i ran out of glass containers (people haven't returned them) and i want to give my boyfriend hot soup for work. Is it safe to put it in a mason jar and put the lid on while hot? And how long might the soup stay hot in it?
For the first part, about pouring hot soup in mason jars, it should be safe, but there's always a risk of shattering if the liquid is too hot.
There are many somewhat related topics regarding putting hot liquid in mason jars.
For the second part, It will not keep the soup hot/warm for a long time, it is not a "thermos" container.
I have had mason jars break when filling with hot items. It is best to warm the jar first, just by running hot water into it. A someone else pointed out, a mason jar is, of course, not insulated. You mention this is to take to work, I assume for lunch or dinner. I also want to point out that you need to consider food safety, as your soup will soon be in the "danger zone." You should read this. It is much safer to chill the soup to refrigeration temperature, pack it in a cooler, or refrigerate at work, then reheat at meal time.