Assuming that the wall socket is providing power (or the switch for the socket is on) it is normal, or rather common, for power adapters of devices to be a bit warm when connected to the wall socket (mains) without a device connected to the other end (iPad, iPhone, laptop, etc.). In many cases, the charger would be considerably warmer while actively charging a device than when it's not.
Here's additional information broken down into bullet points:
- The amount of heat produced primarily depends on the size of the adapter, its charging capacity and its build quality (there are other secondary factors as well).
- Heat is a product of resistance in electrical and electronic circuits, and is observed in every electrical/electronic equipment to varying degrees.
- All power adapters have a "transformer" that first converts the high mains AC (alternating current) voltage, which would be 230V or 110V, to a much lower voltage, which is then converted to DC (direct current) and fed to the device.
- An ideal transformer and power supply would not produce heat when nothing is consuming power on the other end. But practically, all transformers (and power supplies) have losses of different kinds (although this has improved over time), and this not only results in heat, but also results in minuscule amounts of power being used even when no device is connected to the adapter.
- If you find the adapter being too hot to touch, then it's malfunctioning or failing. If it's only a bit warm, there's nothing to worry about except for an increase in your electricity bills (however small).
- As for fire hazard, it's always better to turn off the mains switch (or unplug if there's no switch to control it) on all electrical equipment when not in use and also keep them away from flammable items (or items that may catch fire after prolonged exposure to heat).
- If the charger overheats during usage, Apple's genuine chargers have built-in protection to turn them off.
For more information, see:
- Wikipedia article - Real transformer - deviations from ideal
- iPad charger teardown (Ken Shirriff's blog)