Question

the chapati swells up when kept on hot plate

Answer 1

because the moisture present in it turns into vapour and we all know hot air rises up

Answer 2

We make chapati with wet dough which has water in it. When the wet dough is put on the hot plate, the water in the dough gets converted to steam and expands. The steam lifts the upper layer of the dough trying to escape and thus a chapati swells up when kept on hot plate.