Why was the Hijrah important

The Hijrah is a significant event in the history of Islam in that it signified an important journey that Muhammad and his followers took in 622 CE. Fearing assassination Muhammad and his followers fled Medina and travelled north to the city of Yathrib. Here they established themselves and the city later became known as Medina Al-Nabi or "the City of the Prophet" and was eventually shortened to its current name just Medina. In Medina Muhammad and his followers suffered hardship to reestablish themselves and survive signifying an important step in the survival of the Islamic faith. 

A chemical change because there was a color change