Breakdown of Мы начинаем эксперимент, пока жидкость в колбе холодная.
мы
we
холодный
cold
в
in / to (direction)
начинать
to begin
Questions & Answers about Мы начинаем эксперимент, пока жидкость в колбе холодная.
Why do we use пока here instead of пока не?
In this sentence, пока means 'while' or 'as long as'. It shows that the state of the liquid being cold is happening at the exact same time as starting the experiment. You would use пока не if you wanted to say 'until' a state changes (for example, waiting until it gets cold).
Where is the verb in the second half of the sentence (пока жидкость в колбе холодная)?
The present tense of the verb 'to be' (is/are/am) is omitted in Russian. The clause literally reads 'while the liquid in the flask [is] cold'. If this were happening in the past, you would need to add the verb back in: пока жидкость была холодная.
Why does the adjective холодная take a feminine ending?
It modifies жидкость (liquid). Nouns ending in a soft sign (-ь) can be either masculine or feminine, which can be tricky to remember, but жидкость is feminine. Because it is the subject of that second clause, the adjective takes the feminine nominative ending -ая.
Why is the word колба (flask) written as в колбе here?
The phrase answers the question 'where is the liquid?' (location). When using the preposition в (in) to indicate a static location, the noun must be in the Prepositional case. This changes the standard feminine -а ending to -е.