Police have discovered that the leaked chemical, from one of two lorries that collied on the A1 near Boroughbridge, is a sodium hydroxide called Proxel.
Around three hundred litres were leaked from the lorry.
The leak has been stemmed and officers remain at the scene and are working to get the road open as soon as possible.
In the meantime, drivers are being asked to avoid the area.
Diversions remain in place at junctions 47 and 48.