Q: If all impurities are carefully removed from water and it is then cooled , will it freeze at 0 °C ?
Sol: No , water can freeze only when there are some dissolved impurities to serve as centre of crystallization . If all impurities are removed and the water is cooled carefully , it will not freeze at 0 °C . It will be supercooled to much below 0 °C . If however a little jerk is given or a small crystal of ice is dropped into the supercooled water , it will freeze into a large block of ice .