10th Class Chemistry Notes 2026 — Chapter 1
Class 10 Chemistry — Chapter 1: History of Chemistry
Federal Board
1. Multiple Choice Questions (MCQs)
i. What is the principle of conservation of mass?
(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.
Correct Answer: (c) Mass remains constant during a chemical reaction.
Explanation: Formulated by Antoine Lavoisier, the Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.
ii. What does the peer review process ensure in scientific research?
(a) Faster publication
(b) Accuracy and validity of findings
(c) Higher funding
(d) Reduced experimentation
Correct Answer: (b) Accuracy and validity of findings.
Explanation: Peer review involves independent scientific experts evaluating a study’s methodology, data, and conclusions to verify that the research is sound, accurate, and valid before publication.
iii. Which of the following was an 18th-century chemical paradigm?
(a) Atomic theory
(b) Phlogiston theory
(c) Quantum mechanics
(d) Periodic table
Correct Answer: (b) Phlogiston theory.
Explanation: During the 18th century, combustion and oxidation were mainly explained by the phlogiston theory. It proposed that combustible materials contained a fire-like substance called phlogiston, which was released during burning.
iv. What does the periodic table of elements organize?
(a) Elements by alphabetical order
(b) Elements by their properties and atomic number
(c) Elements by colour
(d) Elements by discovery date
Correct Answer: (b) Elements by their properties and atomic number.
Explanation: The modern periodic table arranges elements in order of increasing atomic number (Z), grouping elements with similar physical and chemical properties together.
v. What does a 95% confidence level mean in scientific reporting?
(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.
Correct Answer: The source does not clearly identify the correct option. The explanation below gives the statistical meaning.
Explanation: A 95% confidence level means that if the same sampling process were repeated many times, approximately 95% of the resulting confidence intervals would contain the true population parameter. It does not mean that an individual result is 95% accurate.
vi. Which model of the atom includes a central nucleus?
(a) Plum-pudding model
(b) Rutherford model
(c) Bohr model
(d) Quantum mechanical model
Correct Answer: (b) Rutherford model.
Explanation: Ernest Rutherford’s gold foil experiment (1911) demonstrated that the positive charge and most of an atom’s mass are concentrated in a tiny, dense central region called the nucleus.
vii. What does repeatability in scientific experiments refer to?
(a) Different results under the same conditions
(b) Same results under the same conditions
(c) Different methods
(d) Multiple publications
Correct Answer: (b) Same results under the same conditions.
Explanation: Repeatability measures the agreement among results when the same experiment is performed by the same investigator using the same equipment, in the same location, under identical conditions.
viii. What is reproducibility in scientific experiments?
(a) Different results under the same conditions
(b) Same results using different methods
(c) Results not verified
(d) Repetition by the same scientist
Correct Answer: (b) Same results using different methods.
Explanation: Reproducibility measures whether the same scientific conclusion can be reached independently by different researchers using different instruments, locations, or analytical methods.
ix. What paradigm replaced the phlogiston theory?
(a) Atomic theory
(b) Theory of combustion
(c) Quantum mechanics
(d) Periodic table
Correct Answer: (b) Theory of combustion.
Explanation: Antoine Lavoisier disproved the phlogiston theory by demonstrating that combustion is a chemical reaction involving oxygen. This became known as the oxygen theory of combustion.
x. Which property does the periodic table help to predict?
(a) Colour of elements
(b) Properties of elements
(c) Weight of elements
(d) Discovery date of elements
Correct Answer: (b) Properties of elements.
Explanation: The periodic table groups elements into families with similar chemical reactivity, valence electrons, ionization energies, and electronegativity patterns.
2. Short Answer Questions
i. Explain the principle of conservation of mass in chemical reactions.
Answer:
The Law of Conservation of Mass states that matter cannot be created or destroyed in an isolated system during a chemical reaction. Consequently, the mass of the reactants before a chemical reaction must equal the mass of the products formed after the reaction.
Atoms are simply rearranged into new chemical combinations.
ii. What is the role of empirical evidence in scientific research?
Answer:
Empirical evidence consists of data collected through direct observation, measurement, and controlled experimentation rather than theoretical speculation or belief.
Its roles include:
Validating or falsifying scientific hypotheses and theories.
