verb
DUBIOUS : Americans can be separated into a number of ethnic groups.
GOOD : Americans can be divided into a number of ethnic groups.
DUBIOUS : England was separated into 650 constituencies.
GOOD : England was divided into 650 constituencies.
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separate = place or keep (people or things) apart from one another: 'Break an egg into a bowl and separate the white from the yolk.'
divide = cause something to consist of (or be seen as) a number of parts, groups, sections etc: 'The manufacturing process is divided into three stages.'