Q: My iphone 5s heat up when I charge

Hello there, adel0_o.

The following Knowledge Base article provides some great information on that very question:

iOS devices: Keeping device within acceptable operating temperatures

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

Particularly:

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.

Thanks for reaching out to Apple Support Communities.

Cheers,

Pedro.