Providing objective, measurable proof to ensure conclusions are repeatable and free from bias.
iii. Describe the peer review process and its importance in science.
Answer:
Process: Scientists submit their experimental findings to a scientific journal. The manuscript is evaluated by independent experts (peers) in the same field who assess the methodology, statistical analysis, logical reasoning, and originality.
Importance: Peer review acts as a quality-control mechanism. It helps identify flawed methodologies, unsupported claims, and personal bias, maintaining the integrity of scientific literature.
iv. How did the phlogiston theory explain combustion?
Answer:
The Phlogiston Theory proposed that combustible substances contained an invisible substance called phlogiston. During burning, phlogiston was released into the air, leaving behind ash or calx (dephlogisticated residual matter).
Burning stopped in closed containers when the surrounding air became saturated with phlogiston.
v. What is the significance of Rutherford’s model of the atom?
Answer:
Rutherford’s model revolutionized the understanding of atomic structure by replacing J.J. Thomson’s homogeneous plum-pudding model. It introduced the following ideas:
An atom has a tiny, dense, positively charged central region called the nucleus, which contains nearly all the atom’s mass.
An atom is mostly empty space, with electrons surrounding the central nucleus.
vi. How does the periodic table organize elements?
Answer:
The periodic table organizes elements:
In order of increasing atomic number (Z), which is the number of protons.
Into horizontal rows called periods and vertical columns called groups or families.
Elements in the same group have similar valence-shell electron configurations, resulting in similar chemical properties and trends.
vii. Define scientific paradigm with an example.
Answer:
Definition: A scientific paradigm is a universally recognized set of concepts, theories, standards, and methods that defines a scientific discipline during a specific era.
Example: The shift from the Phlogiston Theory to Lavoisier’s Oxygen Theory of Combustion is an example of a change in scientific paradigm. Another example is the transition from Newtonian Classical Mechanics to Einstein’s Theory of Relativity.
viii. What does a confidence level in scientific research indicate?
A confidence level indicates how reliable an estimate is in representing the true population parameter.
For example, a 95% confidence level means that if an experiment is repeated many times using the same sampling method, approximately 95% of the calculated confidence intervals will contain the true population value.
ix. Differentiate between repeatability and reproducibility in experiments.
Feature | Repeatability | Reproducibility |
|---|---|---|
Operator | Conducted by the same researcher. | Conducted by different researchers. |
Equipment and laboratory | Uses the same apparatus and laboratory. | Uses different equipment and locations. |
Objective | Measures precision and consistency under identical conditions. | Confirms the validity and general applicability of scientific findings. |
x. Why is skepticism important in the scientific community?
Answer:
Scientific skepticism ensures that new claims, theories, and experimental results are not accepted blindly. It requires researchers to support their claims with strong empirical evidence, thorough testing, and reproducible methods.
It helps prevent false conclusions, pseudoscience, and bias from entering scientific knowledge.
3. Long Answer Questions
i. Discuss the transition from the phlogiston theory to the oxygen theory of combustion and its impact on chemistry.
Answer:
The Phlogiston Paradigm:
In the 17th and 18th centuries, Johann Joachim Becher and Georg Ernst Stahl proposed the Phlogiston Theory. It stated that flammable materials contained phlogiston, an odourless, colourless, and weightless substance released during combustion. When wood burned, phlogiston escaped, leaving ash behind.
Weakness of the Phlogiston Theory:
The theory faced a fundamental contradiction when scientists studied the burning of metals. When metals were burned (calcined), the resulting metal calx (oxide) was heavier than the original metal, even though phlogiston was supposedly released.
Lavoisier’s Quantitative Revolution:
Antoine Lavoisier conducted quantitative experiments using sealed reaction vessels and precise balances. He demonstrated that:
Combustion requires a specific gas present in air, which he named oxygen.
When a metal burns, it gains mass by combining with oxygen gas.
3. The total mass of reactants equals the total mass of products in a closed system, leading to the Law of Conservation of Mass.
Impact on Chemistry:
This change in scientific understanding disproved the Phlogiston Theory and marked the beginning of modern quantitative chemistry. Chemistry moved from a mainly qualitative and alchemy-based framework towards a rigorous, quantitative branch of natural science.
ii. Explain the development of atomic models from the plum-pudding model to the quantum mechanical model.
