Question

why does water curve downward when placed in a glass tube ?

Answer 1

the water curve downward when placed in a glass tube because water has lower force of attraction than solid. so it fills the container in which it is kept.

Answer 2

A concave meniscus, which is  what you normally will see, occurs when the molecules of the liquid are atracted to those of the container. This occurs with water and a glass tube . A convex meniscus occurs when the molecules have a stronger attraction to each other than to the container, as with mercury and
glass.
hope I help you