may i know.. when the experiment to find the formula for zinc chloride, why we can't weigh the zinc chloride when it still hot??

These are guesses:

1) Safety.

2) The reaction may still be proceeding.

3) You have misinterpreted the instructions.

If a salt is still hot, then it may be losing crystallization waters as it cools which can change the weight of the sample significantly. The reverse can also happen if you put some very hygroscopic chemical on a scale exposed to air for a while (especially if you live in a humid climate like me). I've never seen it, but I've heard you can throw a bit of tetra-n-butylammonium chloride on the scale and watch the sample gain mass over the course of a few minutes.

 

I'm sure safety is also a factor though as very much don't won't to get covered in hot wet salt!

 

EDIT: hypervalent_iodine: We posted almost at the same time. I beat you by about thirty seconds with the same answer. I win

Edited June 17, 201114 yr by mississippichem

These are guesses:

1) Safety.

2) The reaction may still be proceeding.

3) You have misinterpreted the instructions.

 

Not quite. It's actually because ZnCl₂ is very soluble in water and will absorb it readily from the atmosphere. So if you weigh it while hot, water will begin to condense in your flask and contaminate your sample. Weighing to cold ensures firstly, that your getting an accurate reading, since you will accrue water mass as the sample cools, and secondly it ensures your sample is at least moderately dry and that it does not react with water to form hydrates.

 

If a salt is still hot, then it may be losing crystallization waters as it cools which can change the weight of the sample significantly. The reverse can also happen if you put some very hygroscopic chemical on a scale exposed to air for a while (especially if you live in a humid climate like me). I've never seen it, but I've heard you can throw a bit of tetra-n-butylammonium chloride on the scale and watch the sample gain mass over the course of a few minutes.

 

I'm sure safety is also a factor though as very much don't won't to get covered in hot wet salt!

 

Well done, sir. You beat me to the punch in a matter of seconds.