Q: iPhone 5 easily hot

Hello there, B.Jian.

Here is a helpful article that explains about the rise in temperature while charging or in use:

iPhone, iPad, and iPod touch (4th generation): Keeping device within acceptable operating temperatures

http://support.apple.com/kb/HT2101

Specifically keep in mind:

When using the device or charging the battery, it is normal for it to get warm. The exterior of the device functions as a cooling surface that transfers heat from inside the device to the cooler air outside.

Cheers,

Pedro D.