chemistry MCQs

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chemistry MCQs

Practice complete Chemistry MCQs covering Matter, States of Matter, Physical and Chemical Changes, Atomic Structure, Chemical Bonding, Periodic Table, Acids, Bases and Salts, Metals and Non-Metals, Chemical Reactions, Electrochemistry, Organic Chemistry, Hydrocarbons, Environmental Chemistry, Polymers, Biomolecules, Nuclear Chemistry, and all other important topics. Includes chapter-wise and exam-oriented multiple-choice questions with detailed answers and explanations for JKSSB, SSC, Banking, Railway, UPSC, CUET, NEET, State PSCs, and other competitive exams.

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Question #661
The pH of normal rainwater is slightly acidic, around:
A. 4.5
B. 5.6
C. 7.0
D. 8.0
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Correct Answer: Option B

Explanation:
Normal rainwater is slightly acidic with a pH of about 5.6. This is due to the natural dissolution of carbon dioxide from the air into the rain droplets, forming weak carbonic acid (CO₂ + H₂O ⇌ H₂CO₃). Acid rain has a pH lower than 5.6 due to pollutants.
This question belongs to: Science chemistry
Question #662
Which of the following is a primary air pollutant?
A. Carbon monoxide
B. Sulphuric acid
C. Ozone
D. PAN (Peroxyacetyl nitrate)
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Correct Answer: Option A

Explanation:
Primary air pollutants are emitted directly from a source. Carbon monoxide (CO), sulfur dioxide (SO₂), and nitrogen oxides (NOx) are primary pollutants. Ozone, PAN, and sulfuric acid are secondary pollutants, formed in the atmosphere by reactions of primary pollutants.
This question belongs to: Science chemistry
Question #663
BOD stands for:
A. Basic Oxygen Depletion
B. Biological Oxygen Demand
C. Biological Ozone Depletion
D. Basic Oxygen Demand
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Correct Answer: Option B

Explanation:
BOD stands for Biological Oxygen Demand. It is a measure of the amount of dissolved oxygen required by microorganisms to decompose the organic matter present in a water sample. High BOD indicates severe organic pollution and poor water quality for aquatic life.
This question belongs to: Science chemistry
Question #664
Minamata disease is caused by the pollution of water with:
A. Cadmium
B. Arsenic
C. Mercury
D. Lead
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Correct Answer: Option C

Explanation:
Minamata disease is a neurological syndrome caused by severe mercury poisoning. It was first discovered in Minamata city, Japan, due to the release of methylmercury in industrial wastewater from a chemical factory, which bioaccumulated in fish and shellfish consumed by the local population.
This question belongs to: Science chemistry
Question #665
Assertion (A): The Montreal Protocol was signed to control the emission of CFCs. Reason (R): CFCs are responsible for the depletion of the ozone layer.
A. Both A and R are true and R is the correct explanation of A.
B. A is true but R is false.
C. Both A and R are true but R is NOT the correct explanation of A.
D. A is false but R is true.
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Correct Answer: Option A

Explanation:
The Montreal Protocol is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances responsible for ozone depletion, most notably chlorofluorocarbons (CFCs). Both statements are correct, and R perfectly explains the purpose of the protocol.
This question belongs to: Science chemistry
Question #666
Which of the following gases is responsible for the formation of photochemical smog?
A. Ammonia
B. Ozone and PAN
C. Methane
D. Carbon dioxide
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Correct Answer: Option B

Explanation:
Photochemical smog is formed in sunny, dry cities due to the reaction of sunlight with nitrogen oxides and volatile organic compounds (from vehicle exhaust). This produces secondary pollutants like ozone (O₃), peroxyacetyl nitrate (PAN), and aldehydes, which make up the harmful smog.
This question belongs to: Science chemistry
Question #667
The phenomenon of global warming is primarily linked to an increase in:
A. Dust particles in the air
B. Ozone layer thickness
C. Greenhouse gases concentration
D. Oxygen concentration
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Correct Answer: Option C

Explanation:
Global warming is the long-term rise in Earth's average temperature, primarily caused by the increased concentration of greenhouse gases (like CO₂, CH₄, N₂O) in the atmosphere. These gases trap outgoing infrared radiation, enhancing the natural greenhouse effect and leading to climate change.
This question belongs to: Science chemistry
Question #668
Which of the following rays has the highest penetrating power?
A. Alpha rays
B. X-rays
C. Gamma rays
D. Beta rays
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Correct Answer: Option C

Explanation:
Gamma rays are high-energy electromagnetic waves with no mass and no charge. They have the highest penetrating power among alpha, beta, and gamma rays, requiring thick lead or concrete to be stopped. Alpha is stopped by paper, beta by aluminum foil.
This question belongs to: Science chemistry
Question #669
Alpha particles are essentially:
A. Helium nuclei
B. Photons
C. Electrons
D. Hydrogen atoms
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Correct Answer: Option A

Explanation:
Alpha particles (α) are positively charged particles consisting of two protons and two neutrons, identical to the nucleus of a helium-4 atom (He²⁺). They are emitted during alpha decay of heavy radioactive elements like uranium and radium.
This question belongs to: Science chemistry
Question #670
Which of the following is a naturally occurring radioactive element?
A. Calcium
B. Iron
C. Aluminum
D. Uranium
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Correct Answer: Option D

