chemistry

Ostwald's rule

Ostwald's rule is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Ostwald's rule rather than just read about it. In short: In materials science, Ostwald's rule or Ostwald's step rule, conceived by Wilhelm Ostwald, describes the formation of polymorphs. The rule states that usually the less stable polymorph crystallizes first.

Key takeaways

Ostwald's rule belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ostwald's rule to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ostwald's rule from memory before moving on to harder problems.

Reference excerpt

In materials science, Ostwald's rule or Ostwald's step rule, conceived by Wilhelm Ostwald, describes the formation of polymorphs. The rule states that usually the less stable polymorph crystallizes first. It also often corresponds to the most soluble phase. Ostwald's rule is not a universal law but a common tendency observed in nature. This can be explained based on irreversible thermodynamics, structural relationships, or a combined consideration of statistical thermodynamics and structural variation with temperature. Unstable polymorphs more closely resemble the state in solution, and thus are kinetically advantaged. For example, out of hot water, metastable, fibrous crystals of benzamide appear first, only later to spontaneously convert to the more stable rhombic polymorph. A dramatic example is phosphorus, which upon sublimation first forms the less stable white phosphorus, which only slowly polymerizes to the red allotrope. This is notably the case for the anatase polymorph of titanium dioxide, which having a lower surface energy is commonly the first phase to form by crystallisation from amorphous precursors or solutions despite being metastable, with rutile being the equilibrium phase at all temperatures and pressures. In the case of calcium carbonate (CaCO3) precipitation from an aqueous solution, a kind of unstable colloidal sol, or gel, forms first as an aqueous suspension, then it evolves at room temperature into vaterite (hexagonal), the less stable and most soluble polymorph of CaCO3, and, depending on the temperature of the solution, into calcite (hexagonal, ρ = 2.7 g/cm3) (T ~ 40 °C) or aragonite (orthorhombic, ρ = 2.9 g/cm3) (T > 70 °C).

See also Chemical crystallography before X-rays Nucleation Crystal growth Disappearing polymorph Ostwald ripening Digestion and ageing of precipitates

References

Further reading Steefel, Carl I.; Van Cappellen, Philippe (1990). "A new kinetic approach to modeling water-rock interaction: The role of nucleation, precursors, and Ostwald ripening". Geochimica et Cosmochimica Acta. 54 (10): 2657–2677. doi:10.1016/0016-7037(90)90003-4.

Worked examples

Example 1 — a first encounter with Ostwald's rule

Start with the simplest possible case. Write down what Ostwald's rule claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ostwald's rule before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ostwald's rule ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ostwald's rule

In research: Ostwald's rule appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ostwald's rule in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ostwald's rule is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chemical engineering thermodynamics, Colloidal chemistry, Crystallographic defects, so understanding it makes those chapters shorter.
In everyday life: Look for Ostwald's rule outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Ostwald's rule” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Ostwald's rule in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ostwald's rule means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ostwald's rule out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ostwald's rule in simple terms?

Why does Ostwald's rule matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ostwald's rule?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ostwald's rule.