar system analogy.

©2024. Everexams.com. All Rights Reserved.