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Egg whites solidify when you boil them.
Literal
Egg [possessive-の] white-part [topic-は] boil-[if-ば] solidify.
The ~ば conditional describes a general cause-effect truth — 'if you boil it, it solidifies [as a matter of law].' 白身 ('white part') is the domain term for egg white, contrasted with 黄身 ('yolk'). 固まる is the intransitive 'become solid' (transitive partner: 固める).