Asubstance will take on the shape of an open container if it is a

A substance will take on the shape of an open container if it is a  Liquid.

Explanation:

The major state of matter are solid, liquid and gas. Liquid usually have a definite volume. Liquid take the shape of the container in which they contain. They are almost incompressible fluid so it is expected they conform to the shape of the container which they occupy.

Liquids have no definite shape . The particles in a liquid flows easily. The atoms or molecules of a liquid are bonded by intermolecular bonds. One example of a known liquid at room temperature is water.

Generally, liquid have a greater freedom of movement than solid. The molecules of a liquid is closely attach to each other.  

frequency or wavelenth of em radiation in a vacuum