What does hot mean? Do you use something to monitor the temperature? You can download iStat Menus (google will lead you the right way), it has 14 days trial I think.
CPU normal temperature should be 90 degrees Celsius at most. If it's less than that, it's fine. Mine is at about 50 while doing a little browsing, some text editing and a bit of chat, but spikes up to 80 or more if I play hd movies or edit some raw images or other intesive things.
Also, you should know the MacBook Air is not a laptop (eg: designed to be used on your lap) but a Notebook. If you keep it on your lap or on a surface that does not allow proper ventilation, it will be hotter than usual.
If you really want to keep it on your lap, you could buy an iLap or something like that, at least that's what I ended up doing because I couldn't stand the heat on my legs any longer