MCQS
i. What is the principle of conservation of mass?
Options:
a) Mass is created during a chemical reaction
b) Mass is destroyed during a chemical reaction
c) Mass remains constant during a chemical reaction
d) Mass can be converted into energy
Answer:
c) Mass remains constant during a chemical reaction
Explanation:
The principle of conservation of mass states that mass is neither created nor destroyed in a chemical reaction; it is only rearranged. This was first formulated by Antoine Lavoisier.
Tips/Tricks:
- Remember the phrase: “Matter cannot be created or destroyed.”
- Eliminate options suggesting mass is created/destroyed (a, b). Option (d) refers to mass-energy equivalence (Einstein’s theory), which is not the conservation of mass.
ii. What does the peer review process ensure in scientific research?
Options:
a) Faster publication
b) Accuracy and validity of findings
c) Higher funding
d) Reduced experimentation
Answer:
b) Accuracy and validity of findings
Explanation:
Peer review involves evaluation by experts in the field to ensure the research is credible, methodologically sound, and free of errors before publication.
Tips/Tricks:
- Focus on the purpose of peer review: quality control. Options (a), (c), and (d) are unrelated to its primary goal.
iii. Which of the following was an 18th-century chemical paradigm?
Options:
a) Atomic theory
b) Phlogiston theory
c) Quantum mechanics
d) Periodic table
Answer:
b) Phlogiston theory
Explanation:
The phlogiston theory (17th–18th century) proposed that a fire-like element (“phlogiston”) was released during combustion. It was later disproven by Lavoisier.
Tips/Tricks:
- Note the time frame: “18th century.” Atomic theory (a) and the periodic table (d) emerged later. Quantum mechanics (c) is 20th-century.
iv. What does the periodic table of elements organize?
Options:
a) Elements by alphabetical order
b) Elements by their properties and atomic number
c) Elements by colour
d) Elements by discovery date
Answer:
b) Elements by their properties and atomic number
Explanation:
The periodic table arranges elements in order of increasing atomic number and groups them by similar chemical properties.
Tips/Tricks:
- Recall that atomic number (proton count) is the primary organizing factor.
v. What does a 95% confidence level mean in scientific reporting?
Options:
a) Results are 95% accurate
b) There is a 5% chance the results are incorrect
c) 95% of scientists agree
d) The experiment is repeated 95 times
Answer:
b) There is a 5% chance the results are incorrect
Explanation:
A 95% confidence level means there is a 5% probability that the observed results occurred by random chance.
Tips/Tricks:
- “Confidence level” relates to statistical probability, not accuracy (a) or consensus (c).
vi. Which model of the atom includes a central nucleus?
Options:
a) Plum-pudding model
b) Rutherford model
c) Bohr model
d) Quantum mechanical model
Answer:
b) Rutherford model
Explanation:
Rutherford’s gold foil experiment (1911) revealed the atom’s dense nucleus, replacing the plum-pudding model (a).
Tips/Tricks:
- Bohr (c) and quantum models (d) came later and built on Rutherford’s nucleus discovery.
vii. What does repeatability in scientific experiments refer to?
Options:
a) Different results under the same conditions
b) Same results under the same conditions
c) Different methods
d) Multiple publications
Answer:
b) Same results under the same conditions
Explanation:
Repeatability means the same team can replicate results using identical methods and conditions.
Tips/Tricks:
- “Repeat” = same conditions. “Reproducibility” (next question) involves different conditions.
viii. What is reproducibility in scientific experiments?
Options:
a) Different results under the same conditions
b) Same results using different methods
c) Results not verified
d) Repetition by the same scientist
Answer:
b) Same results using different methods
Explanation:
Reproducibility means independent teams can achieve similar results with different methods or setups.
Tips/Tricks:
- Contrast with repeatability: reproducibility is broader (different labs/methods).
ix. What paradigm replaced the phlogiston theory?
Options:
a) Atomic theory
b) Theory of combustion
c) Quantum mechanics
d) Periodic table
Answer:
b) Theory of combustion
Explanation:
Lavoisier’s theory of combustion (involving oxygen) replaced the phlogiston theory in the late 18th century.
Tips/Tricks:
- Link phlogiston to combustion. Atomic theory (a) and periodic table (d) are unrelated.
x. Which property does the periodic table help to predict?
