just been given a nalgene flask---can you use it as a hot water bottle? advice please as i do not want to experiment and maybe ruin bottle

You can. Try it!

Are you sure? He's probably referring to the 12 oz. capacity Nalgene PET flask, and not a Lexan bottle or flask. I do have a couple of GSI Lexan flasks that make excellent hot water bottles. I also have Nalgene's PET flask which I use as a pocketable water bottle. I suppose it could work as a hot water bottle as long as the water wasn't too hot. Doesn't seem like enough volume, though.

thanks for replies---it is a 12 oz flask with a plastic protective sleeve---the reason i am asking and not just trying---i had a lovely 12 oz plastic cuban rum bottle---strong, light, never leaked----filled it with hot water and deformed it beyond use

I left one of mine in a hot car with a little bit of water in it. It was not in the blue sleeve. When I came back to it, it was all puffed out. I tried filling it with hot water and then squeezed it back into the sleeve, hoping it would go back to it's normal slim self. It never did. I had to buy another. Not really directly related to your question, but something to be aware of.

Just another observation- my Nalgene flask braves the dishwasher all the time, with no damage so far.

PET will shrink when exposed to temperatures above it's Tg. IE don't put boiling water in it without expecting some deformation.

Bottles that are meant to be "hotfilled" like Gatorade and juices will do better but with repeated exposure will eventually shrink down as well.

The new GSI Lexan flasks come in 10 oz. and 18 oz. capacities. I would think the 10 oz. version might be as compact as Nalgene's flask. I don't know the weight of the GSI flasks but they'd have to be significantly heavier. They would, however, make very good hot water bottles.