"a spacecraft in outer space increases its velocity by firing its rockets. how can hot gases escaping from its rocket engine change the velocity of the craft when there is nothing in space for the gases to push against?

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