Options:
a) Colour of elements
b) Properties of elements
c) Weight of elements
d) Discovery date of elements
Answer:
b) Properties of elements
Explanation:
The periodic table’s arrangement reveals trends in chemical properties (e.g., reactivity, electronegativity).
Tips/Tricks:
- Focus on “properties,” as other options (a, c, d) are not primary purposes of the table.
Short Questions:
i. Explain the principle of conservation of mass in chemical reactions.
Answer:
The principle of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. The total mass of the reactants (substances before the reaction) is always equal to the total mass of the products (substances after the reaction). This law was formulated by Antoine Lavoisier, who demonstrated it through experiments.
Example:
When magnesium burns in oxygen, the mass of magnesium oxide formed equals the combined mass of magnesium and oxygen used.
Key Points:
- Mass remains constant.
- Atoms are rearranged, not destroyed.
- Lavoisier proved this through experiments.
ii. What is the role of empirical evidence in scientific research?
Answer:
Empirical evidence refers to data collected through observation and experimentation rather than just theories. It is crucial in science because:
- Supports or disproves hypotheses – Scientists rely on experiments to verify ideas.
- Ensures objectivity – Results must be measurable and repeatable.
- Forms the basis of scientific laws – Repeated observations lead to established facts (e.g., conservation of mass).
Example:
Lavoisier’s experiments on combustion provided empirical evidence against the phlogiston theory.
iii. Describe the peer review process and its importance in science.
Answer:
The peer review process involves experts evaluating a scientific study before it is published.
Steps:
- A scientist submits research to a journal.
- Experts in the field review it for accuracy, validity, and methodology.
- If approved, it is published; if not, corrections are suggested.
Importance:
- Ensures high-quality, reliable research.
- Prevents false or misleading claims.
- Maintains trust in scientific knowledge.
iv. How did the phlogiston theory explain combustion?
Answer:
The phlogiston theory (17th–18th century) proposed that:
- A fire-like substance called phlogiston was present in combustible materials.
- During burning, phlogiston was released into the air.
- Materials stopped burning when all phlogiston was gone.
Limitation:
- It failed to explain why metals gained mass when burned (later explained by oxygen theory).
v. What is the significance of Rutherford’s model of the atom?
Answer:
Rutherford’s nuclear model (1911) was significant because:
- It disproved the plum-pudding model (which said atoms were uniform).
- It introduced the concept of a dense, positively charged nucleus.
- It showed that most of the atom is empty space with electrons orbiting.
Experiment:
Gold foil experiment—alpha particles were deflected, proving the nucleus existed.
vi. How does the periodic table organize elements?
Answer:
The periodic table organizes elements by:
- Atomic number (proton count) – Elements are arranged in increasing order.
- Groups (columns) – Elements with similar properties (e.g., alkali metals in Group 1).
- Periods (rows) – Shows trends like increasing reactivity.
Example:
- Group 17 (Halogens): Highly reactive nonmetals (e.g., chlorine, fluorine).
- Period 3: Contains sodium (Na), magnesium (Mg), etc.
vii. Define scientific paradigm with an example.
Answer:
A scientific paradigm is a widely accepted framework that guides research.
Example:
- Phlogiston theory (old paradigm for combustion).
- Oxygen theory (new paradigm by Lavoisier).
Key Idea:
When new evidence challenges a paradigm, a scientific revolution occurs (e.g., shift from phlogiston to oxygen).
viii. What does a confidence level in scientific research indicate?
Answer:
A confidence level (e.g., 95%) indicates:
- The probability that results are not due to random chance.
- A 95% confidence level means there is a 5% chance of error.
- It helps scientists assess reliability (e.g., in drug trials).
Example:
If a study says “95% confidence,” it means the conclusion is likely correct 19 out of 20 times.
ix. Differentiate between repeatability and reproducibility in experiments.
Answer:
| Repeatability | Reproducibility |
|---|---|
| Same results when the same scientist repeats the experiment under identical conditions. | Same results when different scientists repeat the experiment using different methods/labs. |
| Example: A chemist replicates their own experiment. | Example: Multiple labs confirm a discovery independently. |
Importance:
- Repeatability ensures consistency.
- Reproducibility validates broader reliability.
x. Why is skepticism important in the scientific community?
Answer:
Skepticism is crucial because:
- It prevents blind acceptance of claims without evidence.
- Encourages testing and verification (e.g., debunking phlogiston theory).
