Reagent bottles must be closed when not in use because they may contain volatile or sublime substances.
Volatile substances are substances that vaporize spontaneously when exposed to the atmosphere. Examples of volatile substances are; methanol, ethanol and chloroform.
Sublime substances are solids that spontaneously turn into vapor without passing through a liquid stage. Examples of sublime substances are; iodine and dry ice.
Also, the reagent may become contaminated or oxidized when exposed to the atmosphere as the reagent bottle is left open thereby making the reagent useless for chemical analysis.
Both sublime and volatile substances will escape from the reagent bottle if the bottles are not closed when not in use. Hence, reagent bottles must always be closed.
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