A postive charge is placed next to an unkown charge. There is a repulsive force between them. What charge is the unknown force and how do you know?

The freezing and boiling point of a substance changes as the air pressure around it changes. for example, at a lower air pressure (higher altitude) it is easier for water molecules to escape from liquid into the air. in a high altitude city such as denver, colorado compared to a sea-level city such as houston, texas, water