A substance occupies one half of an open container. The atoms of the substance are closely packed but are still able to slide past each other.

The description provided suggests that the substance is in the liquid phase. In a liquid, the atoms or molecules are closely packed together but have enough energy to slide past one another, allowing them to flow and take the shape of the bottom of the container. In contrast, gases would fill the entire container and solids maintain a fixed shape. Therefore, the most likely phase of the substance is **liquid**.