Why does my iPhone get hot when I use it?

Your iPhone is basically a computer in its mobile form. No moving parts like fans and hard drives. It also barely has any exhaust holes where heat can dissipate so occasional heating around the device is normal. 

Like a computer, your iPhone kicks out heat. The two main components that generate heat are the system-on-chip (the equivalent to your computer’s CPU), and the battery. When the device creates heat, there’s no fan to cool it directly, and instead, the metal housing acts like one giant heatsink.

Then why is my iPhone is always getting hot to the touch?

A few things could cause this like:

• Playing a demanding 3D game.
• Streaming high quality video for an extended period.
• Using processor-intensive apps like a synthesizer, digital audio workstation, or video editor.
• Making heavy use of GPS, particularly on older devices.
• During charging, especially combined with other activities on this list.

What do you do when excessive heating happens on your iPhone?

• Stop using your iPhone for a while. Yes, common sense suggests that giving your iPhone a rest will certainly help in cooling it back down.
• Turn off your iPhone for a few minutes. If letting it unused for minutes don’t help, try turning it off and leaving it on a cool place.
• Remove any case and help it dissipate heat by holding it on your hand or turning it off.
• If it’s recharging, try not to use it while it is plugged in. 
• If you are suspicious that an app is making all the heating, try closing all open apps and activate Airplane Mode.