[ken3009]

how does the weight vary with temperature and by how much? im talking about normal laboratory glassware like fat flasks for example that are commonly used to take weights

[sjb]

how does the weight vary with temperature and by how much? im talking about normal laboratory glassware like fat flasks for example that are commonly used to take weights

I would not imagine the weight would vary at all. Volumes (both containing and of the glass itself) may vary a bit though?

[CopperSmurf]

I've never seen weight changes in glass unless it was wet or damp, but I've seen little increases in volume with higher temperatures.

[ken3009]

but in our lab we are supposed to dry "fat flasks + boiling chips" (no fat) for 1 hour and then let cool for 1 hour before taking the initial weight.
how about when taking weight with fat in it? should it be room temperature? i think even aoac method says to cool for an hour before taking weight..but WHY??

[ken3009]

okay i recall reading somewhere that if the glass is quite hot, it can cause the air inside the flask to rise a bit, thus giving a false reading on the balance ( we weight to 0.0001 decimals)..could this be true in some way?
maybe i should actually try it practically, weigh at 100C and then at room temp.

[Arkcon]

Yes, in analytical labs, there are rules such as this.  When you've dried something by heating, you've driven off moisture, and driven out air.  If its too hot, you run the risk of cracking the glass (yes, even pyrex) on contact with a cold surface.  If you let it cool too far, you run the risk of allowing ambient moisture to re-condense, adding perhaps thousandths of a gram of weight that wouldn't have been there hours before.  So these are some of the many procedural "rules" analytical labs just follow blindly.  The goal is consistency of how you handle things.  Sometimes that may have minimal effects, and sometimes, at random, say a really humid place or time of the year, really noticeable effects.  So to avoid surprises, everyone follows these rules.

It is not a coincidence that ANALytical chemistry shares the root word that Freud applied to people who's personality traits are tied to "orderliness, stubbornness, a compulsion for control."  Success in analytical chemistry is in some ways driven by the scientist's ability to control even trivial things.

[ken3009]

i tried to weigh straight out of oven (100C) and when i kept it on balance the reading was changing constantly and very fast, up and down.
but i must let u know that it was a round bottom flask so i had to put it in a beaker, which was at room temp before measuring it. maybe something going on there between the hot and cold glasses as well? hmm

[Borek]

i tried to weigh straight out of oven (100C) and when i kept it on balance the reading was changing constantly and very fast, up and down.

Good way of breaking your balance.

Hot objects heat the air, creating updrafts and plenty of turbulences, so your observations are hardly surprising.