Context: The particular contrasts which are phonemic in a language can change over time. At one time, [f] and [v], two sounds that have the same place and manner of articulation and differ in voicing only, were allophones of the same phoneme in English, but later came to belong to separate phonemes. This is one of the main factors of historical change of languages as described in historical linguistics.

Question: What kind of linguistics describes how factors of languages change in history?
Answer: historical

Question: With the passage of time what particular things phonemic in a language are known to change?
Answer: contrasts

Question: In the past sounds that now belong to separate phonemes were allophones of what kind of phoneme in English?
Answer: the same phoneme

Question: What kind of linguistics describes how factors of phenomes change in history?
Answer: Unanswerable

Question: With the passage of phenomes what particular things phonemic in a language are known to change?
Answer: Unanswerable

Question: In the past sounds that now belong to separate phenomes were allophones of what kind  of change in English?
Answer: Unanswerable

Question: What cannot change over time?
Answer: Unanswerable

Question: Where is historical change of time described?
Answer: Unanswerable