What is physically happening when an AC freezes?
Physically, ice forms when the indoor coil's surface temperature drops below 32 degrees Fahrenheit, cold enough that moisture normally condensing off it as liquid freezes onto the metal instead. That coil runs cold in the first place because the refrigerant inside it is boiling at a temperature well below the surrounding air, and the coil surface tracks that boiling point rather than the room's. Freezing is that process going further than it's built to.
The Phase Change That Makes the Coil Cold at All
Refrigerant enters the indoor coil as a low-pressure liquid and boils into a gas as it moves through, and that boiling, that change from liquid to gas, absorbs a large amount of heat from whatever is around it. Warm house air blowing across the coil is what supplies that heat, so as the refrigerant boils it pulls heat straight out of the passing air and carries it off toward the outdoor unit. That's the entire cooling principle in one step, and it's the reason the coil surface runs cold in normal operation, typically somewhere in the 40s Fahrenheit, cold enough to do the job without dipping toward freezing.
None of this happens by accident. The compressor keeps one side of the system at low pressure and the other at high pressure, and dropping the pressure on the indoor side is specifically what lowers the refrigerant's boiling point down into that cold range in the first place. At normal atmospheric pressure the same refrigerant would boil at a much colder temperature than anything useful for cooling a house, so the low-pressure side is engineered to sit exactly where it needs to for the coil to run cold without running so cold it invites trouble on its own.
The One Temperature Line That Decides Water or Ice
That same cold coil runs below the surrounding air's dew point, so water condenses onto the fins and drains off through the pan and condensate line the whole time the system is cooling. That water is exactly what's available to freeze once the coil's surface itself crosses below 32 degrees, the precise point where liquid water turns to ice.
Two different situations can push a coil's surface below that line: not enough warm air moving across it to keep transferring heat into the refrigerant, or a refrigerant charge that has run low, which drops the pressure inside the coil, and a lower pressure means a lower boiling temperature for the coil surface to settle toward. Either way, the coil keeps shedding more heat than it's taking back in, and its surface temperature keeps sliding past the point where condensate would normally drip off. Instead of running down into the pan, that moisture bonds directly to the coil as frost.
Why the Ice Looks the Same No Matter What Started It
From a physics standpoint, once frost starts forming it tends to build on itself, since ice on a surface resists heat transfer in a way bare metal doesn't, and that thickening layer is a self-feeding part of the same process rather than a separate one. A light dusting of frost across the fins looks different from a solid block of ice sealing the coil shut, but both start from crossing that same 32-degree line, just for longer in one case than the other. What actually pushed your coil below freezing in the first place, and what to do the moment you spot ice, is a different question with its own answer specific to your system.
If you're staring at a frozen coil or line right now, the first move is switching your thermostat to off, not just up, to protect the system while it thaws. Beyond that, tracking down which side of the equation caused it calls for licensed AC repair, since telling airflow and refrigerant apart takes gauges and training a homeowner isn't going to have on hand.
Watching Ice Build on the Coil or Line Right Now?
Switch the thermostat off, then call or text (239) 350-5234. Average response is under 3 hours during business hours.