Answer:
The scientific understanding of atomic structure evolved through several important models.
1. Dalton’s Model (1808):
Dalton described the atom as an indivisible, solid sphere, similar to a billiard ball.
2. Thomson’s Plum-Pudding Model (1904):
After the discovery of the electron, J.J. Thomson proposed that an atom was a sphere of uniform positive charge with negatively charged electrons embedded in it, like raisins in a pudding.
3. Rutherford’s Nuclear Model (1911):
Based on the alpha-particle scattering experiment, Rutherford disproved Thomson’s model. He demonstrated that an atom consists of a tiny, dense, positively charged nucleus containing nearly all the atomic mass, surrounded by electrons in mostly empty space.
4. Bohr’s Model (1913):
Niels Bohr improved Rutherford’s model by introducing quantized energy levels. He proposed that electrons move around the nucleus in fixed circular paths called stationary orbits without radiating energy unless they transition between energy levels.
5. Quantum Mechanical Model (Modern):
Developed through the contributions of Erwin Schrödinger, Werner Heisenberg, and Louis de Broglie, this model describes electrons as having both wave-like and particle-like properties.
Instead of fixed orbits, electrons exist in three-dimensional regions of high probability called atomic orbitals. These orbitals are described by quantum numbers.
iii. Analyze how the periodic table serves as a paradigm in chemistry, guiding research and discovery of new elements.
Answer:
The Periodic Table as a Scientific Paradigm:
Dmitri Mendeleev established an important scientific framework by arranging elements according to the periodic recurrence of their chemical properties. This transformed chemistry from an unsystematic collection of facts into an organized and predictive science.
Guiding the Discovery of New Elements:
Gaps and predictions: Mendeleev intentionally left gaps in his original periodic table for undiscovered elements, such as Eka-aluminium, Eka-boron, and Eka-silicon.
He predicted their atomic masses and properties with considerable accuracy.
These elements were later discovered as gallium, scandium, and germanium, respectively.
Guiding Chemical Research:
The modern periodic table helps predict the electronic configurations and chemical behaviour of synthetic elements, including transuranic and superheavy elements such as oganesson and tennessine.
It guides scientists in nuclear experiments involving the synthesis and study of new elements.
The table helps chemists predict reactivity, valency, bonding behaviour (ionic or covalent), oxidation states, and material properties.
These predictions are based on periodic trends such as atomic radius, ionization energy, and electronegativity.
iv. Evaluate the importance of repeatability and reproducibility in maintaining scientific integrity and progress.
Answer:
Maintaining Scientific Integrity
Verification of results: Repeatability ensures that an experiment produces consistent results under identical conditions. It helps identify random measurement errors, sample contamination, and procedural mistakes.
Elimination of bias and fraud: Reproducibility by independent research groups helps identify confirmation bias, theoretical prejudice, and possible data fabrication.
Driving Scientific Progress
Building trust: Science progresses through verified facts. New theories are built upon reliable experimental results. If foundational experiments cannot be reproduced, the theories based on them may become unreliable.
Refinement of theories: When an experiment fails to reproduce under slightly altered conditions, it can lead to scientific advances by revealing unaccounted variables, systematic errors, or new physical phenomena.
v. Describe how levels of confidence and uncertainty are expressed and interpreted in scientific research, providing examples from chemistry experiments.
Answer:
Expressing Uncertainty:
Every experimental measurement carries some uncertainty due to limitations of instruments and environmental factors.
In chemistry, experimental results may be reported as:
Example: A volumetric flask reading may be reported as:
Expressing Confidence Levels:
Confidence intervals quantify the reliability of an estimate. They are commonly expressed at 95% or 99% confidence levels.
The margin of error can be calculated using:
Where:
\(Z\) = Critical value
\(s\) = Sample standard deviation
\(n\) = Sample size
Interpretation in Chemistry Experiments:
Titration example: In an acid-base titration, repeating five trials might yield an average molarity of 0.1002 M with a standard deviation of ±0.0003 M.
At a 95% confidence level, researchers can estimate that the actual molarity lies between 0.0999 M and 0.1005 M, as given in the source example.
Significance: Expressing confidence and uncertainty helps distinguish genuine chemical differences, such as changes in reaction rates, from random measurement errors.
Notes by Sir Dilawar
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