- Leads to better theories (e.g., oxygen replacing phlogiston).
Example:
Scientists questioned the phlogiston theory until Lavoisier provided better evidence for oxygen.
Long Questions:
Long Answer Questions (Detailed Solutions)
i. Transition from Phlogiston Theory to Oxygen Theory of Combustion
1. Phlogiston Theory (17th–18th Century)
- Explanation:
- Proposed by Georg Ernst Stahl.
- Suggested that combustible materials contained “phlogiston” (a fire-like substance).
- During burning, phlogiston was released into the air.
- Limitations:
- Could not explain why metals gained mass after burning (e.g., magnesium oxide).
- Contradicted empirical evidence.
2. Oxygen Theory (Late 18th Century)
- Lavoisier’s Contributions:
- Demonstrated that combustion requires oxygen (not phlogiston).
- Showed that metals combined with oxygen from the air, increasing their mass.
- Introduced the law of conservation of mass.
- Impact on Chemistry:
- Disproved phlogiston theory, leading to modern chemical nomenclature.
- Established quantitative methods in chemistry (measuring reactants/products).
3. Key Example:
- Experiment: Heating mercury oxide produced oxygen, proving it was part of combustion.
Exam Tip: Focus on Lavoisier’s experiments and how they debunked phlogiston.
ii. Development of Atomic Models
1. Plum-Pudding Model (J.J. Thomson, 1897)
- Description:
- Atom as a “pudding” of positive charge with electrons embedded (like raisins).
- Limitation: Could not explain atomic stability.
2. Rutherford’s Nuclear Model (1911)
- Gold Foil Experiment:
- Alpha particles deflected by a dense nucleus.
- Proved atoms are mostly empty space.
- Key Change: Introduced the central nucleus (protons + neutrons).
3. Bohr Model (1913)
- Improvements:
- Electrons orbit in fixed energy levels (shells).
- Explained atomic spectra (e.g., hydrogen’s emission lines).
4. Quantum Mechanical Model (Modern Model)
- Key Features:
- Electrons exist in probability clouds (orbitals).
- Uses quantum numbers to describe electron location.
Exam Tip: Compare each model’s strengths/weaknesses and experimental evidence.
iii. Periodic Table as a Paradigm in Chemistry
1. Organization Principles:
- Atomic Number: Elements arranged by proton count.
- Groups (Columns): Similar chemical properties (e.g., Group 1 = Alkali Metals).
- Periods (Rows): Trends in reactivity and atomic size.
2. Predictive Power:
- Mendeleev’s Predictions: Left gaps for undiscovered elements (e.g., gallium, germanium).
- Modern Applications:
- Predicts reactivity (e.g., fluorine = most reactive nonmetal).
- Guides synthesis of new elements (e.g., synthetic elements like Tennessine).
3. Impact on Research:
- Unified chemistry by classifying elements systematically.
- Enabled discoveries like noble gases (Group 18).
Exam Tip: Highlight Mendeleev’s contributions and modern applications.
iv. Importance of Repeatability and Reproducibility
1. Definitions:
- Repeatability: Same results under identical conditions (same lab).
- Reproducibility: Same results under different conditions (other labs).
2. Role in Scientific Integrity:
- Prevents Fraud: Ensures data is not fabricated (e.g., cold fusion controversy).
- Validates Theories:
- Example: Lavoisier’s experiments were repeated to confirm oxygen theory.
- Failed Reproducibility: Phlogiston theory collapsed when others couldn’t verify it.
3. Case Study:
- Millikan’s Oil Drop Experiment: Repeated globally to confirm electron charge.
Exam Tip: Contrast repeatability (same team) vs. reproducibility (independent teams).
v. Confidence Levels and Uncertainty in Chemistry Experiments
1. Definitions:
- Confidence Level (e.g., 95%): Probability that results are not due to chance.
- Uncertainty: Margin of error in measurements (e.g., ±0.01g).
2. Expression in Research:
- Statistical Tools:
- Standard deviation: Measures data spread.
- p-value: Likelihood of observed results occurring randomly.
- Example:
- A drug trial with “95% confidence” means results are reliable 19/20 times.
3. Practical Example:
- Titration Experiments:
- Report average volume with ± uncertainty (e.g., 24.30 ± 0.05 mL).
- Repeats reduce uncertainty.
Exam Tip: Link confidence levels to real chemistry experiments (e.g., titration).
