Question

how would you confirm that the colourless liquid given to you is pure water

Answer 1

water boil at 100°c .
take colour less liquid in beaker and heat it.
and keep a look on thermometer
and find whether colour less liquid boils at 100°c or not
if it boils at 100°c than colour less liquid is water

Answer 2

water should freeze at 0°
water should boil at 100°
water is a universal solvent which means majority of the solute is miscible in water
water is made of H20