Explanation:
Uranium (U) is a naturally occurring heavy metal that is radioactive. Its most common isotopes are U-238 and U-235. Aluminum, iron, and calcium are stable, non-radioactive elements under normal conditions, though they may have synthetic radioactive isotopes.
This question belongs to: Science chemistry
Question #671
Beta rays are composed of:
A. Positively charged helium nuclei
B. High-speed electrons
C. Electromagnetic waves
D. Neutrons
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Correct Answer: Option B

Explanation:
Beta rays (β) consist of high-speed, high-energy electrons (β⁻) or positrons (β⁺) emitted from the nucleus during radioactive decay. In standard beta decay, a neutron converts into a proton and an electron, and the electron is ejected from the nucleus.
This question belongs to: Science chemistry
Question #672
If the half-life of a radioactive substance is 10 days, what fraction of the original substance will remain after 30 days?
A. 1/16
B. 1/4
C. 1/8
D. 1/2
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Correct Answer: Option C

Explanation:
The number of half-lives elapsed is 30 days / 10 days = 3. The fraction remaining is (1/2)ⁿ, where n is the number of half-lives. So, (1/2)³ = 1/8. After 10 days: 1/2; after 20 days: 1/4; after 30 days: 1/8 of the original amount remains.
This question belongs to: Science chemistry
Question #673
Gamma rays differ from alpha and beta rays in that they:
A. Have no mass and no charge.
B. Have mass.
C. Are deflected by magnetic fields.
D. Have a positive charge.
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Correct Answer: Option A

Explanation:
Gamma rays (γ) are high-energy electromagnetic radiation (photons). Unlike alpha particles (mass, +2 charge) and beta particles (mass, -1 charge), gamma rays have zero rest mass and no electrical charge. Thus, they are not deflected by electric or magnetic fields.
This question belongs to: Science chemistry
Question #674
Assertion (A): Radioactive decay is a nuclear process. Reason (R): It involves changes in the nucleus of an atom.
A. Both A and R are true but R is NOT the correct explanation of A.
B. A is false but R is true.
C. Both A and R are true and R is the correct explanation of A.
D. A is true but R is false.
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Correct Answer: Option C

Explanation:
Radioactive decay is a spontaneous process where an unstable atomic nucleus loses energy by emitting radiation. Since it involves the emission of particles (protons, neutrons, electrons) from the nucleus, it fundamentally alters the nucleus, making it a nuclear, not chemical, process.
This question belongs to: Science chemistry
Question #675
The process in which a heavy nucleus splits into two lighter nuclei with the release of enormous energy is called:
A. Nuclear fusion
B. Radioactive decay
C. Transmutation
D. Nuclear fission
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Correct Answer: Option D

Explanation:
Nuclear fission is a reaction in which the nucleus of a heavy atom (like Uranium-235 or Plutonium-239) splits into two or smaller, lighter nuclei when bombarded by a neutron, releasing a massive amount of energy and more neutrons, potentially causing a chain reaction.
This question belongs to: Science chemistry
Question #676
Who proposed the atomic theory of matter, stating that all matter is composed of indivisible atoms?
A. Dmitri Mendeleev
B. Ernest Rutherford
C. J.J. Thomson
D. John Dalton
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Correct Answer: Option D

Explanation:
John Dalton proposed the atomic theory in 1803. His theory stated that all matter is made of indivisible and indestructible atoms, atoms of a given element are identical in mass and properties, and compounds are formed by combinations of different atoms in fixed ratios.
This question belongs to: Science chemistry
Question #677
The modern periodic table is based on the periodic law proposed by:
A. Henry Moseley
B. Dmitri Mendeleev
C. Niels Bohr
D. John Dalton
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Correct Answer: Option A

Explanation:
Henry Moseley discovered that the atomic number (number of protons) is the fundamental property of an element, not atomic mass. He proposed the modern periodic law: properties are periodic functions of atomic numbers. Mendeleev's law was based on atomic mass, which had anomalies.
This question belongs to: Science chemistry
Question #678
Ernest Rutherford is famous for his discovery of the:
A. Neutron
B. Electron
C. Proton and Nucleus
D. Atomic theory
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Correct Answer: Option C

Explanation:
Ernest Rutherford discovered the atomic nucleus through his gold foil alpha-scattering experiment in 1911. He later discovered the proton in 1919 by bombarding nitrogen with alpha particles. J.J. Thomson discovered the electron, and Chadwick discovered the neutron.
This question belongs to: Science chemistry
Question #679
Niels Bohr modified Rutherford's atomic model by proposing that:
A. Electrons are embedded in a positive sphere.
B. Electrons have wave-like properties.
C. Electrons revolve in fixed, discrete circular orbits called energy levels.
D. The nucleus contains protons and neutrons.
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Correct Answer: Option C

Explanation:
Niels Bohr proposed his model in 1913 to overcome the defects of Rutherford's model. He stated that electrons revolve around the nucleus in specific, discrete circular orbits (shells) with fixed energy, and they do not radiate energy while in these orbits, explaining atomic stability.
This question belongs to: Science chemistry
Question #680
J.J. Thomson discovered the electron using:
A. Alpha scattering experiment
B. Cathode ray tube experiment
C. Oil drop experiment
D. Gold foil experiment
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Correct Answer: Option B

Explanation:
J.J. Thomson discovered the electron in 1897 using a cathode ray tube. He showed that cathode rays were composed of negatively charged particles (electrons) that were much lighter than atoms and were a fundamental part of all matter, leading to his plum pudding model.
This question belongs to: Science chemistry

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