Conservation of mass: in a chemical reaction the total mass of reactants equals the total mass of products (Lavoisier, 18th century).
Scientific paradigm: a theoretical model, a set of ideas, methods and rules that guides research and explains nature. It is replaced when new evidence challenges it.
Peer review: experts check experiments, data and conclusions before work is published, to make sure it is accurate and reliable.
Repeatability: same result when the experiment is repeated under the same conditions, same tools, same place, same operator. Reproducibility: same or similar result under different conditions, equipment, methods or labs.
Confidence interval: range showing how precise a measurement is (0.50 M with a 95% interval of 0.48 to 0.52 M means 95% sure the true value lies in that range). p-value: chance the result is random (0.01 means 1%). Standard deviation or standard error: spread of the data. Bayesian probability: updating confidence in an idea using new evidence. Uncertainty: how reliable a measurement is (0.250 ± 0.005 M).
Must-know points
Phlogiston theory (1700s): burning materials release "phlogiston". Lavoisier proved combustion is a reaction with oxygen, which gave the modern ideas of oxidation and reduction.
Atomic models: Thomson plum-pudding (1904), Rutherford nuclear model (1911, gold foil experiment, small dense positive nucleus), then Bohr (energy levels) and the quantum model.
Mendeleev made the periodic table in 1869 by atomic mass and predicted undiscovered elements. The modern table uses atomic number and electron configuration, which explains periodicity.
Methods of chemistry: conservation laws, observation and logic, controlled experiments (change one variable at a time), peer review, objectivity, skepticism.
Past-paper note
No written question from this unit appeared in the 2026 papers. Expect it only as an MCQ. Likely short questions from the book exercise: state conservation of mass; define a paradigm with an example; differentiate repeatability and reproducibility; why skepticism matters.