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How did Democritus explain the differences between water and iron?

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Bronze bust of Epicurus with a full beard and draped clothing, displayed on a round inscribed base at the National Archaeological Museum of Naples. — Atomism Epicurus, bronze bust from a Greek original, c. 280–270 bce; in the Museo Archeologico Nazionale, Naples.

Democritus explained the differences between substances like water and iron by asserting that all matter is composed of the same eternal, indivisible atoms moving through a void. He argued that qualitative differences—such as hardness or fluidity—are not inherent to the atoms themselves but are merely apparent, resulting from the different configurations and combinations of these atoms.1

According to his theory:

Water: The atoms of water are smooth and round. Because of this shape, they are unable to hook onto one another and instead roll over each other like small globes, resulting in a liquid state.1
Iron: The atoms of iron are rough, jagged, and uneven. These shapes allow the atoms to cling together, which forms a solid body.1

In this atomistic framework, the atoms themselves remain intrinsically unchanged; observable differences in the physical world are simply the result of these purely quantitative variations in shape, arrangement, and motion.21