Questions: The History of Chemistry: From Alchemy to Atomic Theory

Question 1

What was 'phlogiston theory,' and how did Lavoisier overturn it?

Model answer: Phlogiston theory (dominant 17th-18th centuries) proposed that combustible materials contained a substance called phlogiston that was released during burning. It explained why things burned (they had phlogiston) and why some metals formed calxes (ashes) when heated (phlogiston left the metal). Antoine Lavoisier overturned it in the 1770s-1780s through careful quantitative measurements: he showed that substances gained weight when burned (rather than losing phlogiston mass), and identified oxygen as the gas absorbed during combustion. His law of conservation of mass — matter is neither created nor destroyed in chemical reactions — provided the quantitative framework that phlogiston could not accommodate.

The phlogiston-to-oxygen transition is a classic case of scientific paradigm shift. Lavoisier is often called the 'father of modern chemistry' because his quantitative methods and oxygen theory fundamentally reoriented the field.

Question 2

Mendeleev's 1869 periodic table organized elements by atomic weight and left gaps for undiscovered elements. What made this a remarkable predictive achievement?

Model answer: Mendeleev predicted not just the existence of missing elements but their properties: atomic weight, density, physical state, chemical behavior. His predicted 'eka-aluminum' was discovered as gallium (1875) with properties closely matching his predictions; 'eka-boron' became scandium (1879); 'eka-silicon' became germanium (1886). This predictive success — forecasting the existence and properties of unknown elements from a theoretical pattern — was extremely compelling evidence for atomic theory and the periodic organization of chemistry. It demonstrated that the table reflected real structure in nature, not just a convenient filing system.

Mendeleev's predictions were crucial to the acceptance of atomic theory. The periodic table also had to deal with anomalies (some elements didn't fit by weight) that were only resolved when atomic number replaced atomic weight as the organizing principle.

Question 3

Robert Boyle is sometimes called the 'first modern chemist.' What distinction did he draw that was important for chemistry's development?

• A. He discovered oxygen and established its role in combustion
• B. He defined a 'chemical element' as a substance that cannot be decomposed further by chemical means
• C. He invented the periodic table of elements
• D. He formulated the law of definite proportions

Answer: (B)

In 'The Sceptical Chymist' (1661), Boyle argued that the four classical elements (earth, air, fire, water) were not fundamental — a true element was a substance that could not be broken down further by chemical operations. This operational definition of 'element' replaced philosophical speculation with an experimental criterion and provided the conceptual foundation for Lavoisier's later systematic chemistry. Boyle also established many important experimental practices and is associated with Boyle's Law relating gas pressure and volume.

Question 4

Alchemists were charlatans who contributed nothing to the development of chemistry.

• T. True
• F. False

Answer: False

While transmutation of metals was never achieved, alchemists developed essential techniques (distillation, sublimation, crystallization, filtration) and equipment (alembics, crucibles, furnaces) that became the practical toolkit of chemistry. They discovered many substances (sulfuric acid, nitric acid, phosphorus, various metal compounds) through empirical experimentation. Figures like Jabir ibn Hayyan (8th-9th century Islamic world) and Paracelsus (16th century Europe) made genuine contributions to chemical knowledge alongside mystical theorizing. The boundary between alchemy and early chemistry was blurry.

Question 5

What was Dalton's atomic theory, and what problem in chemistry did it elegantly resolve?

Model answer: John Dalton's atomic theory (1803-1808) proposed that all matter consists of indestructible atoms, that atoms of each element are identical in mass and properties, that chemical compounds are formed by combination of atoms in fixed whole-number ratios, and that chemical reactions rearrange atoms without creating or destroying them. This elegantly explained the law of definite proportions (compounds always contain elements in fixed mass ratios) and the law of multiple proportions (when two elements form multiple compounds, the ratios of their masses are small whole numbers). The explanation was beautiful in its simplicity: if atoms combine in fixed ratios, these laws follow necessarily.

Dalton's atomic theory is the foundation of modern chemistry's quantitative character. Though later modified (atoms have internal structure; isotopes exist), the core idea of matter composed of characteristic atomic units remains central.