日本語道場 — Nihongo Dojo

～と化す

~to kasu

turns into, becomes

When to use it

Use ～と化す in written, often dramatic prose to say something has been transformed into something quite different — usually for the worse. It appears in reports, press releases, analyst commentary and news writing, but sounds theatrical in ordinary speech, where ～になる or ～化する is normal. Because it carries a sense of complete, almost irreversible change, it pairs well with nouns like 廃墟, 泥沼, 空論, 死に体.

Formation

Noun + と化す — the only productive pattern: 廃墟と化す, 泥沼と化す, 空き家と化す
Conjugates as a regular 五段 verb: 化す／化した／化している／化しつつある. The resultant-state forms 化した and 化している are by far the most common in writing.
A ～と化する (サ変) variant exists and is even more literary: 廃墟と化する街.
Verbs / i-adjectives / na-adjectives do not attach directly. Nominalize first: 形骸／無用の長物／絵に描いた餅と化す.
Distinguish from the suffix ～化する (国際化する, 形骸化する), which attaches straight to a noun with no と.

Examples

The price negotiations have turned into a quagmire.
The regular meeting has become a mere report-reading session.
With new entrants, the promising market turned into a price-war arena.
One must say the internal rules are becoming an empty shell.
With no agreement reached, the proposal turned into a pipe dream.